> EPA
United States
Environmental Protection
Agency
Fishery Resources and
Threatened Coastal Habitats
In the Gulf of Mexico
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EPA/600/R-05/051
July 2005
FISHERY RESOURCES
AND THREATENED COASTAL HABITATS
IN THE GULF OF MEXICO
By
Darrin D. Dantin
William S. Fisher
Stephen J. Jordan
James T. Winstead
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL HEALTH AND ENVIRONMENTAL EFFECTS
RESEARCH LABORATORY
GULF ECOLOGY DIVISION
1 SABINE ISLAND DRIVE
GULF BREEZE, FL 32561-5299
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Fishery Resources and Threatened
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FOREWORD
The purpose of this document is to lay the groundwork for research into the effects of altered coastal habitats on fish and
shellfish of the northern Gulf of Mexico. The U. S. EPA National Health and Environmental Effects Research Laboratory's
Aquatic Stressors Framework and Implementation Plan for Effects Research (USEPA 2002) states that the principal goal
of this research is to "...provide the scientific basis for assessing the role of essential habitat in maintaining healthy
populations offish, shellfish, and wildlife and the ecosystems upon which they depend." Altered habitat research at the
Gulf Ecology Division (GED) is directed at specifying quantitative relationships between populations of abundant, eco-
nomically important species and essential features of their habitats. The intended application of these stressor-response
models is to support criteria that could prevent or remedy harmful effects of habitat alterations (USEPA 2002).
The ability of our coastal waters to support valued populations of fish and shellfish is frequently degraded when habitats
are altered by human activity. In order to protect habitats and their valuable inhabitants, U. S. EPA Program Offices will
need a solid, scientifically-defensible foundation for quantifying stressor-response relationships between critical habitats
and affected resources. Methods and models must be developed for the measurement and prediction of biotic depen-
dence on different habitat types. Ultimately, information garnered through directed research should lead to recommenda-
tions for habitat-based criteria protective offish, shellfish, and aquatic-dependent wildlife.
The U. S. EPA's Office of Research and Development has planned an Aquatic Stressors Critical Path for Habitat Alter-
ation to address these needs. The first steps of this path, identifying appropriate scales and endpoints for research, have
been completed. A Research Implementation Plan has been prepared to describe research to be performed by four
Ecology Divisions in the National Health and Environmental Effects Research Laboratory. Divisions will investigate
habitat-response relationships and mechanisms for the Pacific Coast (Western Ecology Division), Atlantic Coast (Atlantic
Ecology Division), Great Lakes (Mid-Continent Ecology Division) and the Gulf of Mexico (Gulf Ecology Division) using
similar scales and approaches. The information from each Division will be synthesized into useful models for the devel-
opment of national habitat criteria.
The major objective of ORD's Altered Habitat research is to produce stressor-response models and approaches that will
accurately quantify and predict the effects of altered habitats on valued fish, shellfish, and wildlife populations in lakes and
estuaries, and to elevate these models to (at least) regional scales useful for regulatory application. Research by NHEERL's
Gulf Ecology Division will follow closely the steps outlined in the Critical Path. One of the first goals is to identify the
species and habitats of greatest concern. The following presentation fulfills that goal through examination of life cycles
and habitats of selected economically and ecologically important fish and shellfish inhabiting Gulf of Mexico estuaries.
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Fishery Resources and Threatened
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TABLE OF CONTENTS
Page
Foreword jj
List of Tables jv
List of Figures v
Acknowledgement vj
Executive Summary vii
I. Introduction 1
II. Economic Value of Selected Aquatic Species in the Gulf of Mexico 3
III. Dependence of Valued Species on Essential Habitat 6
IV. Penaeid Shrimp 8
V. Eastern Oyster 14
VI. Blue Crab 20
VII. Sciaenid Fish 24
VIII. Status of Life-supporting Habitats in the Gulf of Mexico 29
IX. Habitat Alterations 35
X. Considerations for Altered Habitat Research 40
Literature Cited 43
in
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LIST OF TABLES
Table 1. Anthropogenic activities capable of affecting estuarine-dependent fish, shellfish,
and wildlife 2
Table 2. Average tonnage and dollar value of shellfish fisheries, 1981 -2000 3
Table 3. Average tonnage and dollar value of commercial and recreational sciaenid fisheries
of the Gulf of Mexico 5
Table 4. Total estimated number of participants in at least one fishing trip per year 5
Table 5. Temperature and salinity tolerance and preference ranges for three species of
penaeid shrimp 12
Table6. Sciaenid species of the Gulf of Mexico 25
Table/. Spotted seatrout preferred prey at each life stage 26
Tables. Target list of habitats for the Northern Gulf of Mexico ecoregion 30
Table 9. Descriptive information for Gulf of Mexico estuaries from Laguna Madre
to Tampa Bay 32
IV
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LIST OF FIGURES
Figure 1. Commercial landings in million kg of all species from Gulf waters, 1950-1996 4
Figure 2. Penaeid shrimp life cycle 9
Figure 3. Gulf-wide ranges for juvenile penaeid shrimp 10
Figure 4. Life cycle of the eastern oyster 15
Figures. Blue crab life cycle 21
Figures. Dependence of selected Gulf of Mexico species on major habitat attributes 29
Figure/. Gulf of Mexico estuaries 31
Figure 8. Existing and predicted loss of Louisiana coastal habitats 34
Figure 9. Evidence of habitat loss in the Pensacola Bay system 35
Figure 10. Standing crop estimates of Breton Sound (LA) oysters from public seed grounds 38
Figure 11. Wetland types in the watershed of the Caloosahatchee River and Estuary (SFWMD 2003) 38
Figure 12. Conceptual diagram of the elements of altered habitat research and their relationships 42
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Acknowledgements
The authors would like to thank the reviewers of the publication. They are: Lawrence Rozas, NOAA, Southeast Fishery
Science Center, Lafayette, LA; Jim Power, U.S. EPA, National Health and Environmental Effects Research Laboratory,
Western Ecology Division, Corvallis, OR; Giancarlo Ciccihetti, U.S. EPA, National Health and Environmental Effects
Research Laboratory, Atlantic Ecology Division, Narragansett, Rl; and Larry Goodman, U.S. EPA, National Health and
Environmental Effects Research Laboratory, Gulf Ecology Division, Gulf Breeze, FL. Also, thanks to Valerie Coseo and
Cecilia Khan, NCBA Senior Environmental Employment Program, for document formatting and typing.
VI
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EXECUTIVE SUMMARY
All life is supported by—and limited by—the physical, chemical, and biological properties of its environment. Organisms,
populations, species, and biotic communities inhabit complex spaces bounded by multi-dimensional ranges of tolerance.
Hutchinson (1959) dubbed these spaces, or niches, "hypervolumes;" we use the simpler, albeit vaguer term, habitat.
Habitat has many definitions, none of them fully satisfactory. Peters and Cross (1992), in attempting to define coastal fish
habitat, determined that the definition (properly) depended upon the context. In this document, habitat generally means
those features of the physical environment without which selected species cannot thrive.
Humans, by altering landscapes and seascapes to extend and modify their habitats, alter other species' habitats in ways
that can shrink their boundaries and weaken their supporting functions. It is sometimes observed, more often inferred,
that such alterations reduce the abundance and productivity of affected species or populations. If essential habitats are
totally destroyed or their functions thoroughly degraded, it is clear that local or global extinctions (depending upon the
scale of destruction) will result. It is much more difficult to determine, in the absence of catastrophe, how population
success is related to habitat extent and condition (USEPA 2002).
The fisheries of the United States are heavily dependent on estuaries and the unique habitat features they provide;
estuarine-dependent species comprise more than 50% of U. S. commercial fisheries landings (Houde and Rutherford
1993). Commercial and recreational fisheries in the northern Gulf of Mexico (western Florida through Texas) have a
combined annual economic value of more than $1 billion (NMFS 2003a, 2003b). These facts have guided the selection
of the following species for initial attention by the Gulf Ecology Division's altered habitat research project: shrimp (three
species), blue crabs, eastern oysters, and spotted seatrout (the last representing several species of sciaenid fishes).
There is strong evidence that all of these species, at some point in their life cycles, depend on specific types of physical
habitats in areas where freshwater and saltwater mix.
A review of the life histories and habitat dependencies of these economically important species indicates a few habitat
factors of major importance for the species of concern. These include (1) sources of freshwater inflow to coastal waters,
(2) tidal marshes, (3) submerged aquatic vegetation, (4) shallow, near-shore soft bottoms, and (5) shell reefs and the
associated oyster communities. Our research is focused on physical alterations of these habitat features. Contamination
of coastal habitats by nutrients, sediments, and toxic contaminants is being addressed by other research teams (USEPA
2002).
VII
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Fishery Resources and Threatened
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I. INTRODUCTION
Many anthropogenic activities adversely influence fish,
shellfish and aquatic-dependent wildlife by altering the habi-
tats in which they live. In fact, habitat alteration is one of
the most important contributors to declines in ecological
resources in North America (USEPA 1990). Habitat loss
and degradation are frequently identified as principle rea-
sons for failure of aquatic systems to achieve their desig-
nated uses in state reports required under Section 305(b)
of the Clean Water Act, and are major causes of species
endangerment (Warren and Burr 1994, Stein and Flack
1997). Therefore, there is a national need to "assess...the
role of essential habitats in maintaining healthy populations
offish, shellfish, and wildlife and the ecosystems upon which
they depend" (USEPA 2002).
Several steps are necessary to provide the scientific basis
and to inform the development of criteria to protect living
aquatic resources from habitat-related losses. Appropriate
stressor and response measures must be selected, tools
developed for their measurement, classification schemes
delineated for critical habitat attributes, and models devel-
oped for extrapolation to a population level of organization
and to larger spatial scales (habitat traditionally has been
studied at the patch scale, whereas USEPA generally ap-
plies regulatory decisions at a regional or ecosystem scale).
National habitat criteria will require data and stressor-re-
sponse models that span several geographic ranges, habi-
tat types and response species. This report provides the
rationale for selecting valued species and habitats from the
Gulf of Mexico.
The Gulf of Mexico is a large, semi-enclosed marine eco-
system that receives runoff from Mexico, Cuba, over half
of the United States and portions of Canada. A great num-
ber and diversity of organisms inhabit the coastal zone,
including those with high economic value (fish, shellfish),
high public visibility (marine mammals, corals, sea turtles)
and ecological significance (submerged aquatic vegetation,
filter-feeding bivalves, detritivorous invertebrates). Survival
and sustainable populations of these living resources re-
quire the supportive infrastructure of a healthy ecosystem.
Yet, increasing human activity along the coast and in the
coastal zone has raised serious questions concerning the
continued integrity of the Gulf of Mexico coastal ecosys-
tem and the many resources it provides.
The most obvious impacts associated with human activi-
ties are those that directly kill and injure biota, such as fish-
ing and by-catch mortality, harmful algal blooms, and toxic
spills (Table 1). Less obvious are the effects of indirect stres-
sors, which include eutrophication, sedimentation, and habi-
tat alteration. Habitat alteration may be the most insidious
of these stressors, often occurring in small, inconspicu-
ous increments that cumulatively generate enormous ef-
fects. Coastal habitats may be altered quantitatively (e.g.,
loss of habitat from dredging, shoreline armoring, marina
development, invasive species) or qualitatively (e.g.,
changes in water temperature, salinity, freshwater flow,
nutrients, contaminants). In either case, the success of
residents in a habitat is threatened when it is altered.
There are perhaps many ways to evaluate and compare
the importance of different types of habitats to the well-
being of society. Evaluation of habitat types for their sup-
port of economically-important species is one of the most
effective options. Such an approach examines the impor-
tance of organisms that are highly regarded by the public
and are continuously appraised for monetary value through
the marketplace. The recorded economic value, which can
reach several decades into the past, provides a basis for
comparing societal value among species and, by exten-
sion, a means to value habitats within a cost-benefit frame-
work. Section II of this report compares the economic value
of selected species from the five states that border the
Gulf of Mexico. The habitat requirements of those with the
greatest economic benefit are then examined in greater
detail.
Declining populations of valued species cannot be linked
arbitrarily to habitat loss or degradation. Even if some
species reside in a particular habitat, they are not neces-
sarily dependent on it. Hence, life-cycle information and
population structures for valued species must be exam-
ined for indications that habitat plays a significant role in
population success. In the context of this report, and the
Altered Habitat Strategy, such habitat is termed 'essen-
tial'. It is also important to recognize which characteris-
tics, or attributes, of essential habitat are most respon-
sible for life support. With this knowledge, we can deter-
mine whether these attributes are influenced by human
activity, and gain a mechanistic tool for classifying habi-
tats and estimating their life-support potential. If habitat
criteria are to be used as the basis for habitat protection,
there is an implied obligation to demonstrate that the habi-
tats (or their attributes) are limiting factors in the success
of the population. In Sections III-VII of this report we ex-
amine the life cycles of four valued species from the Gulf
of Mexico and attempt to identify habitat types and at-
tributes that appear most prominent in the population suc-
cess of each species.
It must be determined whether essential habitat types and
attributes are declining in quality or quantity in the Gulf of
Mexico, and whether they are threatened by human
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activity. As noted previously, many factors can alter es-
sential habitat adversely, and the cumulative results of
these alterations are a significant concern. Declining quan-
tity of essential habitat is at least possible to estimate.
Each of the five U.S. states along the northern border of
the Gulf have some historical records of types of sub-
strate and biota along their coastal shorelines. Aerial im-
agery has been used to document shoreline changes in
many locations. Altered habitat quality would at first ap-
pear even more difficult to evaluate. Yet, freshwater in-
flow to coastal zones is perhaps the predominant factor
in coastal habitat quality, and records of freshwater inflow
to the Gulf of Mexico have been maintained throughout re-
cent history. In Sections VIII and IX of this report, we de-
scribe the status of essential habitats in the Gulf of Mexico
through examination of habitat loss and freshwater inflow.
The final section of this report includes suggestions for the
next steps toward quantitative species-habitat models. Pre-
liminary models could be constructed from existing data,
and refined as directed field and experimental studies gen-
erate additional data and test of hypotheses.
Table 1. Types of anthropogenic activities capable of affecting estuarine-dependent fish, shellfish, and wildlife
(modified from Birkett and Rapport 1999).
Shoreline Development
Construction of jetties and causeways
Dredging and maintenance of shipping
channels
Development of barrier Islands
Amplification of coastal erosion effects
Subsidence from excessive groundwater
Oil and Gas Industry
Offshore oil and gas development
Drilling fluid discharges
Accidental oil spills
Chronic pollution from oil tankers and platforms
Associated navigation and access canals
Physical Restructuring of Wetlands
Flood control levees
Navigation and drainage canals
Wetland impoundments
Drainage for industrial, urban, and agricultural
development
Pollutant Discharges
Organic pollutants from human waste disposal
Point-and distributed-source chemical discharges
Nutrient Loading
Increased concentration of nutrients in riverine outflows
Fisheries
Overharvesting of traditional Gulf species
Overharvesting of oceanic predators
New species fisheries developing
Chemical and harvesting pressure on oyster beds
Introduction of Exotic Species
Nutria (Myocastor coypus), emergent vegetation
herbivore
Insect pests for alligator weed control
Various pathogenic species affecting shellfish stocks
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II.
ECONOMIC VALUE OF SELECTED AQUATIC SPECIES IN THE GULF OF MEXICO
Economic value as a criterion for species selection links
directly to societal values. Moreover, for commercially im-
portant species there are consistent, continuous records
of fisheries statistics over several decades, so we can ob-
serve trends in relative abundance, and for some speci
estimate absolute abundance and biomass. Species of high
economic value are also likely to have high abundance,
biomass, and ecological importance. Other value systems
(e.g., purely ecological) could be applied, but these are not
as simply incorporated into a cost-benefit framework. Fur-
thermore, the primary purpose for examining resources is
to identify and highlight the habitats they depend upon. 11
anticipated that ecologically important species will rely
largely on the same habitats as economically important
species. Some species (e.g., oysters) would be consid-
ered substantial contributors to both categories.
It is the ultimate intent that this economic overview will pro-
vide information applicable to a large portion of the Gulf c
Mexico. Economic comparisons of different species will
depend on geographic boundaries since species distribu-
tions change across eco-regions. Although the northern
Gulf of Mexico falls primarily within the temperate zone
the climate is tropical from the central west coast of the
Florida peninsula southward, and at the southern end of
Laguna Madre, Texas (Hoese 1998). The temperature
barrier at these points is responsible for significant changes
in the abundance and types of aquatic species, althoi
within these climate boundaries, the distribution of dom
nant species is relatively consistent. For this reason tl
species selected for consideration are generally distributed
within the temperate zone of the Gulf of Mexico.
Table 2. Average tonnage and dollar value (in millions per
year) of shellfish fisheries during the decades 1981-2000. Both
Atlantic Ocean and Gulf of Mexico values are shown for eastern
oyster and blue crab fisheries (NMFS 2003a).
Penaeid Shrimp (Gulf Only)
1981-1990 1991-2000
White
Brown
Pink
Total
Tons Dollars Tons
34,452 150 33,750
64,832 240 56,184
8,000 38 8,000
108,804 428 97,934
Dollars
163
240
40
444
Eastern Oyster
1981-1990
Gulf
Atlantic
Total
Tons
9,732
7,146
16,878
Dollars
40.1
38.0
78.1
1991-2000
Tons
9,018
3,223
12,242
Dollars
39.7
33.6
73.3
Blue Crab
1981-199Q
Gulf
Atlantic
Total
Tons
25,255
69,622
94,877
Dollars
19.4
52.0
71.4
1991-2000
Tons
28,928
71,773
100,752
Dollars
38.8
109
148
Records of commercial fishery landings make selection of
most-valued species a simple task. The National Marine
Fisheries Service has documented U. S. fishery landings
and market value for many species since at least 1950
(Figure 1). These data are available in a searchable format
(NMFS 2003). For the Gulf of Mexico, it is apparent that
commercial harvest of shellfish is much greater in eco-
nomic value than commercial harvest of finfish. Penaeid
shrimp are the most valuable commercial species (Table
2), with annual landings worth more than $400 million over
the last two decades. Among the three species of penae-
ids, brown shrimp (Farfantepenaeus aztecus) are the most
valuable fishery, followed by white shrimp (Litopenaeus
setiferus). Pink shrimp (Farfantepenaeus duoarum), al-
though least valuable of the penaeids, support landings
equal in value to eastern oysters (Crassosfrea virginica) or
blue crabs (Callinectes sapidus). Eastern oysters and blue
crabs have generated average annual landings worth nearly
$40 million from Gulf waters over the past two decades.
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Finfish collectively do not have nearly as much commer-
cial value as shellfish (Table 3). Black drum and red drum
fisheries have been valued at $2-3 million per year, and
the spotted seatrout fisheries averaged ~$1 million. Fol-
lowing a near loss of the fishery in the 1980s attributed to
overfishing, the commercial value of red drum dropped
precipitously in the 1990s with the placement of strict har-
vest restrictions on size and quantity in both federal and
state-managed waters. These new restrictions effectively
ended the commercial harvest of red drum in the Gulf of
Mexico. However, the resulting enhancement of the recre-
ational fishery has provided an economic boon for tourism
and the recreational fishing industry. A relatively unbiased
estimate of the economic importance of the recreational
fishery has been provided by the National Marine Fishery
Service's Marine Recreational Fishery Statistics Survey
(NMFS 2003b). High numbers of resident and, more im-
portantly, non-resident recreational fishing trips have been
documented for the Gulf Coast (Table 4). The economic
benefit of out-of-state tourists visiting coastal states for fish-
ing excursions (license fees, hotels, charter boats, food,
etc.) is probably quite significant, albeit difficult to quantify.
1940 1950 1960 1170 1MO 1990 2000
Figure 1. Commercial landings in million kg of all
species from Gulf waters, 1950-1996 (USEPA
1999 citing NMFS data).
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Table 3. Average tonnage (metric) and dollar value (in millions per year) for each U.S. commercial and
recreational major sciaenid fishery during the decades 1981-1990 and 1991-2000 (NMFS 2003b, 2003c).
1981-1990 1991-2000
COMMERCIAL Area Tons Dollars Tons Dollars
Black Drum
Gulf
Atlantic
Total
3,039
96
3,135
2.24
0.60
2.30
2,199
117
2,136
3.69
0.10
3.78
Gulf 1,744 2.77 18 0.05
Red Drum Atlantic 34 0.16 93 0.19
Total 1,878 2.93 112 0.25
Gulf 1,254 2.67 462 1.20
Spotted Seatrout Atlantic 299 0 61 277 0.74
Total 1,554 3.27 739 1.94
RECREATIONAL
Gulf 2,716 4,877
Red Drum Atlantic 770 NA 658 NA
Total 3,486 5,536
Gulf 5,444 5,158
Spotted Seatrout Atlantic 871 NA 942 NA
Total 6,316 6,101
Table 4. Total estimated number of participants in at least one fishing trip
per year by State and Resident Status for 1999 to 2000 (Centner era/. 2001).
Results were not reported for Texas.
State Resident Non-Resident Total
Alabama 222,255 143,374 366,629
Florida (all) 2,153,620 2,282,298 4,435,918.
Louisiana 442,290 90,648 532,938
Mississippi 101,748 74,891 176,639
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III. DEPENDENCE OF VALUED SPECIES ON ESSENTIAL HABITAT
To evaluate the importance of habitat on these valued spe-
cies requires knowledge of whether the species resides in
a particular habitat during critical life stages. Sections IV-
VII include overviews of the life cycle of each valued spe-
cies, characterizations of potentially critical life stages (the
success of which largely determines the success of the
population), geographic distributions and distributions
within different habitat types, and potential life-support at-
tributes of those habitats.
Examination of life histories for valued fishery species il-
lustrates a variety of survival strategies and at least a few
common characteristics. The most consistent, and per-
haps the most obvious for estuarine-dependent species,
is the preference for shallow waters with low or variable
salinity during at least some portion of the life cycle. Al-
though not all species inhabit the same salinity range, there
is sufficient consistency to support a hypothesis that
younger (larval, juvenile) stages succeed in estuarine shal-
lows with low or variable salinities. These smaller, devel-
oping stages undoubtedly benefit from salinity barriers and
vegetated zones to avoid predation, and are likely to uti-
lize detritus and nutrients delivered by freshwater discharge
from the watershed that provides nutrition for rapid growth
and development. For penaeid shrimp, sciaenid fishes,
and female blue crabs, the mature adults ultimately mi-
grate to deeper, higher salinity waters for spawning.
A relatively strong case can be made for the overall im-
portance of freshwater discharge in the success of each
of these organisms. Freshwater discharges provide nutri-
ents, trace elements, and detritus from the watershed, and
in combination with tides, provide energy, mixing, and sa-
linity gradients within the shallows of the estuary. Reach-
ing a similar conclusion, Browder and Moore (1981) pre-
sented a conceptual model of the relationship of freshwa-
ter inflow with fishery productivity. They suggested that
freshwater inflow influenced fishery production in five ways:
(1) transport of nutrients needed to stimulate productiv-
ity of wetland vegetation, phytoplankton, and
seagrasses, all providing food for juvenile fish and
shellfish, either directly or through the food chain;
(2) transport of detritus; the physical force of freshwater
discharge flushes decaying wetland vegetation into
tidal creeks and open waters, where it is processed
by microorganisms into food for benthic organisms
that are eventually consumed in the food chain;
(3) transport and deposition of sediments needed to
build, maintain and counteract erosion of tidal marshes;
(4) reduction of salinity offers euryhaline larval stages pro-
tection from stenohaline predators;
(5) mixing and transport of water masses provides oxy-
genation for decomposition and utilization of detritus,
and transport of larval and postlarval stages through-
out the estuary.
Browder and Moore (1981) argued that the success of the
fishery was so dependent on freshwater inflow that a simple
input-output model could be developed: freshwater inflow
overlaps in time and space with the physical, stationary
components of a habitat (seagrasses, shorelines, tidal
marshes) to create conditions favorable for population suc-
cess. Even though the relationship is modified by a variety
of dynamic factors, such as hydrology, estuarine geomor-
phology, hurricanes, or freezing weather, their model clearly
characterized the dependence of fishery species on the
temporal and spatial overlap between physical habitat and
favorable freshwater inflow during the nursery period. The
overlap should be proportional to the success of a popula-
tion:
(1) growth is related to available food, which is the product
of food concentration and area;
(2) survival and growth rates are density dependent, there-
fore distributing organisms across time and space
within the favorable habitat increases survival and
growth rates;
(3) if favorable habitat is less available, then a greater per-
centage of organisms is forced into poor habitat with
lower survival and growth rates.
From this perspective, it is easy to see how freshwater dis-
charges could influence the duration and spatial area of
favorable habitat. If freshwater flow is too high, then the
range of favorable salinities is pushed beyond the physical
habitat (tidal marshes, seagrasses); if too low, the range of
favorable salinities occurs too far upstream where physi-
cal habitat is limited. Even though different species prefer
different salinities, the most productive freshwater flows
provide favorable salinities across the broadest areas of
physical habitat for the longest period of time during the
nursery period.
Less consistent among the valued Gulf of Mexico species
is preference for physical habitat. Penaeid shrimp, sciaenid
fish larvae and juveniles and early juvenile blue crabs thrive
in vegetated areas, whereas older blue crabs prefer soft,
muddy bottoms and oysters generally require a hard, oral
least physically supporting, substrate. Vegetated areas may
well provide protective cover for vulnerable shrimp and fish
larval and postlarval stages to avoid predation, and may
also serve to trap the nutrients and detritus essential to
their nutrition. Blue crabs may protect themselves from
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predators and increase their predatory capabilities by bur-
rowing in shallow, soft, muddy substrates. For oysters, a
clean surface is favorable for larval setting; even a thin film
of silt or sediment may deter larvae from an otherwise fa-
vorable site. Oyster survival ultimately depends on a sup-
porting substrate as well. Firm substrate prevents sinking
as they grow larger and as subsequent generations of oys-
ters settle on them.
These two factors, freshwater inflow to the estuary and
physical habitat within a favorable salinity range, appear to
be the most consistent requirements of the economically-
valuable species in the Gulf of Mexico. It is likely, although
not addressed here, that numerous other estuarine spe-
cies (e.g., grass shrimp, mud crabs, killifish) have similar
requirements. Success of these species is significant in
terms of ecological functions; many are important second-
ary producers and prey items in the estuarine community
food web. Moreover, they are often essential to the suc-
cess of the economically-valuable species reviewed here.
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IV. PENAEID SHRIMP
Phylum Arthopoda, Class Crustacea, Order Decapoda and Family Penaeidae
Brown Shrimp
Farfantepenaeus aztecus
Pink Shrimp
Farfantepenaeus duorarum
White Shrimp
Litopenaeus setiferus
The Penaeidae are different from other familiar decapods
because they more frequently swim than crawl. Penaeids
are characterized by a laterally compressed body with a
prominent, often compressed and saw-toothed rostrum, a
large, plate-like antennal scale, used as a rudder in swim-
ming and a full set of well-developed swimming append-
ages along the ventral thorax called pleopods (Meglitsch
1972). Recent taxonomic changes have raised all three
Penaeid species in the Gulf of Mexico from subgenera of
the genus Penaeus to a generic level. Now, those com-
monly called white shrimp are Litopenaeus setiferus, brown
shrimp Farfantepenaeus aztecus and pink shrimp
Farfantepenaeus duorarum (Perez-Farfante 1997).
Life History
Gametogenesis
All three species are sexually dimorphic (Williams 1955,
Cook and Lindner 1970) and attain sexual maturity at av-
erage lengths of 135 mm, 140 mm and 100 mm, respec-
tively (Williams 1955, Renfro and Brusher 1964, Perez-
Farfante 1969). In general, mature females tend to be larger
than males of the same age (Williams 1955).
Gametogenesis for white shrimp and brown shrimp be-
gins in early spring and continues into the fall (Lindner and
Anderson 1956, Renfro and Brusher 1963), whereas pink
shrimp gametes are produced year round in the Dry
Tortugas area of Florida and between early spring and fall
in the more northerly regions of the Gulf (Eldred et al. 1961,
Joyce and Eldred 1966).
Spawning
Adult penaeid shrimp live and spawn in high salinity off-
shore waters at 20-30°C temperatures (optimum 25-27°C;
Jones etal. 1964, 1970, Subrahmanyam 1971, Pattilloet
al. 1997). White shrimp and brown shrimp spawn in wa-
ters of 7-31 m and 46-109 m depth, respectively (Lindner
and Anderson 1956, Renfro and Brusher 1963, Bryan and
Cody 1975), and initial spawning is coincident with rapid
warming of bottom temperature (Lindner and Anderson
1956, Perez-Farfante 1969). Pink shrimp spawn year round
at depths of 4-48 m in the Dry Tortugas area of Florida
and more northerly latitudes (Eldred et al. 1961, Perez-
Farfante 1969). For all three species, the number of indi-
viduals spawning at any one time appears correlated with
changes in water temperature, especially those shrimp in
the more northern latitudes (Idyll and Jones 1965). Also,
individuals from all three species are likely to spawn more
than once during the spawning season (Linder and Ander-
son 1956, Perez-Farfante 1969, Martosubroto 1974).
Fertilization
Males deposit spermatophores on females during copu-
lation, leading to external fertilization of eggs in all three
species (King 1948). Eggs are demersal (Perez-Farfante
1969) and hatch into planktonic larvae 10-24 hr after
spawning (Cook and Murphy 1969, Turner and Brody 1983,
Pattillo et al. 1997) at temperatures of 19-30°C, depend-
ing on the species.
Larval development
Over a period of 10-25 d (Johnson and Fielding 1956, Cook
and Murphy 1969), larvae pass through five nauplial, three
protozoeal and three mysis stages in the high-salinity off-
shore waters (Anderson et al. 1949, Perez-Farfante 1969).
Feeding from the water column begins during the first
protozoeal stage when larvae cease to live on yolk (Dobkin
1961, Cook and Murphyl 969). Larvae are omnivorous and
feed on both phyto- and zooplankton (Perez-Farfante 1969,
Van Lopik et al. 1979). Growth rates vary and are depen-
dent on temperature, season, size and sex (Linder and
Anderson 1956, Perez-Farfante 1969). Thus, growth is
-------
faster when summer water temperatures are warmer (Klima
1974, St. Amant et al. 1962, Ford and St. Amant 1971).
Females grow more rapidly and attain a larger final length
and weight than males (Parrack 1978, Turner and Brody).
Postlarvae and Juveniles
The last larval mysis stage is followed by the first
mastigopus or first postlarval stage in all three species of
shrimp (Perez-Farfante 1969, Muncy 1984). The first
postlarvae are still planktonic and live offshore but gradu-
ally move inshore with tidal currents toward estuaries
(Perez-Farfante 1969, Whitaker1983b)(Figure 2). Depend-
ing on the species, postlarval stages enter estuaries from
January to fall (Perez-Farfante 1969, Turner and Brody
1983, Mulhalland 1984) and after reaching shallow inshore
waters settle to the bottom, usually at the marsh-water in-
terface or in seagrass beds, to become benthic postlarvae
(Costello and Allen 1970). Four to six weeks after entering
estuarine nurseries, postlarvae transform into juveniles and
begin moving into deeper water (Perez-Farfante 1969,
Mulhalland 1984). The morphological difference between
postlarval and juvenile stages has not been clearly defined,
although Perez-Farfante (1969) considered small Penaeus
to be juveniles when they attained the ultimate rostral tooth
formula. Juveniles spend 2-6 months in nursery areas as
they develop into subadults (Costello and Allen 1970), and
at a particular point in time, emigrate from estuaries back
Marsr
arger
Juvenile
Postlarval
Stage -
Protozoea
Stage
Naupilus
Stage
Barrier
Island
Open
Ocean
Adult
Figure 2. Penaeid shrimp life cycle
(South Carolina DNR 2003)
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
into the sea (Figure 2). The movement from estuaries back
into the Gulf is governed largely by body size, age and en-
vironmental conditions such as precipitation, temperature
and tides (Klima et al. 1982, Shipman 1983)
Adults
Once offshore, subadult shrimp develop into non-spawn-
ing adults and spend much of their time in near coastal
waters (Perez-Farfante 1969, Wenner and Wenner 1989,
Pattillo et al. 1997). As non-spawning adults begin to ma-
ture into spawning adults, they migrate even farther off-
shore over the coastal shelf. Although there is some varia-
tion among species, the majority of shrimp complete this
life cycle in about 12 months (Joyce and Eldred 1966, Ander-
son 1966).
Critical Life Stages
Larval populations
Fecundity increases in almost direct proportion to body
weight, and studies have estimated individual pink shrimp
and white shrimp can produce up to 624,000 and 1,000,000
ova, respectively (Anderson etal. 1949,1965, Martosubroto
1974). Hatching success of shrimp in captive experiments
averaged 50% (white shrimp) and 77% (brown shrimp),
with those metamorphosing to protozoea averaging 53 and
44%, respectively (Chamberlain and Lawrence 1983).
There are no estimates of the percentage of larvae that
survive to postlarval stages, but it is probably small. Preda-
tion by fish and invertebrates (planktivores), periodic physi-
cal catastrophes (e.g., hurricanes, floods), and disease are
probably the major causes of natural mortality (Tabb et al.
1962, Perez-Farfante 1969, Couch 1978). Instantaneous
mortality rates in white shrimp are estimated to be 0.02-
0.25 for fishing, 0.21-0.56 for natural causes and 0.24-0.80
total. Weekly mortalities range from 13 to 51% with the
lower rates typical for both juveniles and adults (McKenzie
1981).
Postlarvae and juveniles
Once inside estuaries, postlarval and juvenile shrimp con-
gregate in shallow estuarine areas in seagrass beds or at
the marsh-water interface. These habitats apparently pro-
vide both food and protection from predation by fish, blue
crabs and seabirds (Turner and Brody 1983, Mayer 1985,
Minello and Zimmerman 1985). Vegetated cover appears
to be essential habitat for shrimp through these stages,
which are considered the most critical for ultimate recruit-
ment into the fishery (Williams 1955, Perez-Farfante 1969,
Thayeretal. 1978, Mulhalland 1984).
Subadults
As juveniles grow and mature they begin to emigrate into
deeper water and move into open bays as subadults. These
9
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
shrimp, while still preyed upon by fish, are less likely to be
consumed because of their larger size and improved abil-
ity to escape predation (McKenzie 1981, Turner and Brody
1983).
Adults
The average life span for penaeid shrimp is about one year
(Anderson 1966), but recapture studies have shown that
some white shrimp live as long as four years (Klima et al.
1982). Although large fish may feed on adult shrimp, adults
are generally less susceptible to predation because of their
larger size and offshore existence (Divita et al. 1983, Sherid-
an and Trimm 1983). Hurricanes cause major losses of
shrimp in the Gulf of Mexico by increasing salinity, destroy-
ing vegetative cover and food supplies, dispersing and
stranding the shrimp, or generating high turbulence in es-
tuaries (Kutkuhn 1962). Hurricanes Carla (1961) and
Camille (1969) caused 61% and 88% decreases in the
white shrimp catch, respectively (Barrett and Gillespie
1973). Unlike anthropogenic disruption of shrimp habitat,
which is generally permanent, natural physical disturbances
are usually temporary so that populations can eventually
return to pre-disturbance levels.
Distribution
Geographic range
Penaeids occur in both deep and shallow waters, but they
are more abundant in the littoral region, where they may
swarm in huge numbers (Meglitsch 1972). White shrimp
occur along the Atlantic coast from Fire Island, New York,
to St. Lucie Inlet, Florida. In the Gulf of Mexico, they are
distributed from the Ochlockonee River of Apalachee Bay,
Florida west and southward around the coast to Ciudad
Campeche, Mexico (Perez-Farfante 1969). Highest den-
sities occur off the Louisiana coast at depths of <9 m (Klima
et al. 1982). Brown shrimp are distributed from Martha's
Vineyard, Massachusetts, through the Gulf of Mexico to
the Yucatan Peninsula, Mexico, with maximum densities
along the Texas-Louisiana coast at depths ranging from
18-100 m (Lassuy 1983). Pink shrimp range from the lower
Chesapeake Bay southward along the coast to the Florida
Keys and Gulf of Mexico. Gulf of Mexico populations range
from the Dry Tortugas along the Gulf coast of the United
States and through the coastal waters of Mexico to Cape
Catoche and south of Isla Mujeres (Figure 3). Dense popu-
lations occur off southwestern Florida and in the south-
west portion of the Golfo de Campeche at depths of 11-65
m (Perez-Farfante 1969, Huff and Cobb 1979).
Dispersal
Larvae and postlarvae are planktonic, live offshore, and
are carried inshore by favorable tidal currents (Perez-
Farfante 1969, Whitaker 1983b). They enter estuaries,
settle to the benthos, and develop into juveniles (Williams
1965, Anderson 1966, Costello and Allen 1970). Juveniles
spend 2 to 6 months in the estuary and, as they develop,
gradually move towards deeper water (Costello and Allen
1970). Juvenile white shrimp tend to move farther up into
freshwater portions of estuaries than do brown shrimp or
pink shrimp. White shrimp juveniles have been found as
far inland as 160 km in Louisiana and 210 km in northeast
Florida (Perez-Farfante 1969). Maturing shrimp migrate
out of estuaries to offshore waters where they congregate
in large numbers. Adult shrimp are strong swimmers and
can migrate long distances, sometimes assisted by cur-
rents. There is a southerly mass migration of white shrimp
from North Carolina to Florida in the fall and a return mi-
gration in the spring (Joyce 1965). Recapture studies show
white shrimp can migrate as far as 516 km from their nurs-
ery areas (Anderson 1966) and pink shrimp as far as 278
km (Costello and Allen 1966). Other studies have shown
that individual white shrimp can move from 1.8 to 6.9 nau-
tical miles per day (Shipman 1983).
Figure 3. Gulf-wide ranges for juveniles of valued spe-
cies of penaeid shrimp. Pink shrimp map also shows adult
area (NOAA-NMFS 2003).
10
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Dependence on Habitat
Penaeid shrimp reside primarily in two habitat types. Adult
shrimp live in high salinity offshore waters (Subrahmanyam
1971), where they spawn and migrate back and forth along
the coast. In contrast, postlarval and juvenile life stages
reside in vegetated areas of enclosed estuaries. It is the
latter habitat that plays a significant role in the success
and maturation of the critical postlarval and juvenile stages
(Mulhalland 1984). Vegetated habitats appear to provide
food resources and protection from predators until the
shrimp migrate to offshore waters (Kutkuhn 1966, Perez-
Farfante 1969, Thayeretal. 1978, Turner and Brody 1983,
Mulhalland 1984).
Shrimp postlarvae appear to depend on at least two at-
tributes that vegetated estuarine habitats provide: nutri-
tional resources and protection from predation. Penaeids
are omnivorous and feed mostly on the organic matter
present on or within the benthic substrate. Postlarvae in
estuaries feed primarily at the vegetation-water interface
(Mulhalland 1984). They indiscriminately ingest the top
layer of sediment which contains plant detritus, algae and
microorganisms (Jones 1973). For protection, it appears
that postlarvae use the physical structure of grass beds
and marshes to hide from predators. Laboratory and field
experiments have shown the usefulness of restrictive
spaces (e.g., physical structure provided by grasses) for
protecting prey from larger predators (Chamov et al. 1976,
Vince et al. 1976). Studies of pinfish (Lagodon rhomboides)
and croaker (Micropogonias undulatus) predation on ju-
venile brown shrimp have demonstrated lower rates of pre-
dation in salt marsh vegetation compared to unvegetated
areas (Minello and Zimmerman 1982).
Some investigators have concluded that the yield of adult
Penaeid shrimp is directly limited by the available quantity
and quality of marshes and submerged vegetation (Gunter
1956, Doi et al. 1973, Turner 1977, Turner and Brody 1983).
A significant association between young pink shrimp and
seagrass beds (de Sylva 1954, Phillips 1969, Saloman
1965, Allen and Hudson 1980) has led some to conclude
that loss of seagrass beds is the primary reason for a de-
cline in the Tampa Bay pink shrimp fishery (Saloman 1965).
Not only habitat loss, but habitat alterations are suspect.
Studies in Florida, Louisiana and Texas have shown that
landfill, dredging and impoundments adversely affect
shrimp production (Christmas and Etzold 1977, Etzold et
al. 1983). When marshes are separated from the estuary
with levees or bulkheads, adequate food (rich organic ma-
terial) becomes unavailable and shrimp densities can be
reduced as much as 85% (Mock 1967, Lindall et al. 1973,
Trent etal. 1976).
11
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Life-supporting Attributes of Habitat
Penaeid shrimp postlarvae and juveniles appear depen-
dent on vegetated estuarine zones and tidal marshes. Nu-
trition and predator protection are most often considered
the life-support attributes of these habitats, but ultimate
survival may depend on multiple factors, such as the physi-
cal-chemical environment, substrate and biological inter-
actions. Some of these factors are considered below.
Temperature
Water temperature influences shrimp spawning, growth,
habitat selection, osmoregulation, movement, migration and
mortality (Muncy 1984). Spawning for all three species is
triggered by increasing water temperature in the spring and
inhibited by colder fall temperatures (Lindner and Ander-
son 1956). All three species burrow when water tempera-
tures drop, and severe cold fronts have caused mass mor-
talities in white shrimp and brown shrimp in shallow waters
(Gunter and Hildebrand 1951, Zein-Eldin and Renaud
1986). A comparison of temperature tolerance and prefer-
ences among the penaeid species is presented in Table 5.
White shrimp growth rates are increased at temperatures
up to, but inhibited beyond, 20°C (Etzold and Christmas
1977). Mortalities occur below 8°C, and essentially no white
shrimp survive 3°C or lower (Joyce 1965). Laboratory stud-
ies have demonstrated 50% mortality within 24 hrs for white
shrimp held at 36-37°C, and have demonstrated that
postlarvae and juveniles are slightly more resistant to higher
temperatures than larger juveniles (Wiesepape 1975). Adult
white shrimp appear to be more susceptible than juveniles
to cold temperature (Whitaker 1983a). White shrimp ap-
pear to be more tolerant of high temperatures but less tol-
erant of low temperatures than brown or pink shrimp (Etzold
and Christmas 1977). However, studies indicate that white
shrimp postlarvae are less tolerant of higher temperatures
than are brown shrimp postlarvae (Muncy 1984).
Temperatures below 4°C cause mass narcosis and mor-
talities of brown shrimp, whereas temperatures above 33°C
can cause severe stress (Gunter and Hildebrand 1951,
Zein-Eldin and Aldrich 1965, Kutkuhn 1966, Swingle et al.
1971). Larval development is optimal at 28-30°C (Cook and
Lindner 1970) and peak growth rates are around 25°C for
juveniles and adults (Zein-Eldin and Aldrich 1965).
Temperature is also an important parameter in the growth
and survival of pink shrimp (Perez-Farfante 1969). Adults
grow optimally above 25°C (Cummings 1961, Costello and
Allen 1970), whereas larvae and pre-settlement postlarvae
tolerate 15-35°C with optimum survival above 30°C, de-
pending upon salinity (Ewald 1965, Jones et al. 1970). Al-
though pink shrimp appear to survive relatively low tem-
peratures, like brown shrimp they become completely nar-
cotized below 10°C (Williams 1955).
-------
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Table 5. Temperature and salinity tolerance and preference ranges for three species of penaeid shrimp.
SALINITY (ppt)
tolerated
Life stage
Eggs
Larvae &
Post larvae
Juveniles
Adults
White
20-45
27
0.4-40
10-15
0.4-40
<10
0-45
27-35
preferred
Brown
No data
24-36
10-20
0-70
2-40
Q-7Q
27-35
Pink
No data
0.5-43
10-22
0-65
10-30
Q-65
25-45
TEMPERATURE (°C)
tolerated
White
No data
8-38
13-31
8-38
13-31
8-38
>15
preferred
Brown
Hatch>24
10-37
28-30
7-35
15-30
10-37
15-25
Pink
Hatch>27
15-35
>30
6-38
24-28
16-31
>25
References
1,2,3,4,5,6 3,7
1. Tabbetal. 1962
2 Cook & Murphy 1969
3. Zein-Eklin & Renaud 1986
4. Perez-Fa rfante 1969
5. Joyce 1965
6. McKenzie 1972
7. Gunter 1964
1.8,9,10,11
12,13,14,15 13,16,17 4,18,19,20,21
8. Bursey and Lane 1971
9. Allen etal. 1980
10. Williams 1960
11. Higman1972
12. St. Amant and Lindner 1996
13. Venkataramaliah 1977
14. Etzold and Christmas 1977
15. Muncy1984
16. Cook and Under 1965
17. Zein-Eldin and Aldrich 1965
18. Cummings 1961
19. Costello and Allen 1970
20. Ewald 1965
21. Costello et al. 1986
In general, white shrimp are found in waters of lower salin-
ity than are brown or pink shrimp (Copeland and Bechtel
1974). Adult shrimp of all three species prefer 27-35 ppt,
even though they survive within a range of 0-45 ppt (Tabb
etal. 1962, Cook and Murphy 1969, Zein-Eldin and Renaud
1986). Table 5 shows a comparison of salinity parameters
for all three species. As white shrimp postlarvae and juve-
niles move up into the estuary, their affinity for lower salini-
ties increases (Zein-Eldin and Renaud 1986), tolerating
even a reported 0.42 ppt in the northern Gulf of Mexico
(Perez-Farfante 1969). It is not unusual to find juveniles
far upstream living in salinities around 1 ppt (Joyce 1965).
However, growth appears to be optimal at 10-15 ppt (Zein-
Eldin and Griffith 1969, McKenzie 1981) and shrimp move
back towards higher salinity waters as they mature (Harris
1974).
Brown shrimp appear to have a greater salinity tolerance
than white shrimp (Zein-Eldin and Renaud 1986). Larvae
do not appear to tolerate as wide a salinity range as do
postlarvae and juveniles. Postlarval brown shrimp have
been raised in laboratory water of 1 ppt (Venkataramaiah
1971), but most studies indicate they prefer salinities of
10-20 ppt (Gunter et al. 1964).
Pink shrimp are more tolerant of higher salinity than white
or brown shrimp (Tabb et al. 1962), and laboratory studies
show that postlarvae and juveniles can survive 0-65 ppt
(Bursey and Lane 1971, Allen and Hudson 1980). Also, for
most postlarva and juveniles, survival is optimum when
salinity is 10-30 ppt (Williams 1960). Shrimp size and sa-
linity are positively correlated; smaller juveniles inhabit the
low salinity portions of an estuary, and incrementally larger
animals occupy areas nearer marine salinity (Williams 1955,
Tabbetal. 1962).
Temperature -salinity interactions
All three species of shrimp tolerate a wide range of tem-
perature and salinity combinations, and tolerance ranges
vary at different life stages. However, these interactions
are pronounced at the tolerance extremes (Muncy 1984).
Thus, during periods of extreme temperatures, shrimp have
a harder time adapting to salinity changes and during times
of extreme salinities they have a harder time adapting to
temperature changes (Zein-Eldin and Aldrich 1965, St.
Amant etal. 1966, Venkataramaiah etal. 1974). Variability
in shrimp harvests can reach 100% and, in several cases,
was correlated with temperature and salinity extremes dur-
ing the postlarval and juvenile stages in the estuary (Turner
12
-------
and Brody 1983). Also, freshwater inflow may affect coastal
water temperatures which affect growth rates and migra-
tion of shrimp (White and Boudreaux 1977, Shipman 1983,
Muncy 1984). The brown shrimp harvest in Louisiana is
directly correlated with the temperature of estuarine water
in mid-April and acreage of marsh found in areas with sa-
linities above 10 ppt Barrett and Gillespie 1973, Barrett and
Ralph 1976). Studies of pink shrimp show that osmotic regu-
latory ability decreases with temperature and is significantly
impaired at temperatures less than 9°C (Williams 1960).
Dissolved oxygen
Adult white shrimp and brown shrimp, as well as postlarvae
and juveniles, cannot tolerate low oxygen and are stressed
at concentrations of 2 ppm or less (Brusher and Ogren
1976, Zein-Eldin and Renaud 1986). Postlarvae and juve-
niles avoid water with 1.5 ppm and mortality occurs at 1
ppm or less (Zein-Eldin and Renaud 1986, Minello et al.
1989). Persistent hypoxia (< 2 ppm) in summer may cause
mass mortalities in juvenile and sub-adult brown shrimp
(May 1973, Turner and Allen 1982). Pink shrimp are even
less tolerant of low dissolved oxygen. Postlarvae and juve-
niles avoid waters with less than 2.5 ppm, but can tolerate
diurnal lows of 0.2 ppm for several hours (Brusher and
Ogren 1976, Sheridan et al. 1997). Also, for all three spe-
cies, the oxygen requirement increases with temperature
(Zein-Eldin and Renaud 1986)
Substrate
All three species are generally associated with specific
bottom types. White shrimp and brown shrimp both prefer
shallow, muddy bottoms rich in the food materials that make
up the bulk of their diet (Williams 1955, 1958, Mock 1967,
Van Lopik et al. 1979). Brown shrimp postlarvae are found
in the greatest numbers in or near marshes or seagrass
beds growing on soft muddy substrate (Christmas et al.
1966). White shrimp and brown shrimp postlarvae prefer
substrates of loose peat and sandy mud (Williams 1958).
Juvenile white shrimp and brown shrimp avoid coarse sub-
strate and seek food on soft bottoms (Williams 1958). In
contrast, pink shrimp prefer substrates composed of cal-
careous mud and sands or mixtures of shell and sand
(Hildebrand 1954, Springer and Bullis 1954, Brusher and
Ogren 1976). Laboratory experiments corroborate the pref-
erence of hard, shell-sand substrate by juveniles and, un-
like white and brown shrimp, pink shrimp can burrow into
extremely coarse sediment (Williams 1958, Fuss and Ogren
1966). Temporal and spatial differences among the three
species of shrimp for specific substrate types rprobably re-
duces interspecific competition (Lassuy 1983).
Vegetation
One of the most valued attributes of the habitat for
postlarvae and early juveniles of white, brown and pink
shrimp is vegetation (Mulhalland 1984). Vegetation provides
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
these life stages with an abundant food source and protec-
tive cover to avoid predators (Thayer et al. 1978). The type
of vegetative habitat preferred by each species varies, but
in general, white shrimp and brown shrimp densities are
highest around the salt marsh edge, marsh ponds and sub-
merged aquatic vegetation (Williams 1958, Giles and
Zamora 1973, Pattillo et al. 1997, Minello et al. 2003). In
contrast, highest densities of pink shrimp are found within
submerged aquatic vegetation, especially Halodule and
Thalassia (Higman et al. 1972, Brusher and Ogren 1976,
Kennedy and Barber 1981, Costello et al. 1986, Peterson
and Turner 1994). In developed penaeid shrimp industries
throughout the world, yields are highest in areas where in-
tertidal wetland area is high (Doi et al. 1973, Turner 1977).
In addition to intertidal wetland area, density of vegetation
is also important for all three species. More shrimp are found
in estuaries where the vegetation density is high
(Zimmerman et al 1982).
13
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
V. EASTERN OYSTER
Phylum Mollusca, Class Bivalvia, Order Mytiloidea, Family Ostreidae
Crassostrea virginica (Gmelin)
Eastern oysters differ from other bivalves in that they pos-
sess filibranch gills with inter-lamellar junctions, a small foot,
a reduced anterior adductor muscle and no siphon. East-
ern oysters have a well-developed promyal chamber. They
are dioecious (Galstoff 1964, Bahr and Lanier 1981) and
generally protandrous, often exhibiting male before female
characteristics. They are also alternate hermaphrodites,
often changing gender between spawning cycles (Galstoff
1961).
Life History
Under optimal environmental conditions, eastern oysters
in estuaries of the Gulf of Mexico can become sexually
mature and reproductively active 4 weeks after metamor-
phosis, or within 2 months of fertilization (Menzel 1951).
However, these very small oysters do not contribute signifi-
cantly to the year class because of low gamete production
(Hayes and Menzel 1981). Gamete production increases
with oyster size until the second spawning season. There-
after, fecundity in oysters of the same size depends on physi-
ological condition (Galtsoff 1964). Gametogenesis usually
occurs during the early spring, but development of new eggs
may proceed throughout the summer simultaneously with
spawning (Figure 4).
Spawning
Rising seawater temperatures are usually considered the
stimulus for release of eggs and sperm from mature go-
nads. Most spawning is initiated when water temperature
reaches a relatively consistent 20°C (Butler 1949, Loosanoff
1953, Schlesselman 1955, Hofstetter1977,1983) and when
salinity is greater than 10 ppt. Phytoplankton may also stimu-
late oysters to spawn (Nelson 1955). For individuals, spawn-
ing may be seasonal (spring, summer, or fall) and of short
duration or long. For oysters as a population, however,
spawning occurs almost continuously in Gulf of Mexico es-
tuaries from spring through late fall.
Fertilization
Eggs are fertilized by sperm in the water outside of the oys-
ter (external fertilization). Hence, proximity of males and
females and simultaneous release of sperm and eggs into
the water are essential for successful reproduction. Some-
times males are more responsive to rising water tempera-
ture (Dupuy et al. 1977) and spawn before the females.
Chemicals released by the sperm provide additional stimu-
lation for the females. During heavy spawns, the water over
an oyster reef might appear milky from gametes. Spawned
eggs are denser than water and quickly sink to the bottom.
Water currents distribute the embryos during the relatively
brief period of embryogenesis (several hours).
Larval development
Four to six hours after fertilization, a trochophore larval stage
is reached, during which a ciliated girdle is formed that al-
lows swimming. After 24-48 h, a velum develops and the
oyster becomes a veliger. The velum protrudes between
the shells to assist in swimming (large cilia along the mar-
gin) and to capture food. Food particles are passed along
the smaller cilia at the base of the velum to the mouth.
14
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SPAWNING
Adult oysters
Figure 4. Life cycle of the eastern
oyster (Tansey 2003)
Additional larval phases display conspicuous morphologi-
cal changes; appearance of the umbo (umbo stage), eye-
spot, (eyespot stage) and foot (pediveliger stage). The last
of these stages, considered the mature larva, possesses a
well-developed foot that is projected outward during swim-
ming. All larval stages are pelagic and, although larval swim-
ming may exert some influence, dispersion of larvae is
largely determined by tides and currents interacting with
the salt wedge created by mixing of salt and fresh water.
The pediveliger is believed to seek out, or at least investi-
gate, different surfaces in pursuit of environmental condi-
tions suitable for attachment and ultimate survival. Some
attribute the general tendency of larvae to settle in dark or
shaded areas to sensory capabilities of 'eyespots' that ap-
pear in veliger larvae.
Spaf
Larvae settle (set, strike) on hard substrate and creep along
pulled by the foot until a suitable attachment point is found.
Shells of live oysters are the optimal substrate for setting,
resulting in layers of oysters growing atop one another to
form a reef structure. Once settled, oysters (now called
"spat") attach to the substrate with a cementing fluid. Oys-
ter spat almost immediately metamorphose to an adult form
and an adult existence. The velum, foot, and 'eyespots'
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
are lost at metamorphosis, and the oyster remains per-
manently sedentary.
Critical Life Stages
Larva/ populations
Estimates of population-scale abundance are nearly im-
possible and rarely attempted for early developmental
stages. Fecundity and spawning is so highly variable in
eastern oysters that some females produce more eggs in
a single spawn (up to 100 million eggs) than others do in
an entire spawning season (Davis and Chantey 1955). Also,
larvae are distributed by currents and tides in such a man-
ner that their final disposition is impossible to discern. Most
likely, the majority of larvae do not survive predation and
environmental challenges long enough to reach the ma-
ture veliger stage. Because of the vast numbers of larvae
that are spawned in relation to the number that success-
fully settle, most investigators believe that oyster mortality
is greatest for the planktonic larval stages (Davis 1958,
Davis and Calabrese 1964, Hidu etal. 1974, Kennedy and
Breisch 1981). Regardless, the vast numbers of spawned
larvae likely offset any adverse impacts of high larval mor-
tality on population structure and dynamics.
Larval setting
Those larvae that do survive must find suitable substrate
usually near a source of fresh water, upon which to settle*
Even suitable substrates experience wide variation in an-
nual setting, a likely consequence of variability in fecun-
dity, spawning, larval distribution patterns, and freshwater
discharge at the time of setting. Nonetheless, there is little
doubt that larval setting and early survival is the most
critical life stage for oysters. Even the earliest investiga-
tors emphasized this fact:
"Spatting is the ail important event. The value of the oys-
ter harvest does not depend upon the number of eggs
spawned, nor upon the number of larvae in the water but
upon the number of successful spaf (Stafford 1913).
" The most important and critical period in the life history of
the oyster is that during which the fully developed larva
cements itself to some clean submerged surface such as
old shells or stones and then undergoes metamorphosis
into a spat and adult oyster* (Prytherch 1934).
Spat
Having successfully set, the oyster remains at the mercy
of predators and environmental conditions. Finucane and
Campbell (1968) reported that oyster mortalities were
greatest during the first 2 months after settlement. Others
have estimated spat mortalities ranging from 15 - 100%
(Loosanoffand Engle 1940, Mackin 1961, Hofstetter 1977).
Spat mortality is higher in dense due to crowding and
15
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
increased predation (Webster and Shaw 1968, Hofstetter
1977, Chatryetal. 1983).
Adults
For adults, which are the harvestable oyster stocks, natu-
ral mortality has rarely been evaluated except when cata-
strophic losses due to floods, hurricanes or epizootics were
experienced (Galtsoff 1930, May 1972, Little and Quick
1976, Hofstetter 1981, Berrigan 1988). An exception is the
recent publication of annual natural and fishing mortality
rates for the entire stock of harvestable and sub-market-
sized oysters in Maryland's Chesapeake Bay from 1991-
2001 (Jordan et al. 2002). Natural non-catastrophic mor-
tality occurs less rapidly and is often undetectable because
of continuous recruitment and rapid growth. Regardless,
estimates of annual mortality rates can be very high (50 -
95%) among subadult and adult oysters (Menzel et al.
1966, May 1971, Quick 1971, Little and Quick 1976^
Swingle and Hughes 1976, Hofstetter 1977, Quasi et al'
1988, Berrigan 1990, Jordan et al. 2002). Much of the
mortality is attributed to diseases. These losses represent
a substantial economic loss to the oyster industry and re-
main the principle limiting factor of commercial harvesting
in many regions. Natural mortality on intertidal reefs, in
particular, is often so high as to preclude commercial har-
vesting.
Distribution
Geographic range
Eastern oysters inhabit coastal waters ranging from the
Gulf of St. Lawrence in Canada through the Gulf of Mexico
to the Bay of Campeche, Mexico, and into the West Indies
(Stenzel 1971, Abbott and Alcolado 1978, Andrews 1979).
They occur in every major bay system along the U.S. Gulf
of Mexico, but are not evenly distributed among or within
bays. They are most abundant in shallow, semi-enclosed
water bodies (<40 ft. depth), usually with salinities moder-
ated by freshwater discharge.
Dispersal
Oysters are mobile only during planktonic larval stages,
and although not well understood, larval movement ap-
pears to be primarily dictated by water currents. Even
though larvae swim continuously, their dispersal and ulti-
mate fates are strongly dependent on current regimes and
flushing rates of estuaries (Andrews 1983). This dispersal
mechanism, coupled with the abundance of larvae pro-
duced, ensures population survival in favorable areas of
an estuary, even if traditional reef areas become unaccept-
able. Planktonic dispersal also ensures oyster survival in
the event of adverse climatological conditions such as flood-
ing and drought.
Habitat
Throughout much of the range of eastern oysters, two types
of habitat experience the greatest spatfall and subsequent
success of oysters. Beaven (1955) summarized:
"Two distinct types of high setting areas appear to be
present. One type occurs where rich runoff enters directly
into high salinity waters producing a body of water exhibit-
ing a fairly steep salinity gradient.... A second type of high
setting area consists of a semi-enclosed body of water
where the salinity is fairly uniform and where there is a
comparatively slow rate of exchange of water overlying
the oyster beds" (Beaven 1955, p. 34).
The first habitat type is more commonly recognized be-
cause it generates deep, subtidal, high-density oyster reefs
epitomized by reefs found in the Chesapeake Bay in the
late 1800s. But the second type is also wide-ranging; in-
cluded are intertidal oysters on Cape May Shore (NJ) of
Delaware Bay, Wachapreague, Virginia (Nelson 1955), and
the tidal creeks of South Carolina (Lunz 1955). Both types
comprise the reefs in the Gulf of Mexico. The subtidal oys-
ter beds of Barataria Bay (LA), Pensacola Bay (FL) and
Apalachicola Bay (FL) are examples of the first habitat type,
whereas the shallow, intertidal beds of West Bay
(Galveston, TX) and Laguna Madre (TX) are examples of
the second.
Dependence on Habitat
Eastern oysters are marine organisms; they can survive in
high salinity sea water but can survive only short durations
in fresh water. Nonetheless, three major factors appear to
limit oyster distribution to zones within estuaries and near-
shore locations. (1) An association between low salinity
and larval setting success, (2) avoidance of predation, and
(3) avoidance of disease. Each of these factors can be
linked to a zone of influence created by freshwater dis-
charges.
16
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Eastern oysters can tolerate brief exposures to fresh water
and are generally dependent on freshwater intrusions for
reproduction and setting. Most observers have found fresh
water to be a common factor in oyster distribution:
"A study of the location of natural oyster beds reveals at
once the fact that they are found only in coastal areas where
salt water of the ocean is considerably diluted by drainage
of fresh water from the land. " (Prytherch 1934, p. 49).
Large harvestable populations of oysters in the Gulf of
Mexico are located near freshwater discharges such as
rivers, creeks and bayous. Oyster reefs extend farther and
deeper in regions with the highest rates of freshwater dis-
charge. Consequently, it appears that the majority of oys-
ter reefs in the U.S., including those in the Gulf of Mexico,
are restricted to zones that experience some freshwater
intrusion. Even oysters that settle in areas removed from
obvious freshwater sources are probably subject to the
same restriction. These oysters occur in semi-enclosed
embayments, and their distribution in shallow, intertidal ar-
eas immediately adjacent to land is commensurate with
the reduced rates of freshwater discharge from relatively
small watersheds.
Larval setting success
Many observers have associated larval oyster setting with
low or variable salinity (Menzel 1954, Beaven 1955,
Loosanoff 1953, Ulanowiczetal. 1980). However, Prytherch
(1934) demonstrated that, even though salinity affected the
time required for setting, it exerted no influence on the ca-
pacity to set. Rather, he determined that setting and meta-
morphosis of oyster larvae required copper in the water
column. Sea water has almost undetectable levels of cop-
per, but the metal is transported from terrestrial sources to
estuaries and near-coastal zones by streams and rivers.
The availability of copper, then, is in complete accord with
the distribution of oysters, which extends farther and deeper
with the zone of (copper-laden) freshwater influence. The
perception that low salinity is a requirement for setting is
contraindicated by heavy setting in some areas of high
salinity These areas are adjacent to land, which probably
serves as a nearby source of copper by direct runoff, small
tributaries, or groundwater discharge. For larval setting,
then, the zone of freshwater influence may be determined
by the distance and depth that copper can be transported
from land at the time mature oyster larvae are present.
The zone may be dictated by runoff from large rivers and
streams or from timely runoff of rain collected in small wa-
tersheds.
Avoidance of predators
Several studies have demonstrated the devastating effects
of oyster predators. Many protozoans, coelenterates, bar-
•* .«*.*.
"'. ••--
nacles and molluscs prey on oyster larvae (Berrigan et al.
1991). Even adult oysters ingest larvae (Andrews 1983).
Numerous species of gastropods, crustaceans and fish prey
on spat, juveniles and adult oysters (Gunter 1955, Berrig-
an et al. 1991). The most devastating predator on subtidal
oyster reefs in the Gulf is Thais haemastoma, the southern
oyster drill (Butler 1954). Although euryhaline, Thais pros-
pers at salinities above 15 ppt (Butler 1954, Pollard 1973,
Cooley 1978). In fact, Gunter (1955) concluded that the
principal predators of oysters, including oyster drills, stone
crabs and black drum, were most abundant at higher sa-
linities. Consequently, any zone of freshwater influence pro-
tects oysters by reducing salinity, at least intermittently,
thereby temporarily driving the predators off the reefs. For
those less common oysters that live in medium-high salin-
ity estuaries, survival may be restricted to only those oys-
ters that are intermittently exposed to air (i.e., intertidal).
Emergence of oysters at low tide, like influx of fresh water,
drives predators off the reef.
Avoidance of disease
Since 1948, eastern oysters in the Gulf of Mexico have
been ravaged by an epizootic caused by a protozoan patho-
gen, Perkinsus rnarinus (Fisher 1996). More than 90% of
oysters are infected with P. marinus, and estimated losses
of harvestable stock are generally around 50%. However,
lower infection intensity and greater ability to survive infec-
tion are commonly associated with low salinity. These ef-
fects are due primarily to a high-salinity optimum for P
marinus, and a narrow differential between the low-salinity
thresholds of host and pathogen. Intermittent discharges
of fresh water are believed to reduce salinity sufficiently to
eradicate some of the disease agents, at least temporarily.
The foregoing discussion has implicated a spatial limita-
tion for oysters, i.e. to coastal zones that experience fresh-
water influence. Oyster habitat is limited in at least one
additional aspect. Oyster setting, and thus ultimate survival,
is restricted to clean, hard substrates. Considerable
amounts of silt and mud are deposited when streams empty
17
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
into bays, sounds and estuaries. If oysters settle on this
soft material, they may sink and smother, whereas hard
surfaces will physically support the growing oyster. Setting
on a hard substrate may also serve to aggregate oysters at
a common site. Proximity is essential to the reproductive
success of an organism that relies on external fertilization.
Oyster larvae settle best on existing oyster shell, as long
as it is not covered with silt or mud. This phenomenon,
younger oysters growing on older shells, generates an oys-
ter reef that presents a very favorable option for setting
larvae. It provides a hard substrate located in environmen-
tal and biological conditions that support adult growth and
survival, reproductive success, and, ultimately, population
sustainability.
Life-supporting Attributes of Habitat
Although oysters can grow and survive in a wide range of
environmental conditions, there are physical, chemical and
biological attributes that oyster habitat must provide. These
are outlined below.
Temperature
Oysters are poikilothermic, thus their tissues generally
maintain ambient temperature. Optimal temperatures for
eastern oyster reproduction and development are 20-23°C
(Stanley and Sellers 1986), even though they can tolerate
temperatures within the range of 1-36°C (Galtsoff 1964).
Oysters in Redfish Bay and Harbor Island (TX) have expe-
rienced mass mortality when exposed to temperatures of
37°C (Copeland and Hoese 1966). Atlantic coast stocks
tolerate partial freezing of their tissues (Loosanoff 1965),
but Gulf stocks do not (Cake 1983). Water temperature
also influences growth rate and developmental rate of oys-
ters. In combination with salinity, temperature determines
when gravid oysters spawn (Hopkins 1931, Galtsoff 1964),
the rate of larval maturation (Loosanoff and Davis 1963)
and spat setting (Davis and Calabrese 1964, Hidu et al.
1974), all of which influence the mortality rate of develop-
ing larvae (Menzel 1951, Hidu et al. 1974, Hayes and
Menzel 1981). Larvae held at 30°C under laboratory con-
ditions began setting 10-12 days after fertilization, whereas
larvae at 24°C set only after 24-26 days, and few larvae
held at 20°C set within 35 days (Loosanoff and Davis 1963).
Salinity
The salt content of water is often considered the single
most important factor influencing the distribution and abun-
dance of oysters (Berrigan et al. 1991). Since they are
osmoconformers, internal salinities of oysters generally re-
flect ambient conditions. The ability to exclude ambient
water with tightly closed valves provides oysters short-term
protection from extreme changes in salinity. Oysters nor-
mally occur in Gulf of Mexico estuaries at salinities of 10-
30 ppt, but may survive extremes of 3-44 ppt (Gunter and
Geyer 1955, Copeland and Hoese 1966). Atlantic Coast
oyster populations found at the upper and lower limits of
this range exist under marginal conditions that inhibit growth
and reproduction (Loosanoff 1953, Galtsoff 1964). How-
ever, oysters in South Bay (TX) are known to tolerate hy-
persaline conditions or higher than sea water. The effects
of salinity, both positive and negative, on oyster popula-
tions depend largely on the range and rate of salinity fluc-
tuations (Berrigan et al. 1991).
Salinity influences spawning, setting of spat, and feeding.
Salinities less than 10 ppt through the spring and summer
inhibit spawning and reduce larval survival. When salini-
ties greater than 15 ppt dominate, mature larvae are abun-
dant, but survival of recently set oysters may be poor due
to increased numbers of fouling organisms and predators.
An association of spat setting with lower salinity has been
documented (Menzel 1954, Beaven 1955, Loosanoff 1955,
Ulanowicz et al. 1980), and optimal setting salinities ap-
pear to range from 16-22 ppt (Hopkins 1931, Chatry et al.
1983). However, this association may be indirect, since
lower salinity co-occurs with freshwater discharges that
provide copper to mature larvae (Prytherch 1934). In fact,
some investigators have found no association of setting
with salinity (Prytherch 1934, Hidu and Raskin 1971). Labo-
ratory studies using oysters acclimated to 27 ppt showed
that feeding ceased in salinities below 3 ppt (Loosanoff
1953). Salinities from 0-15 ppt can benefit oysters by re-
ducing the abundance of some predators. Oyster drills and
stone crabs can pose serious threats to oyster populations,
but they cannot tolerate salinities lower than 11 ppt (Wells
1961, Menzel et al. 1966). Even a short-term decrease in
salinity can help control these predators and extend oyster
survival.
Dissolved oxygen
Oysters are facultative anaerobes, and although they pros-
per at concentrations of oxygen above 4 ppm, are able to
tolerate hypoxic conditions (<2 ppm) and survive brief ex-
posures to anoxic conditions. For example, Sparks et al.
(1958) found that oysters survived dissolved oxygen levels
below 1 ppm for up to 5 days. Laboratory experiments indi-
cate the oxygen consumption rate for oysters is 303 mL/
kg/hrfor wet tissue (Hammen 1969). However, oxygen re-
quirements vary with salinity and temperature. Between
water temperatures of 10-30°C, and salinities of 7-28 ppt,
oxygen consumption increases with increasing tempera-
ture and decreasing salinity (Shumway 1982).
Copper
A constituent of fresh water that may be extremely impor-
tant to eastern oysters is copper, which has been found
essential to both setting and metamorphosis. Prytherch
18
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
(1934) found that removal of copper from the medium will
suspend both of these developmental activities. It appears
that copper concentrations in larval oysters must reach a
threshold before setting can occur. Copper may 'activate
cells involved in the metamorphic process. Copper origi-
nates from terrestrial sources and is transported through
the watershed in streams and rivers. The dissolved ele-
ment forms complexes with ions in seawater, making it bio-
logically unavailable, so oyster setting can occur only in
areas where sufficient copper remains in the water column.
Both copper and zinc are accumulated to high concentra-
tions in adult oyster amebocytes, and these elements may
play critical roles in oyster defense and shell deposition
Freshwater discharges supply nutrients for phytoplankton
(oyster food) production, enhance estuarine circulation, and
reduce predation. A copper requirement for oyster setting
may have evolved as a cue to ensure that oysters were
setting in areas where freshwater discharges provided nu-
nents and refuge from predators.
Substrate
Substrate is critical for setting of oyster larvae; a relatively
:lean, stable surface is required, and virtually any hard
surface will suffice, including glass, concrete, rock, metal.
wood, rubber or other materials Usually the most suitable
cultch (substrate material) is a clean un-encrusted shell on
the surface of an established, adequately elevated oyster
reef (Truitt 1929, Hopkins 1931, Gunter 1938, Lunz 1958,
St Amant 1961) In the Gulf region, oysters are successful
in shallow bays and on mud flats where they can survive
as long as the mud is relatively dense and firm enough to
support their weight Soft mud and shifting sand are the
only substrates unsuitable for oyster communities Exist-
ing oyster reefs generally provide the best and most at-
tractive substrate for mature oyster larvae to set (Truitt
1929) The reef-building proclivity of oysters stems from
the attachment of spat to the shells of older oysters (Gunter
1972) Live oysters may be found in several layers on a
well-elevated reef, with the youngest oysters forming the
top layer (Gunter 1972, Hofstetter 1977). This type of growth
leads to thick deposits of aggregated shells that elevate
new spat and prevent sinking and inundation with silt.
Phytoplankton
Oysters are filtration feeders, obtaining nourishment from
phytoplankton, and possibly bacteria, removed from sus-
pension in the water column (Newell and Langdon 1996).
Food particles are highly variable in both quantity and quality
"n the estuarine environment, and may be composed of
complex mixtures of living microorganisms, particularly
bacteria and phytoplankton, detritus and inorganic particles
(Soniat et al 1984, Berg and Newell 1986). Oyster larvae
collect particles on the cilia of the ciliated girdle of the ve-
lum and transport them to the mouth. Spat and adult oys-
ters sweep particles into the shell cavity with currents gen-
erated from beating cilia along the gills and mantle folds.
Once inside the oyster shell, cilia on the labial palps select
particles for ingestion (transported to the mouth) or for re-
jection (transported to the anterior shell cavity of the oyster
for elimination in the psuedofeces). Particle selection may
be based on nutritional value or merely on size (Baldwin
and Newell 1995, Newell and Langdon 1996). Although
detritus, zooplankton, and bacteria can provide some of
the nutritional needs of oysters (Langdon and Newell 1996),
phytoplankton are considered the principle food source
(Haines and Montague 1979, Hughes and Sherr 1983).
Predators and pathogens
The oyster habitat must provide some protection from the
numerous oyster predators and from the disease caused
by the protozoan pathogen, Perkinsus marinus. These are
addressed in a previous section.
Fresh water influx
Nearly every life-support attribute of oyster habitat is di-
rectly or indirectly related to freshwater influx. Freshwater
provides copper for setting, nutrients (N and P) for
phytoplankton growth, intermittent low salinity for protec-
tion from predators and pathogens, and currents for oxy-
genated water, dissipation of wastes and dispersal of lar-
vae.
Requirements for water flow or current velocity are poorly
understood For maximum feeding, current velocity must
be high enough to exchange the water above a reef 3 times
every hour (Galtsoff 1964). Oyster feeding rates were re-
duced at very low current velocities (<2 cm s '), but were
uncorrelated at higher velocities (Jordan 1987). Perhaps
oysters respond negatively to food depletion or accumula-
tion of metabolites when water is not exchanged rapidly
enough
19
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
VI. BLUE CRAB
Phylum Arthropoda, Class Crustacea, Order Decapoda, Family Callinectidae
Callinectes sapidus (Rathbun)
Blue crabs (Callinectes sapidus Rathbun) inhabit estuar-
ies and coastal waters from Nova Scotia to northern Ar-
gentina (Steele and Bert 1994). They have been introduced
into Asia and the Mediterranean Sea. Where most abun-
dant, from the mid-Atlantic through the northern Gulf of
Mexico, they support important commercial and recreational
fisheries. A complex, migratory life history requires that each
successful crab is exposed to a wide variety of habitats
and environmental conditions during its lifetime (Van
Heukelem 1991). The larval stages are planktivorous. Ju-
venile and adult blue crabs are omnivorous and cannibal-
istic, feeding on detritus, algae, vascular plants, inverte-
brates, fish, and other blue crabs (Hay 1905, Baird and
Ulanowicz 1989, Van Heukelem 1991). On average, they
feed high on the food chain. Baird and Ulanowicz (1989)
assigned Chesapeake Bay blue crabs a trophic ranking of
3.5 (primary producers = 1.0), 10lh highest of the 36 pro-
ducer and consumer groups in their model. Blue crabs are
preyed upon by many other species; predation on juvenile
crabs by estuarine fishes may be particularly important in
trophic pathways and population regulation. Their abun-
dance and large biomass, wide geographic distribution,
ubiquity within estuaries, and central position in food webs
imply that blue crabs have critical ecological importance in
many U. S. estuaries.
Life History
The life cycle of C. sapidus (Figure 5) begins with hatching
from one of up to millions of eggs, carried in a mass
(sponge) under the abdomen of an adult female. Hatching
may take place at any time of year in lower latitudes, dur-
ing summer in cooler climates, and generally in higher sa-
linity water near the mouths of estuaries. After a brief pe-
riod of attachment to the egg mass, the zoea (early larval
stage) becomes free and joins the plankton. Most of the
larvae are transported in surface currents seaward from
estuaries onto the continental shelf.
The larvae spend their early lives in the plankton, undergo-
ing 6-7 molts. With the last of these molts, the larva meta-
morphoses into a megalops, the second larval, or postlar-
val, form. Megalopae are capable of swimming and crawl-
ing, and are more demersal in habit than the planktonic
zoeae. The tendency for megalopae to prefer deeper wa-
ter is believed to facilitate their movement back into and
upstream in estuaries as they mature. Net landward
counter-flows of more saline, denser bottom water are typi-
cal of stratified and partially mixed estuaries. Megalopae
also move off the bottom into the water column during flood
tides, another adaptation that favors their movement to
estuaries. In shallow, well-mixed estuaries and lagoons,
this is the principal means of entry for megalopae. There is
only one megalopal stage at the end of which the megal-
ops settles onto the substrate and metamorphoses into a
juvenile crab. At this stage, the crab is about 2.5 mm across
the carapace, from point to point, i.e. from the tip of one
lateral spine to the other. From hatching to settlement takes
45-60 days or more, depending on temperature (Van
Heukelem 1991).
As the juvenile crab grows, it continues to molt and contin-
ues its upstream migration. Within a year, crabs have molted
several times and have dispersed widely into estuaries.
Their tolerance for low salinity apparently increases as they
grow, so that older juvenile and mature crabs (mostly males)
can be found even in freshwater reaches of estuaries. Males
and females become sexually mature within 1-2 years
20
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\V k. Marsh 0
Figure 5. Blue crab life cycle
(South Carolina DNR 2003).
after hatching. Time to maturity is shorter in the Gulf of
Mexico where crabs can grow and molt year round, than
in more northern areas where they are dormant during the
winter. Adult crabs range from about 120 mm to more than
200 mm in carapace width. Crabs up to 260 mm have
been encountered, albeit infrequently.
Mating is a complex and sensitive part of the life cycle.
Female crabs approaching their pubertal molt seek out
mature males, and males may compete for mates. The
male crab, his normally cannibalistic habits somehow sup-
pressed, clasps and protects the female before and through
the molting process. After shedding her hard shell, the fe-
male is ready for copulation. The male then deposits sper-
matophores, and continues to clasp and protect the fe-
male until her new shell hardens. The whole process of
mating may require several days, during which the crabs
are continuously in contact. After mating, the mature fe-
males migrate to spawning grounds in high salinity wa-
ters, usually near the mouths of estuaries. Ovulation and
fertilization, hence embryonic development and hatching,
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
may be delayed for months, with the timing dependent on
season (temperature) and the length of the migration. Fe-
males spawn two or more times, using the sperm stored
from their sole mating for fertilization. For most female
crabs, the pubertal molt is the last, although a few survive
and grow through one or more additional molts. Male crabs
can continue to grow and molt for up to 8 years, based on
evidence from tagging studies (Miller 2001), although few
crabs live more than 3 years.
Critical Life stages
The blue crab population of the northern Gulf of Mexico
(western Florida to Texas) is considered by fisheries sci-
entists to be a single stock (Guillory et al. 1998). Some
adult crabs migrate hundreds of km (Guillory et al. 2001),
and the early larval stages should be expected to disperse
and mix widely during the weeks they spend in the open
waters of the Gulf. Genetic studies generally have not found
systematic differentiation among blue crab populations ei-
ther within the Gulf of Mexico or between the Gulf and the
Atlantic Coast. Genetic patchiness within estuaries, attrib-
uted to post-settlement selection, is of the same magni-
tude as large scale genetic variation (reviewed by Guillory
etal. 2001).
Although fisheries statistics are available and Gulf state
agencies conduct fishery independent monitoring that pro-
vides indices of relative abundance, we have not found
quantitative estimates of abundance or biomass for the Gulf
population. Commercial landings give a general idea of
magnitude: from 1980-1994, reported hard crab landings
in the Gulf averaged 25.7 x 106 kg, with a peak of 35.9 x
106 kg in 1988 (Guillory et al. 1998). In addition, there is
substantial recreational catch, estimated at 4-20% of com-
mercial landings, softshell and peeler crab fisheries (re-
ported landings averaged about 0.1 x 106 kg), and by-catch
of blue crabs in trawl fisheries. Commercial landings, es-
pecially the peeler crab component, and by-catch are un-
der-reported in unknown but probably significant quanti-
ties.
A crude estimate of population biomass can be made from
knowledge of landings, fishing mortality rates, and the size
structure of the population. Taking average landings for the
Gulf of Mexico to be -30 x 106 kg (combining the various
fisheries, by-catch, and some upward bias for under-re-
porting), the instantaneous rate of fishing mortality (F) as
ranging from 0.5 to 1.0 (within the range of estimates for
the Chesapeake Bay blue crab fishery; Rugoloetal. 1998),
and assuming that pre-recruit biomass is equal to recruited
biomass, we obtain a rough estimate of total population
biomass for the Gulf of about 100-150 x 106kg.
21
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Densities of juvenile crabs have been reported from spe-
cific habitats (Castellanos and Rozas 2001), in at least one
case exceeding 50-rrr2 in seagrass (Thomas et al. 1990).
Density estimates from quantitative sampling gear can be
extrapolated to estimate total abundance and biomass for
spatially defined sub-populations. For example, Minello and
Rozas (2002) estimated total abundance of blue crabs for
a 437 ha marsh complex in Texas. Mean density was 26,000
crabs per ha; total abundance was 11.3 x 106 individuals.
Guillory et al.(2001) estimated total instantaneous mortal-
ity rates (Z) for each of the Gulf states from size-frequency
data, but did not subdivide Z into natural mortality (M, i.e.
mortality from predation, diseases, environmental hazards,
etc.) and fishing mortality (F) rates. Rugolo et al. (1998)
estimated Z for Chesapeake Bay blue crabs from fishery-
independent data, then used assumed values for M in or-
der to calculate F by subtraction. Explicit estimates of M
will be essential for investigating the effects of altered habi-
tats. Although this is a difficult problem, size-frequency data,
collected at sufficiently close intervals in space and time,
can be used to estimate natural mortality rates: (1) in the
absence of a fishery, (2) if fishing mortality rates are known,
or (3) for pre-recruit crabs not subject to fishing mortality.
Judging from the distribution of research effort, the life his-
tory phase from megalopal settlement through the first few
juvenile molts (Moksnesa et al. 1997) has been consid-
ered the most critical in the crab's life cycle. A test of this
hypothesis would require determining which part of the life
cycle is most strongly correlated with recruitment of ma-
ture adults over a long period of time, or large spatial scale.
Large scale population studies, which have been oriented
toward fishery management (Rugolo et al. 1998, Uphoff
1998, Guillory et al. 2001), have shown stable or increas-
ing recruitment in the Chesapeake Bay and the Gulf of
Mexico, suggesting that cumulative effects of habitat alter-
ation in these major centers of blue crab production have
not negatively affected blue crab recruitment. Other poten-
tially critical life stages include the zoeae whose distribu-
tion and fate strongly depend on coastal ocean currents,
winds, and storms, adults during the vulnerable molting and
mating sequence, and mature females from their seaward
migration through ovulation, incubation and spawning.
Distribution
Blue crabs range throughout the coastal waters of the north-
ern Gulf of Mexico, from the continental shelf to tidal fresh-
water rivers. There are no clear patterns of abundance along
the coast from western Florida to Texas, although com-
mercial landings are dominated by the Louisiana fishery.
The general pattern of distribution follows the life history:
zoeae passively transported into and within the coastal
ocean; megalopae moving on flood tides through the passes
into bays and sounds, then settling in muddy bottoms, sea
grass beds, flooded marshes, and other habitats; juvenile
and adult crabs migrating upstream in search of food, ref-
uges from predators, and mating habitats; mature females
migrating seaward prior to spawning. Some crabs spend
time in the intertidal zone, where they may bury in mud to
prevent dessication, hide from predators, and ambush prey
(Hay 1905).
Dependence on habitat
Spawning grounds at estuary mouths seem to be critical
habitats for blue crabs in relation to fecundity, natality, and
early larval survival and dispersal, although little research
has been directed to whether there are essential qualities
of these habitats other than salinity >20 ppt. Tidal marsh-
water interfaces, beds of seagrasses and macroalgae, and
shallow muddy bottoms are important habitats for settling
megalopae and small juvenile crabs (Heck et al. 2003,
Minello et al. 2003). Cover or structured habitats are ben-
eficial, but not required, for molting and mating of mature
crabs during these episodes of vulnerability. The typical
migration of mature crabs into brackish and freshwater
habitats suggests an important role for fresh water in sup-
porting the life cycle. Because blue crabs eat virtually any-
thing organic, the density of food supply should be more
important than the availability of specific types of edible
substances.
The nursery hypothesis of Beck et al. (2001) states:
"A habitat is a nursery for juveniles of a particular species if
its contribution per unit area to the production of individu-
als that recruit to adult populations is greater, on average,
than production from other habitats in which juveniles oc-
cur" (p. 635).
22
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Although the literature generally reports that physically
complex (structured) habitats favor higher density, survival
and growth of early juvenile blue crabs (e.g., Hovel and
Lipcius 2001, Pile etal. 1996, Moksnesa et al. 1997, Minello
et al. 2003), the nursery hypothesis apparently has not
been fully tested for any specific habitat. Major episodes
of seagrass loss have not been strongly associated with
declines in the large scale relative abundance of blue crabs
(Heck et al. 2003). A comparative analysis of blue crab
density or abundance in estuaries with different physical
and biotic characteristics apparently has not been con-
ducted, although sufficient data may exist.
Life-supporting Attributes of Habitat
The habitat requirements of blue crabs have been reviewed
in detail, for the Chesapeake Bay by Van Heukelem (1991)
and for the Gulf of Mexico by Guillory et al. (2001). Salinity
>20 ppt is a critical factor for hatching, development and
survival of zoea larvae (Van Heukelem 1991). As crabs
mature, their salinity tolerance increases. Juveniles and
adults inhabit areas with salinity from 0-40 ppt or more.
Temperature tolerance is dependent on acclimation, rang-
ing broadly from 0-39 C. Tolerance for extreme tempera-
tures is greater at higher salinity. Blue crabs are sensitive
to hypoxia, and avoid areas with low concentrations of dis-
solved oxygen (DO). Van Heukelem (1991) recommended
that DO should be >3 mg-L'1 to maintain suitable condi-
tions for blue crabs.
Cover and habitat complexity favor avoidance of preda-
tion and cannibalism, especially for settling megalopae and
early juveniles, but no specific qualities of seagrasses,
marshes, or macroalgae seem to be required. The prefer-
ence for structured habitats diminishes as crabs mature.
Blue crabs also create their own refuges by burying in soft
bottoms. Shallow, vegetated or un-vegetated muddy bot-
toms, including tidal marsh edges, have shown positive in-
fluences on survival, growth and recruitment in several stud-
ies (Guillory etal. 1998, Van Heukelem 1991, Minello 1993).
The extent of these habitats and alterations by bulkheading,
dredging, and rising water levels should receive close at-
tention for their potential effects on blue crab populations.
Fresh water supplies to estuaries are important for older
crabs, which favor low salinity habitats, at least in some es-
tuaries.
Blue crab zoeae through the second molt depend on very
small planktonic food items. They have been reared suc-
cessfully in the laboratory on rotifers and polychaete larvae.
Older larvae probably feed mainly on copepods (Van
Heukelem 1991). As grazers, predators, detritivores, sus-
pension-feeders and cannibals, juvenile and adult blue crabs
do not depend critically on specific types of prey or other
food sources. Where crab densities are high, food quantity
could limit growth, survival and fecundity, but it is unlikely
that food availability or quality is a critical habitat issue for
blue crabs in most areas.
Blue crabs are significant contributors to the diets of many
estuarine predators. High rates of predation on juvenile blue
crabs by abundant populations of predatory fish has been a
concern in Chesapeake Bay at a time when some species
of forage fish are at low abundance. Maintaining balanced
food webs and predator-prey ratios in estuaries could pre-
vent excessive predation on blue crabs.
23
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
VII. SCIAENID FISH
Phylum Chordata, Class Osteichthyes, Order Perciformes, Family Sciaenidae.
Representative species, Cynoscion nebulosus Cuvier
FWC 2001
The sciaenid fish family (Table 6) is commonly termed the
'croakers' or 'drums' because of the audible sounds pro-
duced during mating rituals or when distressed Hoese
(1998) reported nine species of sciaenids as common in-
habitants of the inshore waters of the northern Gulf of
Mexico. Ten additional species either maintain a predomi-
nantly offshore or reef existence or reside elsewhere
Of the primarily inshore sciaenid species, the spotted
seatrout, Cynoscion nebulosus Cuvier, was selected for
examination of an inshore-based finfish species to assess
dependency on spatially explicit estuarine habitats. In com-
parison with the other valued populations of sciaenids, the
primary reference materials (e.g. ichthyology texts, regional
management documents, and scientific literature sources)
generally confirm that habitat requirements and critical life
cycle stages of spotted seatrout are similar to those of other
Sciaenid species. However, the spotted seatrout is unique
in that it spends its entire life-cycle inshore, utilizing the
attributes provided by many estuary habitats (Bortone 2003,
Gold 2003, Pattilloetal. 1997, Hoese 1998, Kostecki 1984).
Life History
Egg stage
Following spawning by mature adults in open water, fertil-
ized eggs are either buoyant or demersal, depending upon
salinity (VanderKooy 2001). Hatching is normally expected
within 16-20 hours after fertilization at 25 C (Fable et al.
1978) and may occur at a wide range of salinities (15-50
ppt). However, salinity tolerance may be influenced by the
salinity at spawning (Holt and Holt 2003). Fertilized eggs
range from 0.60 mm (Holt et al. 1988) to 0.85 mm (Fable
et al. 1978) in diameter, and egg size may also be signifi-
cantly affected by salinity (Holt and Holt 2003).
Larval stages
After hatch, both larvae and post-larvae are carried by tidal
currents into shallow waters (Patillo et al. 1997). Post-hatch
larvae range from 1.30-1.60 mm SL (standard length) (Fable
et al. 1978) and are primarily pelagic (VanderKooy 2001).
Larval stages appear to continue for up to 14 days until
they reach 11-12 mm (Fable et al. 1978, Hildebrand and
Cable 1934, Pearson 1929).
Young larvae are usually found in both mesohaline and eu-
ryhaline waters while older larvae and post-larvae tend to
be more euryhaline in character (VanderKooy 2001, Patillo
et al. 1997). Due to their short hatch time, post-hatch lar-
vae normally settle near the vicinity of their spawning loca-
tions. Most often, they are found in the lower layers of
seagrass, in areas of high detrital content or within shell
rubble (VanderKooy 2001). Like the fertilized eggs, salinity
tolerances for larvae may also be influenced by spawning
salinity. Holt and Holt (2003) suggest that this characteris-
tic may represent local adaptations for specific salinity re-
gimes. From he initial feeding stages, spotted seatrout are
characterized as opportunistic carnivores with diets that shift
from invertebrates to nearly 100% fish over the successive
life stages (Table 6)
Juvenile stage
Juvenile spotted seatrout range from 12-200 mm SL with
individuals progressing in size and pigmentation toward the
24
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Table 6. Sciaenid species of the Gulf of Mexico (Hoese 1998, MacEachran and Fechhlem 1998). Species are listed by
location of the primary population range or eco-region. Bold denotes valued species of the inshore water of the northern
Gulf.
Inshore bays and estuaries
of the northern Gulf
Gulfshore or reef inhabitants
of the northern Gulf
Tropical Ares of Gulf or
along the Atlantic coast
Atlantic croaker1
Micropogonias undulatus
Black Drum1
Pogonias cromls
Red drum; redfish2
Sciaenops ocellatus
Sand trout; white trout
Cynoscion arenarius
Silver perch
Bairdiella chrysoura
Southern kingfish
Menticirrhus americanus
Spot; flat croaker
Leiostomus xanthurus
Spotted seatrout; speckled trout
Cynoscion nebulosus
Star drum
Stellifer lanceolatus
Northern kingfish; king whiting
Menticirrhus saxatallis
Gulf kingfish; Gulf whiting
Menticirrhus littoralis
Reef croaker
Odontoscion dentex
Silver seatrout
Cynoscion nothus
Banded drum/croaker
Larimus fasciatus
Cubbyu
Equetus umbrosus
Sand drum; roncador
Umbrina coriodes
Southern kingfish; sea mullet
Menticirrhus americanus
Blackbar drum
Equetus iwamotoi
Jacknife fish
Equetus lanceolatus
Spotted drum
Equetus punctatus
1 Common in Gulf, but valued as fishery primarily along Atlantic states.
2 Primary value is as an inshore recreational fishery. However, offshore "bull" reds are sought as a sport fish,
but are typically not often taken due to a perceived lower quality flesh (Hoese 1998).
adult form (VanderKooy 2001). Juvenile seatrout are char-
acterized as euryhaline (Patillo et al. 1997) with a reported
salinity preference of 8-22 ppt (Baltz et al. 2003). Although
estimates of growth are highly variable and may be re-
gionally dependent (Bumguardner and Maciorowski 1989,
Murphy and McMichael 2003, Colura et al. 1991), growth
rates of wild caught juveniles have been best estimated at
13-18 mm per month (McMichael and Peters 1989). The
variability in growth may also result from differences in
food type and abundance or confounding factors such as
recruitment (Murphy and McMichael 2003, VanderKooy
2001). Regardless, there ia a pattern of higher growth
rates for early or mid season (July-August) juveniles vs.
late season (fall) juveniles as both temperature and prey
abundance decline.
Adult stage
In a review (VanderKooy 2001) of published data for spot-
ted seatrout collected from 24 bay systems across the
northern Gulf of Mexico, the average size of a one year-
old adult seatrout was 214 mm total length (low = 114 mm,
Biloxi Bay MS; high = 270 mm, Baldwin County, AL). At
age 5, adult seatrout averaged 483 mm TL (low = 392 mm,
Matagorda Bay TX; high = 631 mm, Apalachicola Bay FL)
(VanderKooy 2001). With much uncertainty across stud-
ies due to the number of fish sampled at any given time
and the possibility of differences across the geographic
range, life expectancy for the spotted seatrout is reported
as ranging from 4-12 years for both males and females.
However, as seatrout reach the maximum age for their
population or resident location, a shift in the sex ratios in
most locations appears to indicate that females have a
longer life expectancy than the males (VanderKooy 2001).
As adults, spotted seatrout and the other Cynosion sp.
populations are generally considered piscivorous
(VanderKooy 2001, McMichael and Peters 1989, Hoese
1998). With maturity and continued growth, their depen-
dence on invertebrate populations decreases significantly
25
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
(Table 7) relative to the numbers of penaeid shrimp and
other invertebrates taken during the juvenile stages
(Gilmore 2003, VanderKooy 2001, Patillo et al. 1997). This
shift towards piscivory appears to occur in coincidence with
the development of two canine-like upper teeth and a ter-
minal mouth structure that is better adapted for catching
small fish (VanderKooy 2001, Hoese 1998).
In contrast, the adult stages of other common Sciaenids,
such as the red drum, Atlantic croaker (Micropogonias
undulatus), and the silver perch (Bairdiella chrysura), main-
tain a sub-terminal mouth that is better adapted for bottom
feeding and, therefore, invertebrates persist through all life
stages as a large component of the diet. Some sciaenids
with sub-terminal mouths (e.g. black drum, croaker) have
barbels under their lower jaw that act as feelers to help
seek out burrowing crustaceans (Hoese 1998, Patillo et al.
1997).
Spawning
Unlike single-spawning season sciaenids, spotted seatrout
are characterized as "multi-spawners", meaning they are
able to spawn multiple times across seasonal periods from
early spring to early fall in most areas (Holt and Holt 2003,
VanderKooy 2001, Patillo et al. 1997). Extended spawning
seasons occur at the warmer temperature regimes of the
Gulf region (Holt andHolt 2003, VanderKooy 2001, Hoese
1998), but consistency with temperature and salinity toler-
ances remain. In comparison to other valued sciaenids,
and especially red drum (the second most important
sciaenid fishery), spotted seatrout spawn at lower salini-
ties and nearly always within estuarine waters. A known
exception occurs offshore of coastal Louisiana, where spot-
ted seatrout are known to venture into the surf zones of
barrier islands to seek the higher salinity water needed for
spawning. Red drum spawning occurs under high salinity
conditions, usually offshore, in or near tidal passes. There-
after, fertilized eggs, larval and juvenile stages follow the
same or similar habitat associations as those of the spot-
ted seatrout. Although differing accounts report spawning
activity occurring within a wide range of salinity, (Saucier
and Baltz 1993), controlled laboratory studies have indi-
cated that successful spawning is generally limited to sa-
linities less than 20 ppt (Holt and Holt 2003).
Brown-Peterson et al. (1988) report that spawning does
not occur in waters less than 23°C in Texas, with others
reporting 22°C as the minimum for Louisiana and Florida
estuaries (Neiland et al. 2002). The frequency of male ag-
gregations and the intensity of their drumming patterns at
temperatures of 30-31 °C were cited by Saucier and Baltz
(1993) as evidence of optimal spawning conditions for spot-
ted seatrout. The lack of spawning aggregations above 33°
C found by the same investigators appears to define the
upper temperature limit for spawning.
Table 7. Spotted seatrout preferred prey at each life stage
(source data from stomach contents information compiled in
Patillo 1997 and published in Gilmore 2003).
Larval Seatrout
copepods
Early Juvenile Seatrout
planktonic schizopods
mysids*
copepods
isopods
amphipods*
gastropods
bivalves
Caridean shrimp*
penaeid shrimp*
fish
Late Juvenile Seatrout
Caridean shrimp*
penaeid shrimp
fish*
Adult Seatrout
Fish*
Clupeiformes
bay anchovy, Anchoa mitchilli
menhaden, Brevoortia patronus
Shad, Dorosoma $pp.
Alopiformes
inshore lizard fish, Synodus foetens
Batrachlidiformes
gulf toadfish, Opsanus beta
Mugiliformes
striped mullet, Mugil cephalus
Atheriniformes
silversides, Menidia spp.
hardhead silverside, Atherinomorus stipes
Cyrprinodontiformes
sheepshead minnow, Cyprinodon variegatus
rainwater killifish, Lucania pan/a
goldspotted killifish, Floridichthys carpio
Gasterosteiformes
pipefish, Syngnathus spp.
Perciformes
gray snapper, Lutjanus griseus
snapper, Lunjanus spp.
pigfish, Orthopristis chrysoptera
silver jenny, Eucinostomus gula
Atlantic croaker, Micropogonias undulatus
spotted seatrout, Cynoscion nebulosous
code goby, Gobiosoma robustum
naked goby, G. bosc
clown goby, Microgobius gulosus
"* = most prominent prey by relative abundance
(Gilmore 2003)."
26
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Spawning behavior usually begins when mature males con-
gregate at dusk in open, relatively deep inland waters, and
initiate acoustical signaling that lasts for approximately
three hours (Gilmore 2003). There is a strong correlation
of sound emission with egg and larval abundance (r = 0.92
to 0.98) and hydroacoustic monitoring has become a val-
ued technique for documenting the spatial and temporal
trends of spawning activity for sciaenids. Sounding activi-
ties related to spawning usually are most intense in April
or May in warm locations (Gilmore 1994), or as late as
June in cooler locations, and are sustained into August
(VanderKooy 2001, Patillo et al. 1997, Gilmore 1994).
Spawning occurs in all lunar phases, but is most prevalent
during and shortly after a full moon (Gilmore 1994). Males
typically are able to spawn earlier in the season than fe-
males (VanderKooy 2001). It is suspected that the move-
ment of the males to spawning grounds and the initiation
of courtship soundings eventually leads to mass gather-
ings of males and ripe females (VanderKooy 2001). As a
school congregates, intensity of activity also increases and
much side-to-side contact occurs, causing the release of
sperm and ova (VanderKooy 2001).
Fecundity
As with many species that utilize a multiple spawning re-
productive strategy, fecundity estimates are highly variable
(VanderKooy 2001). In general, fecundity estimates vary
with the size and age of fish as well as geographic area
and season. Seasonal variability and timing of environmen-
tal conditions (such as salinity, temperature, moon phase,
and the differences between growth rates and maturity
schedules) also contribute to the variability of fecundity es-
timates for populations of spotted seatrout in the Gulf. Un-
der normal conditions, spawning has been reported to oc-
cur every 16-21 days from April to September, about 8-11
times per year in Louisiana (Saucier and Baltz 1993).
Spawning frequency appears somewhat altered by latitude
and age, with older females (3-4 years old) reproducing
more frequently than younger ones (VanderKooy 2001).
Spotted seatrout have been shown to produce 451 ± 41
eggs/g female for each spawn (Brown-Peterson et al.
1988). Using this value, VanderKooy (2001) concluded that
a 2-pound spotted seatrout spawning eight times in a sea-
son would produce about 3 million eggs.
Critical Life Stages
From post-hatch larvae through the juvenile life stage, there
is a strong association of abundance with aquatic vegeta-
tion in the forms of tidal marsh or seagrass meadows
(Bortone 2003, VanderKooy 2001, Hoese 1998, Patillo et
al. 1997). The structural complexity of these habitats ap-
pears to provide a foraging area, within which exists a con-
centrated source of prey (relative to open water, mud or
sand bottoms) while also offering some degree of protec-
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
tion from predation for the early life stages of spotted
seatrout. As juveniles, spotted seatrout (and other
sciaenids) begin to move into a greater range of depth (up
to 3.0 m), while still maintaining an association with veg-
etation. When extensive areas of vegetation are not present
large numbers of juveniles and young adults can be found
in back-water habitats, such as "bayous, tidal creeks, slow
flowing rivers" (Patillo et al. 1997). Although presence/ab-
sence data appear frequently in the literature for all
sciaenids, reports demonstrating that adult populations are
dependent on larval or juvenile success are not available.
Distribution
Estuarine-dependent spotted seatrout are widely distrib-
uted, extending from the coastal waters of Massachusetts
into the Bay of Campeche and along the Yucatan Penin-
sula (Gold 2003, Hoese 1998, Patillo et al. 1997). The popu-
lation is most abundant in the northern Gulf of Mexico. Some
genetic variation exists among different geographic popu-
lations, but there is apparently sufficient exchange of ge-
netic material throughout the range to exclude distinction
of separate species (Gold 2003, VanderKooy 2001 Hoese
1998).
Although other northern Gulf species of Sciaenidae are con-
sidered estuarine residents with the majority of the valued
harvest coming from inshore areas, the spotted seatrout is
the only one that spends its entire life-cycle within the es-
tuarine habitats of inshore waters (Bortone 2003, Hoese
1998, Patillo et al. 1997). Red drum (Sciaenops ocellatus)
occupy estuaries through the adult stage, but spawning oc-
curs primarily offshore and the largest of the individuals
(termed 'bull' reds) move offshore, where a second fishery
is supported (Patillo et al. 1997). Spotted seatrout are gen-
erally euryhaline and appear to require specific salinities
>20 ppt only during spawning (Saucier and Baltz 1993). As
with the other Gulf sciaenids, spotted seatrout are also tol-
erant of wide temperature ranges; both larvae and juve-
niles have been found in temperatures from 5-36°C
(Bortone 2003, VanderKooy 2001, Patillo et al. 1997). |n a
review of numerous tagging studies, VanderKooy (2001)
points out that spotted seatrout are basically non-migra-
tory, with only rare observations of trout moving more than
32 km from their natal estuary.
Dependence on Habitat
Post-hatch early life stages of spotted seatrout exhibit a
strong preference for the attributes provided by aquatic
vegetation (VanderKooy 2001, Patillo et al. 1997, Rakocinski
et al. 1992). High abundance of aquatic vegetation, in the
forms of both tidal marsh or seagrass meadows, are con-
sistently reported for larval and juvenile stages (Bortone
27
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
2003, VanderKooy2001, Hoese 1998, Patilloetal. 1997).
However, reports concluding that aquatic vegetation is a
requirement for larval or juvenile survival have not been
found.
Underwater acoustic monitoring technology has been used
to identify deeper open water areas as preferred locations
for seatrout spawning (Gilmore 2003, Baltz 2002, Collins
et al. 2002). While meeting the minimal requirements for
spawning, the areas were generally characterized as de-
void of freshwater river and canal discharges, physical
structures (bridges), and fast flowing channels, passes
open to ocean or Gulf water (Baltz 2002, Collins et al. 2002,
VanderKooy 2001, McMichael and Peters 1989). As with
the preferences for vegetated habitats during the early life
stages, there appears to be no evidence supporting limi-
tation of harvestable populations to preferred spawning
habitats.
Life-supporting Attributes of Habitat
Vegetated Habitats
Larval and juvenile stages of spotted seatrout and other
sciaenids have routinely been associated with aquatic veg-
etation, either as seagrass or among the flooded depths
of emergent marsh (Rooker and Holt 1997, Rozas and
Minello 1997, Rooker etal. 1998, Patilloetal. 1997, Bortone
2003). A review of available habitat-specific density data
and growth experiments showed that seagrass beds and
Spartina altemiflora marsh adjacent to the shoreline (marsh
edge) support higher populations of these fishery species
than other habitats (Minello et al. 2003). Young sciaenids
may be able to maximize their growth rate (Rooker et al.
1998) due to higher abundance and diversity of prey than
in the interior of tidal marshes, large seagrass meadows,
or non-structured habitat such as open water, mud, or
sandy areas (Minello et al. 2003). This relationship has
led to the commonly used description of vegetated areas
as nursery habitats. However, data to indicate the links
between these potential attributes provided by specific
habitat types and the success of the valued population
are lacking.
Water quality
Another attribute that may be critical for the success of
spotted seatrout population is access to preferred salinity
and temperature ranges necessary for spawning and egg
survival (Brown-Peterson 2003, Holt and Holt 2003). Al-
though not as critical for sciaenids that normally spawn
offshore, spotted seatrout may be limited in their access
to spawning sites following periods of heavy rainfall or
near significant flows of freshwater. With an extension of
their spawning range into tidal passes or even offshore
beaches (as in coastal Louisiana), the distance required
for transportation of fertilized eggs and post-hatch larvae
is also increased, which may affect recruitment. Likewise,
salinity appears to be an important factor for insuring the
buoyancy of eggs and post-hatch larvae by providing an
increased probability that tidal flow will allow them to hatch
and settle in or near areas of structural complexity (e.g.
aquatic vegetation).
Freshwater flow
In contrast to salinity requirements (>20 ppt) for spawning
and egg buoyancy, it has been suggested that inshore popu-
lations of finfish were significantly enhanced following a
period of wet years in the early 1990s. The implication is
that high freshwater discharge, with higher concentrations
of nutrients, results in increased prey abundance. These
benefits may outweigh any reduction in spawning activif-
due to lower salinity.
28
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
VIII. STATUS OF LIFE-SUPPORTING HABITATS IN THE GULF OF MEXICO
It can be determined from Sections IV-VII that certain physi-
cal habitats are critical or important for shrimp, oysters,
blue crabs and sciaenids (Figure 6). In the most general
terms, it appears that shallow, near-shore, estuarine ar-
eas, with ample inputs of fresh water and physical struc-
ture in the form of reefs, marshes or seagrass beds sup-
ply critical habitat needs for these economically important
resources. In this section, we briefly review habitat classi-
fications relevant to coastal aquatic eco-systems, describe
the recent status of important habitat types in the northern
Gulf, and summarize information on historical and recent
spatial trends.
Habitat Classifications and Descriptions
The wetlands classification system of Cowardin et al.
(1979) is a hierarchical approach that assigns wetlands to
categories and is supplemented by descriptive material
and lists of representative plant species for each wetland
type. The definitions of Cowardin et al. are used in the U.
S. Fish and Wildlife Service's National Wetlands Inven-
tory (Dahl 2000), which is a periodic assessment of status
and trends of tidal and non-tidal wetlands within the United
States. The U. S. Department of Agriculture uses this sys-
tem in its National Resources Inventory, which includes
data on wetlands status and trends. Gulf of Mexico coastal
habitats include three of the Cowardin et al. (1979) major
wetland systems: Marine, Estuarine and Riverine-Tidal
subsystem. The Estuarine system is divided into Subtidal
and Intertidal subsystems, each of which is divided into
several classes and subclasses (e.g., class Reef is divided
into subclasses Mollusc and Worm). At the most specific
level of classification (subclasses) the Cowardin system
would recognize nearly 100 types of coastal habitats in the
Gulf of Mexico. Despite its specificity, the system does not
include altered habitats per se, nor does it address habitat
quality within subclasses.
A comprehensive "Marine and Estuarine Ecosystem and
Habitat Classification" system was proposed by Allee et
al (2000). This system is complex, with 13 levels of hier-
archy. The lowest level of classification ('eco-types,1 e.g.,
salt marshes, seagrass beds, mud flats, beaches, oyster
reefs) best corresponds to the habitats of concern for this
project.
Stevenson et al. (1986) classified coastal marshes based
on sediment accretion rates and expected responses to
sea level change. Five of their six classes occur in the
northern Gulf of Mexico: Submerging Coastal, Estuarine,
Submerged Upland, Floating Mat, and Tidal Freshwater.
Except for floating mats, the long-term stability of all of
these marsh types depends on the balance of allochthono-
us sediment supply, autochthonous sediment production,
land subsidence, and sea level change. Floating mats, com-
mon in Louisiana marshes, are not directly susceptible to
rising water levels, but could succumb to increasing salin-
ity as sea level rises. Although the Stevenson et al. (1986)
classification is for marshes only, it illustrates dynamics of
habitat change that apply to all shallow-water estuarine
habitats. Shorelines, marshes, seagrass beds, and oyster
reefs are subject to continual changes, even in the ab-
sence of human intervention.
SPECIES
CO
<
I
i' II !• I I
I .litfill
oystei
o
Blue
ci.il,
JJ
0
0
feh
0
o
0
II
=; ;
o
Key
Oitic.il
Secoinl.iiy <>i hypollielk.il
Figure 6. Dependence of selected Gulf of Mexico
species on major habitat attributes.
29
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Table 8. Targeted list of habitats for the northern Gulf of Mexico ecoregion, modified from Beck et al. (2000). Items in
brackets are suggested additions relevant to altered habitat research. The primary habitats of concern for this project
are indicated with asterisks (*).
Habitats
*Seagrass
High Relief (10-70 cm tall)
Low Relief (< 10 cm tall)
Tidal Freshwater Grasses
'Oyster reefs
tidal/intertidal]
[high relief/ low relief]
[managed/un-managed]
*Salt marsh
Polyhaline Saltmarsh
Mesohaline Saltmarsh
Oligohaline Saltmarsh
Sponge and soft corals
Tidal Flats
*Tidal Fresh Marsh
Intertidal Scrub/Forest
*Muddy-bottom Habitats
Coquinia Beach Rock
Beaches and Bars
Serpulid Worm Reefs
Some Characteristic Species
Thalassia testudinum, Syringodium filiforme, Halodule wrightii
Halophila spp.
Vallisneria americana, Potamogeton spp., Ruppia maritime
Crassostrea virginica
Spartina altemiflora, Juncus roemerianus, Distichlis spicta
S. altemiflora, D. spicata, S. patens, Scirpus americanus
Paspalum vaginatum, S. Patens, Eleocharis spp., Sagitiaria lancifolia
Loggerhead sponges, vase sponges, sea fans, small hard corals
Algae, polychaetes, bivalves
Scirpus spp., Typha spp., Cladium spp.
Avicennia germinans, Iva spp., Baccharis spp.
Polychaetes, amphipods, isopods
Donax spp.
Shorebirds, mole crabs, amphipods and isopods
Family Serpulidae
The Nature Conservancy (TNC) developed a list of north-
ern Gulf of Mexico habitats targeted for conservation be-
cause of their biodiversity (Beck et al. 2000). Like the clas-
sification systems discussed above, TNC's classification
ignores altered habitats, but it is simpler and more relevant
to the needs of this project (Table 8). We are concerned
here with alterations of a few specific habitat types of major
importance to economically valuable species. We will need
to define some habitat types more specifically than in the
Cowardin et al. (1979) or Allee et al. (2000) systems. At the
same time, we are not immediately concerned with many
of the habitat types that are applicable to the Gulf of Mexico.
Peters and Cross (1992) concluded that there was no uni-
versal definition of coastal fish habitat, but suggested that
habitat should be defined as "...the structural component of
the environment that attracts organisms and serves as a
center of biological activity." This definition is consistent
with usage in this document, if we accept that mixing of
fresh and salt water in coastal systems is a structural prop-
erty.
Locations of Different Habitats in the Gulf of Mexico
The Gulf of Mexico is a semi-enclosed sea, bounded by
the southern United States on the east, north and north-
west, Mexico on the south and southwest, and Cuba on
the south (Figure 7). It is surrounded by low-lying coastal
plains covered mostly by unconsolidated marine and flu-
vial sediments. The Big Bend area of northern Florida,
where limestone bedrock outcrops along the shore, is an
exception. The surface area of the Gulf is 1.6 million km2;
the shoreline, including bays and estuaries, extends 27,000
km within the United States. Estuarine habitats along the
U.S. coast of the Gulf include about 2.3 million ha of inter-
tidal vegetated wetlands (primarily salt marshes), and 1.5
million ha of submerged aquatic vegetation, including
macroalgae (Guillory et al. 2001). Acreage of marshes and
submerged aquatic vegetation is available by state (Guillory
et al. 2001), and by estuary (Table 9). The U.S. Geological
Survey's National Wetlands Research Center (NWRC) has
compiled National Wetlands Inventory maps for much of
30
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•*-.-- • r
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
V
lachee Bay ^
Anclote Key '*
Tampa Bay
Charlotte Harbor
Bay System
* Linked Coastal Louisiana Estuaries
Calcasieu and Mermentau River Basins
Bayou Teche, Vermillion and
Atchafalaya River Basins
-Western half ofTerrebonne Bay
4 - Eastern half of Terrebonne Bay ^juM^
5 - Barataria Basm
6 - Mississippi River .
7 - Breton Sound Basin
Lower Lake Pbntchartrain Basin
"""er-Lake Pontchartrain Basin
Figure 7. Gulf of Mexico Estuaries. Base map modified from Nipper et al. 2003. Descriptive information for
each system located in Table 9.
the Gulf Coast. These maps show coastal wetlands and
changes in wetland acreage from 1976-1996. Seagrass
data for selected areas of the Gulf Coast are also available
from NWRC.
Losses and Alterations of Gulf of Mexico Habitats
"Indeed it has been noted that estuaries may represent the
most anthropogenically-degradedhabitats on earth... Loui-
siana has seen the greatest loss of coastal habitats; in this
century there has been a net conversion of 4,000 square
kilometers of wetland to open water... A peak loss rate of
about 108 square kilometers of wetland habitat per year
occurred during the 1958-1974 period, but continues pres-
ently (1990 estimate) at about 66 square kilometers per
year... Oyster reefs have also declined in the Gulf..." (Beck
etal.2000).
31
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Table 9. Descriptive information of the Gulf of Mexico bay systems from Laguna Madre to Tampa Bay. Data source
compiled from GMFMC 1998 and, as noted, by Nipper et al. 2003.
Bay System
(West-East)
Laguna Madre
(upper & lower)
Corpus Christi Bay
Aransas Bay
San Antonio Bay
Matagorda Bay
Galveston Bay
Sabine Lake
Totals of Linked
Estuaries across
Louisiana Coast
Major Estuaries of
Louisiana4
West to East
values for surface
water area
Area Values for Essential Habitat
(hectares)
Surface Tidal Sea- Oyster
water
113,746
43,288
45,257
55,123
98, 921
155,403
22,605
2,900,000
73,688
174,413
35,721
69,235
28,571
46,245
79,050
303,275
183,053
Marsh grass reef
101,150 73,088 minimal
10,115 9,955 350
18, 207 3,327 340
10,115 4,289 2,913
48,552 1 ,550 "many"
93,624 113 3,046
171,955 sparse "few"
NA
1,580,999 8,094 53,825
primarily
Freshwater Benthic Salinity
Influence (m3/s) Description (ppt)
low, no major mud, silt,
discharges sand, gravel
34 (Nueces mud, sand,
River) silt
28 (from sleet
flow & 3 riverine
systems)
116 (Guadalupe mud, sand,
and San Antonio shell
Rivers)
not published; prior
to1980'swas87
m3/s primarily from
3 small rivers and sand, shell,
before diversion of silt, clay
Colorado River
430 (Trinity River,
HoustonShip mud, shell,
Channel) clay
487 m'/s (Sabin mud and silt
and Neches Rivers)
20,456 m3/s/ mud, sand,
(Mississippi and silt
Atchafalaya Rivers)
24-50
26-37
11-30
7-26
17-31
10-21
2-10
low and
variable
Calcasieu and Mermentau River Basins
Bayou Teche, Vermillion and Atchafalaya River Basins
Western half of Terrebone Bay
Eastern half of Terrebone Bay
Barataria Basin
Mississippi River Delta
Breton Sound Basin
Lower Pontchartrain Basin
Upper Pontchartrain Basin
Mississippi Sound
1235 m3/s (as com-
bined drainage of
479.2001 36,019 49,420 NA Pearl and Pascagou-
la Rivers and sources
in Biloxi and St. Louis
Bay systems)2
silt clay 24 <3>
32
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Table 9. Continued
Area Values for Essential Habitat
(hectares)
Bay System Surface Tidal Sea- Oyster
(West-East) water Marsh grass reef
Coastal Zone of 160,809
Alabama2
Pensacola Bay 51 ,005
System3
Choctawhatchee 34,8000
St Andrew Bay 27,900
Apalachicola Bay 55.4001
System
Apalachee Bay to 1 02,300
14,008 NA 2,039
(mainly
Mobile Bay)
3598 2664 162
1140 1214 566
4200 3500 57
large
239.6002 36001 amt's
(NA)
75,000 53,420 162
Freshwater Benthic Salinity
Influence (m3/s) Description (ppt)
1659
328 '(from three
coastal watersheds)
200 (Chocta-
wahatchee River)
1271
824 (main source
Apalachicola
River)1
150 (into Apalachee
, ... "highly „
mud, silt variable
0-36
0-30
fine-coarse
quartzose
sand with 0-30
silt and clay
in center of
bays
18-33
clays, hard
muds, 0-32
oyster
10-30
Anclote Key
Bay only1); rests NA
Tampa Bay
System
Charlotte Harbor
137,841
80,500
987
(9,025 as
mangroves)
252,500
608
14,970 scattered
21,400 31.6
68 (five major river
systems; flow for
three are managed)
1.0 (primary source,
Caloosahatchee
River, is managed)
mud and
muddy sand 27
(avg.1)
mud, salt,
sand and 0-38
shell "highly
variable"
11nformation form Nipper et al. 2003
2 Inclusive of Mississippi Sound (far eastern side), Mobile Bay and Delta, Perdido Bay and Little Lagoon.
3 Inclusive of Pensacola, Escambia, East, and Blackwater Bays and Santa Rosa Sound.
4 Complete information not available for individual estuaries. Habitat coverage is generally similar across Louisiana.
33
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Figure 8. Existing and predicted loss of Louisiana coastal habitat (America's Wetland 2003).
Intertidal vegetated wetlands (marshes)
The disappearing Louisiana marshes (Figure 8) are the
largest and most prominent manifestation of habitat loss
in the northern Gulf, but other areas have also lost wet-
lands to development and other causes. For example, the
Pensacola Bay estuary complex lost 7-8% of coastal wet-
lands from 1979-1996, based on NWRC data (Figure 9).
More than 90% of the loss was attributable to wetlands
becoming uplands. Less than 10% of the change was due
to wetlands that became open water. Stedman and Hanson
(2003) reported that coastal wetlands in the Gulf states
were being lost to development at much higher rates than
were inland wetlands.
Submerged aquatic vegetation
With a few positive exceptions, Gulf estuaries have lost
20-100% of seagrass acreage since the mid-20th century
(Handley 1995). Seagrass coverage and distribution from
year-to-year, but there is no consistent or comprehensive
seagrass monitoring in the northern Gulf so trends are dif-
ficult to quantify. In addition, documenting the condition and
loss of SAV coverage is inherently difficult in brackish and
oligohaline waters where the beds are known to retreat
and recover based on normal cyclical climatic conditions
(e.g. drought or flood cycles). Regardless, best available
techniques will be used during aerial surveys of the north-
ern Gulf planned for 2003-2004, to be conducted by the
EPA and USGS. The data will be compared to earlier (1992)
overflight data to estimate trends in seagrass area and dis-
tribution,
Shallow, soft-bottom habitats
These habitats generally are not subjects of directed sur-
veys. Shoreline maps, bathymetric charts designed for navi-
gation, and available aerial photography could give indica-
tions of the extent and degree of alteration of shallow bot-
toms. It might be assumed that in the absence of human
interventions, this type of habitat would have existed adja-
34
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Pensacola Bay System
Habitat Change
1979-1996
Backwater River
Habitat
Chacge3
Acres
Escambia River
Habitat Legend
Upland
Upland >W«te
Upland > Wetland
VfttH
Water > U pl«d
Wafer > Wetland
Wetland
W»iand> Upland
Wettind> Wattr
4069017
45433
355688
9892100
59705
349.25
238S257
3286.97
62244
Figure 9. Evidence of habitat loss in the Pensacola Bay system.
cent to nearly all of the 27,000 km of Gulf shoreline. The
extent of shallow, soft-bottom habitat that has been altered
by structures, dredging, or other modifications probably could
be estimated from existing data. We have not found such
estimates.
Oyster reefs
The Gulf's bays, sounds and estuaries include significant
areas of natural and constructed oyster reefs. Summary
statistics on the acreage of oyster reefs are not available
for the northern Gulf as a whole, but state fishery manage-
ment organizations maintain various levels of relevant in-
formation. Among the Gulf states, Louisiana has the largest
extent of oyster reefs, a total of about 165,000 hectares of
leased bottom and an undetermined area (> 4000 ha) of
public seed and longing grounds (Louisiana Dept. of Natu-
ral Resources 2003). Charts of oyster reefs in Mississippi
are available on the internet (Mississippi Dept. of Marine
Resources 2003). Charts of oyster reefs in Florida, Ala-
bama, Louisiana and Texas apparently have not been pub-
lished.
We did not find data that apply directly to large-scale trends
in the areal extent or quality of oyster reefs. Gulf of Mexico
oyster harvests have been dynamically stable for at least
40 years (NMFS 2003 a-c), suggesting that there have not
been major changes in habitat quantity or quality, although
these properties may vary greatly within individual estuar-
ies.
35
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
IX. HABITAT ALTERATIONS
The overview of life histories (Sections IV-VII) illustrated
that freshwater discharge and its influence on salinity was
a consistent, and often dominant factor in the success of
each of the selected valued fishery species in the Gulf of
Mexico. A second factor was the availability of physical
habitat; seagrasses and tidal marshes for fish and shrimp,
clean and supportive substrate for oysters, and shallow,
near-shore soft bottoms for early juvenile blue crabs. The
type of physical habitat may be linked to freshwater dis-
charge, since different plant species are successful within
different salinity ranges. Section VIII examined distribution
of these physical habitats across the Gulf of Mexico. Here,
we begin to examine past and current alterations to fresh-
water discharge and physical habitats, alterations that have
already, or may eventually, affect the productivity and
sustainability of Gulf fisheries.
It is important to reemphasize that freshwater discharge
and physical habitat are both important to the survival of
each species. Critical life stages occurring in estuarine shal-
lows are not successful outside their salinity range, outside
areas that provide cover for protection, or outside areas
that provide essential nutrition. Successful recruitment de-
pends on favorable freshwater discharge co-occurring with
favorable physical habitat and food availability during the
nursery period. It would be counter-productive to consider
habitat criteria that focused on only one of these factors.
Freshwater Discharges into the Gulf of Mexico
There are numerous rivers and streams that discharge fresh
water into the Gulf of Mexico. The largest is the Mississippi
River, with a watershed estimated at more than 2/3 of the
continental U.S. and part of Canada. River discharge pro-
vides nutrients, detritus, sediment, mixing and variable sa-
linity to estuarine habitats, characteristics that have been
used to advantage in the life cycle strategies of estuarine
species. Freshwater discharges change in frequency, du-
ration and scope with changes in rainfall patterns in the
watershed. Over geologic time, freshwater inflows have
been created or disrupted by shifts in land masses, land
subsidence or upheaval, or other geologic changes in the
watershed. Changes in river systems, even if transient or
negligible on a geological scale, have far-reaching effects
on the biological populations at the river mouth (Gunter
1960). Altered freshwater flow regimes can bring devastat-
ing losses or unparalleled opportunities, depending on the
habitat requirements of different species.
Among the factors that alter the quantity, quality or timing
of freshwater discharge into the Gulf of Mexico are numer-
ous activities of humans, including dams for irrigation and
power, diversions, canals in uplands, deforestation, live-
stock grazing, road construction, and paving for urban de-
velopment (Browder and Moore 1981). These anthropo-
genic activities do not necessarily affect the overall amount
of freshwater entering the estuaries, but may affect the tim-
ing. For example, deforestation, road construction and
paving all reduce the natural delay between rainfall and
runoff. This effect increases peak flow after a rainfall and
decreases dry season flow, both factors that determine the
amount of time and area where favorable discharge co-
occurs with physical habitat.
Mississippi RiverDelta
The Mississippi River formed when streams throughout cen-
tral North America were diverted southward by growth of
the North American Ice Cap. The Mississippi River water-
shed covers 3,185,000 km2 (exceeded only by the Amazon
and the Congo), or approximately one-third of the United
States plus 33,000 km2 of Canada. The average annual
rainfall over this area is about 762 mm, of which about one-
fourth travels to the sea via the Mississippi River. Water
discharge at the mouth of the Mississippi has been esti-
mated at 5.8 x 1011 m3 annually with an estimated load of
0.9-1.8 x 106 kg of sediment a day. An alluvial valley was
formed by river sediment that overflowed the river banks
during changes in sea water level associated with melting
of glaciers. The alluvial valley terminates in the Deltaic Plain,
which was formed over the last 5,000 years by a series of
prograding and overlapping deltaic lobes composed of sedi-
ments transported by the River and its distributaries. The
Deltaic Plain covers ~38,000 km2 of the Louisiana coast
and extends from the head of the Atchafalaya River south
224 km to the Gulf of Mexico and southeast 346 km follow-
ing the course of the Mississippi River to the tip of Louisi-
ana. Because of this huge discharge and sediment load,
the Mississippi River is the dominant factor in the geology
and geomorphology of the Mississippi Delta and the Gulf of
Mexico (Russel and Howe 1935). It may also be the domi-
nant factor in the abundance and diversity of organisms
inhabiting the coastal regions of the Gulf (Gunter 1960).
Levees and dams along the Mississippi River Delta are com-
mon (Gunter 1960). Because of the vast watershed, river
flows can be extraordinarily high. Peak river flows occur in
the spring (March-May) as snow and ice melts in the north-
ern reaches. The thaw leads to overflow from the banks of
36
-------
the river, a common occurrence noted as early as 1543 by
De Soto and 1684 by La Salle. To settle the city of New
Orleans, located in the center of the delta region, levees
were constructed to block the natural overflow of the river
banks. The levee started in 1717 to encircle the French
Quarter, was the first of many levees, banks and dams
constructed to protect the human community from the sea-
sonal overflow of the River. During early settlement, the
King of France ordered that all land owners along the River
must provide levees or lose their land. By 1850, the U.S.
Federal Government gave 3.4 x 106 ha of land to States to
sell if they used the proceeds to build levees. The Missis-
sippi Valley Commission was established in 1879 to ex-
pand the levee system, often for the purpose of aiding ship
navigation. Following the disastrous flood of 1927, the Fed-
eral Government took complete charge of the Mississippi
River levees. During the period from 1880 to 1935, the levee
system grew from nearly 1,600 to over 3,200 km in total
length. As the levee system grew and the River was cut off
more and more from its flood basins, the peak river flows
grew higher. The rise in flood levels demanded higher and
more extensive levees, some reaching 10.6 m high. In spite
of, or perhaps because of this extensive network, Elliott
(1932) reported 58 major floods between 1717 and 1929.
Prior to human efforts to tame overflow in the Mississippi
Delta, there were several natural flood outlets, or distribu-
taries. The larger of these included: Bayou Manchac, head-
ing east into Lake Maurepas and Lake Pontchartrain;
Bayou Plaquemine, heading 128 km south into the Gulf at
Atchafalaya Bay; Bayou Lafourche, which travels 144 km
to the Gulf; and the Atchafalaya River, which also empties
into Atchafalaya Bay. Of these four major flood distributar-
ies, three were closed by construction. Bayou Manchac
was dammed in 1828, Bayou Plaquemine was blocked by
construction of the west bank levee in 1868 and a naviga-
tion lock added in 1909, and Bayou Lafourche was cut off
from the river in 1904. As these three distributaries were
blocked by dams and levees, water volume in the Atchafala-
ya River quintupled between 1858-1927 (Elliott 1932). Now,
the Atchafalaya carries one-fifth of the Mississippi River
water to the Gulf, a volume similar to the Arkansas, Mis-
souri or Ohio Rivers. In roughly 100 years, the Atchafalaya
changed from a simple stream to one of the major rivers
of America.
Historically, oysters were produced in Atchafalaya Bay. By
the 1970s, with the influx of additional fresh water to the
Bay, oyster production moved offshore to Marsh Island and
west to the mouth of Vermilion Bay, areas with higher sa-
linity (Hoese 1981). In contrast, Gunter (1960) reported
numerous examples of increased salinity in Louisiana
coastal zones that were disconnected from freshwater flow
by the levee system. High salinity completely eradicated
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
once-flourishing oyster populations. Throughout the many
years of levee construction, oyster growers and harvesters
continually asked for cuts in the levees to allow fresh water
to enter their oyster beds. The political pressure to provide
appropriate amounts of fresh water to oyster beds remains
today. The effects of freshwater diversions, both positive
and negative, on oyster stocks in Breton Sound, LA, have
been dramatic (Figure 10).
Gunter (1960) suggested that two anthropogenic forces,
(1) deforestation in the watershed, and (2) the delta levee
system, created the most important alterations of the Mis-
sissippi River Delta, increased sediment delivery and higher
flood stages. Deforestation eliminated plant cover that had
previously attenuated runoff and provided a more continu-
ous flow throughout the year. Browder and Moore (1981)
characterized a similar scenario; their list of anthropogenic
alterations included (1) dams for irrigation and power; (2)
diversions; (3) canals in uplands; (4) deforestation; (5) clear
cutting; (6) grazing; (7) road construction; and (8) paving
(as in urban development). The major effect of several of
these activities was to increase the flashiness (amplitude
of variation) of river flows. Major reservoirs (5,000 -25,000
acre feet, or 6-30 x 106 m3, storage capacity) along the
Mississippi River drainage basin increased from 69 to 948
between 1910-1988(Judd 1995). Reservoirs generally de-
crease peak flows but, depending on the release sched-
ules, can increase or decrease dry season flows (Browder
and Moore 1981). Although it is difficult to discriminate ef-
fects of any single alteration, or anthropogenic alterations
from natural variability in rainfall, freshwater discharge into
Gulf coast waters increased about 1,380 m3s'1 between
1945-1990 in both of the hydrologic segments affected by
the Mississippi River. The Mississippi River segment in-
creased from around 11,000 to 12,400 m3s-1 and the
Atchafalaya River segment increased from about 4,800 to
6,200 m3s-1.
Ca/oosaftafcnee River
The Caloosahatchee River basin is the primary source of
freshwater for the Charlotte Harbor Estuary (Table 9) lo-
cated in southwestern Florida. The watershed extends from
the Gulf of Mexico to the Everglades (Gunter and Hall
1962).The Caloosahatchee River and Canal, along with the
St. Lucie Canal, are the two primary outlets used by the
South Florida Water Management District (SFWMD) to
regulate water levels in Lake Okeechobee under the South-
ern Florida Flood Control Project (Figure 11). The
Caloosahatchee River once followed a natural watercourse
that extended approximately 82 km from Lake Flirt to San
Carlos Bay on the Gulf of Mexico. In 1884, the
Caloosahatchee Canal was built between the river head-
waters and Lake Okeechobee for water control and navi-
gation (Gunter and Hall 1962, Chamberlin and Doering
37
-------
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
3500
^ 3000
ra 250°
tfl
O 2000
••^
CO 1500
LU
{J£ 1000
m 500-
Freshwater flow is
temporarily restored after
high rainfall events in 1973,
1979, and 1991.
In 1991, Caernarvon
Freshwater Diversion
Structure begins operation,
permanently restoring
annual flow of freshwater.
1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1986 1986 1937 1988 1989 1990 1991 1992 1993 1994 1995'
YEARS
Figure 10. Standing crop estimates of Breton Sound (LA) oysters from public seed grounds (Kumpf et al. 1999).
pre-1880's location
of Lake Flirt
Fort
eyers, FL
Franklin
Lock and Dam
| | East-West Basin Boundary
Wetland Classification
m Aquatic Bed
Emergent
|0| Forested
m Open Water
| Scrub-Shrub
/ Caloosahatchee River and SW Florida Coastline
Figure 11. Wetland types in the watershed of the Caloosahatchee River and Estuary (SFWMD 2003).
38
-------
1988). During the ensuing years, a number of different
management decisions were made that altered freshwater
flow to the Gulf. By 1937 (after the River and Harbor Act of
1930 authorized Federal improvement of the Caloosahatch-
ee), extensive channelization, bank stabilization and three
lock-and-dam structures were completed (Chamberlin and
Doering 1988). The downstream structure, Franklin Lock
and Dam, demarcates the beginning of the Caloosahatch-
ee Estuary; it also controls water levels upstream, dis-
charges freshwater into the estuary and acts as a barrier
to salinity and tidal exchange which, before the dam was
built, extended upstream nearly to Lake Flirt (Chamberlin
and Doering 1988).
^n intricate system of canals within the watershed, corn-
Dined with regulatory releases from Lake Okeechobee, has
drastically altered the Caloosahatchee River and Estuary
(Chamberlin and Doering 1988). The estuary experiences
large fluctuations in freshwater inflow volume, frequency
of inflow events, and timing of discharges. Unfortunately,
these physical and hydrologic alterations have adversely
affected the ecosystem and economy of the region
(Chamberlin and Doering 1988). For example, historical
accounts report that oysters were very prominent in the
lower regions of the estuary (Shell Point), and early set-
tlers had "difficulty surveying channels through the numer-
ous oyster bars that obstructed the lower portion of the
river" (Sackett 1888). The oyster bars were severely re-
duced by the changes in freshwater inflow, hydrodynamics
and shell mining (Chamberlin and Doering 1988). Chang-
es in the quality and quantity of freshwater flow were held
responsible for the collapse of the bay scallop (Argopecten
irradians) fishery in the 1960's (Chamberlin and Doering
' 388) and for significant declines in seagrasses in the deep-
: areas of the estuary (Harris et al. 1983).
lysical habitat
~ abitats are altered by human activity and by natural pro-
cesses. Draining and filling marshes, physical disturbance
and eutrophication in SAV beds, and non-sustainable har-
vesting of oyster reefs are examples of impacts that can
be directly tied to human activities. Intense storms and geo-
logical processes are entirely natural forces that also can
alter habitats, sometimes drastically. Coastal habitats can
be influenced by the interactions of both anthropogenic and
natural factors. Patterns of sedimentation and erosion—
natural processes in estuaries—can be disrupted by struc-
tural modifications such as bulkheads, seawalls, and rip-
rap. These shoreline structures focus wave energy and in-
crease the slope of the shore, possibly degrading or elimi-
nating habitat valued for SAV and blue crabs.
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
Increasing sea level is another example of how human
and physical forces can interact to alter the distribution and
quality of habitats for coastal living resources. Relative sea
level can increase as a result of land subsidence brought
on by excessive pumping of oil, gas and water from sub-
surface deposits. Natural geological processes also cause
subsidence in coastal lands; rivers transport the products
of highland weathering to the coastal zone where sediments
accumulate sufficiently to depress the Earth's crust. Ac-
cretion of autochthonous and allochthonous sediments in
marshes contributes to this process (Stevenson et al. 1986).
Degradation of soft bottom habitats by dredging, persis-
tent contaminants, and eutrophication are serious concerns,
especially when they occur simultaneously. Shoreline al-
terations such as bulkheads, seawalls, rip-rap, docks, boat
basins, and canals replace natural shallow-water and lit-
toral habitats with unnatural deepening and hardening of
the land-water interface that influence the dynamics of wa-
ter and sediment. The scale of discrete shoreline alter-
ations typically is small relative to the vast amount of Gulf
shoreline, but cumulative effects on coastal ecosystems
could be large.
The geographical extent of oyster reefs can be reduced,
and their ecological values degraded, by harvesting, silt-
ation, and altered freshwater discharges. These anthro-
pogenic impacts, especially in combination with the lethal
effects of parasitic oyster diseases, can deplete oyster
populations and diminish their economic and ecological
value. Often, harvested populations are maintained or re-
stored by deploying oyster shells to stimulate spat settle-
ment.
In addition to physical alterations and changes in fresh-
water discharges, the quality of habitats can be altered by
variations in the constituents and properties of the envi-
ronment. Ambient temperature, water quality, sediment
quality, and biological communities all vary under the influ-
ences of natural and anthropogenic processes. Contami-
nation of habitats by toxic substances can affect the health,
survival, and recruitment of living resources in ways that
can be obvious or subtle, depending on the types and con-
centrations of contaminants. With increasing frequency,
coastal habitats are being invaded by foreign species that
can deplete populations of native species by predation,
competition, or pathogenesis.
39
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Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
X. CONSIDERATIONS FOR ALTERED HABITAT RESEARCH
The principal reason for EPA's interest in conducting re-
search into the effects of habitat alterations is to support
eventual development of regional criteria for habitat pres-
ervation and restoration. Habitat criteria will be most use-
ful and meaningful if they quantitatively relate abundance
or biomass of important species to changes in habitat quan-
tity and quality.
Whatever the causes and nature of habitat alterations-
natural, anthropogenic, physical, chemical, or biological-
populations of desirable species will be depleted if the al-
terations shift the balance between reproductive capac-
ity, growth, and recruitment with natural mortality. To quan-
tify the effects of habitat alterations on populations will re-
quire estimating these vital rates in the presence and ab-
sence of important habitat types and attributes, and apply-
ing these differential rates in simulations of the popula-
tions over time.
Since we have chosen to focus on economically important
species, one way to define populations would be in terms
of harvestable stocks. For the purposes of fisheries sci-
ence and management, a stock is the harvestable propor-
tion of a population that exists within a defined geographic
area. Usually, this area is defined biologically, by the limits
of a reproductively unified population. Stocks sometimes
are defined more arbitrarily, by state boundaries or other
management units. In the northern Gulf of Mexico, shrimp
(3 species), blue crabs, and spotted sea trout, all migra-
tory during some phase of their life cycles, could be con-
sidered as single stocks ranging from western Florida to
south Texas. Populations also could be defined more lo-
cally, depending upon the scale of analysis. For both bio-
logical and practical reasons, oyster stocks or populations
might be defined most conveniently at the scale of indi-
vidual estuaries, or perhaps by state boundaries, even
though such distinctions might not be reflected in genetic
differences.
40
-------
State agencies and the National Marine Fisheries Service
maintain long-term data on total catches and fishing effort
for harvested marine and estuarine species. Although these
data have many weaknesses, they can be used in combi-
nation with other information to estimate absolute abun-
dance, biomass, and fishing mortality rates. States also
conduct fishery-independent surveys to estimate popula-
tion parameters such as spawning stock biomass, fecun-
dity, recruitment, size distributions, sex ratios, and natural
mortality. These data can be used in the development of
population models, which generally require (at least) esti-
mates of total stock abundance or biomass at one or more
points in time, along with average or time-variable rates of
recruitment and mortality. Conceptually, if a population
model is available or can be constructed, population pa-
rameters can be adjusted to predict the effects of altered
habitats. For example, recruitment of young animals to a
harvestable stock requires that they survive and grow to a
minimum legal orcatchable size. If differential probabilities
of growth and survival, or their net effects on recruitment,
can be estimated for specific habitat types, then the effects
of habitat on the stock or population can be simulated. The
simulated population need not be the harvestable stock-
other life stages, age classes, or sizes could be simulated.
Large-scale population models based on fisheries data, no
matter how well-constructed, are likely to have large un-
certainties for predictions. There are other possible ways
to specify species-habitat stressor-response models. Field
observations of differential survival, growth, and recruitment
of critical early life stages could be used to form hypoth-
eses about species-habitat relationships. These hypotheses
could then be tested experimentally, in mesocosms or in
field manipulations. The observed relationships could then
be extrapolated to appropriate regional scales. Although
the concept is simple, estimating habitat dependencies will
be a difficult challenge. Habitat dependence is species-
specific and operates at small spatial scales within a back-
ground of great variability in physical, chemical, and bio-
logical properties of the environment. It is encouraging to
note that in studying habitat selection by blue crabs, Meise
and Stehlik (2003) suggested that habitat factors that ap-
peared to be significant at fine temporal scales might not
be significant at larger scales. The same principle may ap-
ply to spatial scales, suggesting that cumulative effects of
habitat alterations over large spans of space and time are
the major concern. This idea is consistent with our large-
scale view of Gulf of Mexico fish and shellfish populations.
Habitat dependencies typically have been inferred from dif-
ferential densities of organisms in, for example, vegetated
and unvegetated habitats, but these observations may re-
flect preference rather than dependence. Some studies
have correlated gross productivity (fishery yields) of impor-
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
tant estuarine species with shallow-water vegetated habi-
tats and freshwater discharges over large spatial scales
(e.g., Boesch and Turner 1984); such analyses can gener-
ate predictive hypotheses about species-habitat relation-
ships. Nevertheless, estimating vital rates such as survival
and growth within specific habitat types, and applying them
at population and regional scales, will provide the stron-
gest support for criteria development.
This document identifies five features of coastal habitats
for special attention: (1) vegetated intertidal wetlands, (2)
shallow subtidal vegetated bottoms, (3) shallow subtidal
unvegetated bottoms with soft sediments, (4) oyster reefs,
and (5) areas of freshwater discharge into coastal waters.
These features of coastal habitats are not entirely inde-
pendent; in fact they could be considered jointly as a sea-
scape complex that supports high productivity of desirable
species. Beyond the simple presence or absence of par-
ticular habitat types, it may be necessary to consider the
values of other attributes. How much freshwater discharge
is enough, or too much? What densities and species of
aquatic plants are most favorable? Are the benefits of tidal
marshes best judged by their areal dimensions or by the
linear extent of marsh-water boundaries (Minello and Rozas
2002)?
Fortunately, some data and research information are avail-
able for the northern Gulf of Mexico that relate distribution,
growth, and survival of selected fish and invertebrates to
various types of physical habitats. Similar bodies of data
are available for the life histories and population attributes
of the species we have chosen for study. It is likely that,
with low confidence, stressor-response models of the ef-
fects of habitat quantity and condition on populations of a
41
-------
Fishery Resources and Threatened
Coastal Habitats in the Gulf of Mexico
few economically important species can be generated from
existing data; the major effort would be to acquire and man-
age these dispersed and disparate data. Although such
coarse models could be informative, additional field and
possibly experimental studies will be required to fill data
gaps, improve confidence and verify the models. For ex-
ample, there is a large quantity of data on nekton densi-
ties and vital rates in marsh, seagrass, and unvegetated
habitats of the northwestern Gulf of Mexico, but similar
data are sparse for the eastern Gulf. Data for abundance,
mortality, and recruitment of oyster populations appear to
be scarce or nonexistent for much of the Gulf.
To achieve the goals of altered habitat research will re-
quire a coordinated program of field studies, data acquisit-
ion, and modeling, possibly supplemented by meso-scale
or field-scale experimental studies (Figure 12).
Besides the fishery-based population data discussed above,
habitat-specific data such as densities (relative abundance)
of target organisms, size and age distributions, and per-
haps secondary population characteristics such as sex ra-
tios and health indicators, will be needed. Measures of
physical habitat should include areal, linear, and vertical
dimensions of key habitat types and alterations, along with
data for freshwater discharges to estuaries. Indicators of
habitat quality should include species and densities of
aquatic plant and oyster reef communities, plus basic en-
vironmental data such as salinity, water temperature, and
dissolved oxygen.
POPULATION
DATA
HABITAT
DATA
DATA
ACQUISITION
HABITAT -
POPULATION
MODELS
MODEL
VERIFICATION
EXPERIMENTAL
STUDIES
^ I
FIELD
STUDIES
Figure 12. Conceptual diagram of the elements of altered habitat research
and their relationships
42
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