65
Oyster Population Estimation in Support of the Ten-Year Goal for Oyster
Restoration in the Chesapeake Bay: Developing Strategies for Restoring and
Managing the Eastern Oyster.
Roger Mann7, Steve Jordan13, Gary Smith2, Kennedy Paynter4, James Wesson5,
Mary Christman4, Jessica Vaniska2, Juliana Harding', Kelly Greenhawk2, Melissa Southworth1
1 Virginia Institute of Marine Sciences,
2MD Department of Natural Resources,
3current address, US-Environmental Protection Agency,
4 University of Maryland,
5Virginia Marine Resources Commission
ABSTRACT
The dramatic decline of the eastern oyster, Crassostren virginica (Gmelin, 1791), population in the
Chesapeake Bay during the last century has impacted the ecosystem and economy of the region. A collaborative
project was initiated between researchers in Maryland and Virginia with the goal of quantifying the baseline
oyster population, and establishing the monitoring, data management, and data analysis frameworks for
measuring progress toward the Chesapeake 2000 oyster restoration goal of a ten-year, ten-fold increase. Using
fishery independent dredge surveys in Maryland, oyster abundance from 1994 to 2002 ranged from 1.60 to 5.55
x 10B oysters, with a mean of 4.14 x 10" oysters and with a current standing stock of approximately 279,245
bushels. Intensive patent tong surveys in the James River, Virginia between 1993 and 2000 indicate a fairly
stable population of 4.41 and 6.30 x 10" oysters with a current standing stock of approximately 365,000 bushels.
The grand total estimates for the Virginia productive bottoms using both patent tong and dredge data vary
from 5.31 x 10' to 6.00 x 10" oysters.
INTRODUCTION
The dramatic decline of the eastern oyster,
Cmssostrea virginica (Gmelin, 1791), population in the
Chesapeake Bay during the last century has impacted
the ecosystem and economy of the region. This decline
has reduced the filtration capacity of the oyster
population and has caused a shift from a benthic to a
pelagic-dominated ecosystem (Baird and Ulanowicz,
1989). Accordingly, the Chesapeake 2000 Agreement
established a 10-fold increase in the biomass of the Bay
oyster population by 2010 as one of the principal goals
of the Bay restoration effort. A collaborative project
was initiated between researchers in Maryland and
Virginia with the goal of quantifying the baseline oyster
population, and establishing the monitoring, data
management and data analysis frameworks for
measuring progress toward the oyster restoration goal.
Two estimators of the size of the oyster population in
the Bay were developed as goals, these being the
absolute number of oysters, and their biomass. The
absolute number is valuable as the index of population
size, but it can be misleading in that it varies
enormously and does not differentiate between the
ecological contribution of a large versus small oyster.
In assessing ecological function, oyster biomass is a
more useful barometer of population size. Biomass
estimates focus on one year and older oysters. Young-
of-the-year oysters, or spat, are exposed to high levels
of predation. Both their biomass and absolute
abundance experience large fluctuations both annually
and inter-annually.
Both Maryland and Virginia support oyster
fisheries, however the Maryland fishery is almost
exclusively a public access fishery while in Virginia
there is an historical mix of a direct public access fishery
and a private fishery based on leases of bay bottom.
Unlike the public fishery in Virginia, the leased private
grounds can be planted with spat and harvested
throughout the year once they reach the legal
marketable size. Landings in Virginia illustrate a long-
term decline and are at a level <1% of the historical
high of the 1880's (Fig.l). Landings in Maryland have
also declined rapidly from historic levels, then
stabilized in the early 1990's, but are again in a state of
decline (Fig.l). In both states the cumulative impacts
of the diseases, MSX (Haplsporidium nelsoni) and Dermo
(Perkinsus mnrinus) on harvest have been and continue
to be substantial. Fishery dependent data contributes
to population estimation in Maryland, however, the
collapse of the fishery severely limits the use of such
data in Virginia.
S9B»l,Ste^uJonnf'\Gar* Smith> Kemedy Paynter, James Wesson, Mary Christman, Jessica Vanisko, Juliana Harding, Kelly Greenhawk and
• Oyster Population Estimation in Support of the Ten-Year Goal for Oyster Restoration in the Chesapeake Bay: Developing
?nnq L,,„°r t	^rmging the Eastern Oyster, In: Chesapeake Bay Fisheries Research Program: Research Project Symposium, March 24,
03, Patuxent Wildlife Refuge Visitor's Center, Laurel, Maryland. NOAA Cheseapeake Bay Office, Annapolis, MD. Pp. 65-69. {ERL.GB X1056).

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66
Oyster Assessment
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Figure 1, Oyster fishery production from 1985 - 2001.
OBJECTIVES
The past two years have been largely
dedicated to developing the framework with which to
evaluate the ten-year restoration goal, These objectives
included: (1) Examining the extant datasets in MD and
VA for their utility in estimating the Chesapeake Bay
oyster populations. (2) Developing a compatible, geo-
referenced, shared archive of MD and VA data;
developing the protocols to provide consistent data
acquisition in the future, (3) Identifying sentinel
populations to act as proxies for changes in the oyster
population. (4) Offering review of emerging restoration
efforts. The current project builds off the data produced
by MD and VA over the previous two years. The data
will be used improve our understanding of oyster
population parameters such as growth, recruitment,
and natural and fishing mortality, establish biological
reference points, and develop the tools with which to
evaluate the efficacy of various management options.
These continuing objectives are: (1) To quantify the
efficacy of various management options for restoring
oyster populations. (2) To develop management
recommendations based on the most biologically
effective combination of options. (3) To develop concise
recommendation for managing commercial oyster
fisheries consistent with the restoration goals set
forward.
METHODS
In Maryland, the Department of Natural Resources
conducts an annual modified fall dredge survey at 43
sites that have been used to track changes in the oyster
population as well as disease intensity and prevalence
from 1990 to present. These were selected because they
were represents live of the diverse types of oyster bars,
Tracking towards the ten-year goal is focused on
designated sentinel sites in each state. Sentinel sites
are a subset of survey data for which historical data
typically exists and lor which long term monitoring
will continue as part of the 2010 goal evaluation. These
sites include both historically well sampled locations
and more recent sanctuary and restoration sites. The
Maryland dredge survey 43 core sites have been
supplemented with four additional sites that are closed
to the fishery, and comprise Maryland's sentinel sites.
The distance the dredge is towed as well as the total
volume of material sampled by the oyster dredge has
been measured since 2001. In Spring and Summer 2002,
field studies were conducted to determine dredge
sampling efficiency within various oyster habitat types.
This data, plus additional data to be collected in
Spring/Summer 2003, will serve to develop a suite of
oyster dredge efficiencies that can be used to estimate
the oyster population. To estimate the oyster
population prior to 2001, a conversion factor was
developed from the dredge track measurements and
volumetric samples taken in 2002. In Virginia a
combination of long term dredge data covering
extensive areas has been combined with mare focused
patent tong surveys using stratified random sampling
designs. The patent tong surveys provide a continuous
data series for the James River for the period 1994-
present. Slightly shorter time frames are covered for

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Oyster Assessment
67
the Great Wicomico and Piankatank Rivers, and more
recently extended surveys have been added in the
Rappahannock River, as part of the Oyster Heritage
Program, and other sites as restoration activities
continue. Long term data from the James using both
dredge and long data were examined to develop a
conversion function for application in estimating
absolute abundance from dredge data. This conversion
is the focus of continuing work. Virginia has identified
32 sentinel sites for long term monitoring of progress
towards the 2010 goal, these being a combination of
both old sites for which data exist, and new sites in
recent restoration areas.
Both Maryland and Virginia have reported
and archived their data for this project in the common
currency of oyster per area. Data has been found to be
highly compatible between the two states. In the
Maryland dredge survey, the oysters in a one bushel
sub-sample are counted and measured to the nearest
5 mm size class. Virginia's fishery independent data
is collected from both dredge and patent tong surveys
Virginia fishery independent Size frequency data for
oysters measured to the 5mm size class is available
from Virginia dredge and patent tong surveys. Size
frequency data is being used in a length-based analysis
of individual oyster growth. Oyster growth provides
an essential link between spat settlement and the
fishable stock. In order to evaluate potential oyster
management options, it is essential to understand
population dynamics and quantify parameter such as
growth, fishing and natural mortality, and recruitment.
Additionally, size frequency data will be used to
estimate natural mortality in Virginia, where harvest
is negligible and in Maryland within fishery closures.
These data can then be compared to box (articulated
oyster shells) counts that comprise present annual
mortality estimates in Maryland, but are believed to
be biased high due to dead oyster shells that fail to
disarticulate within a year. Refined oyster population
parameters will then be used within a framework to
examine the efficacy of various restoration and
management options.
Identification of oyster habitat within
Maryland and Virginia remains an important aspect
of this project. The incorrect estimate of habitat can
result in a gross over or under-estimation of oyster
population size within the Bay. The continued
incorporation of refined oyster habitat data through
acoustic surveys will serve to refine these population
estimates as this project continues.
RESULTS
Mean oyster density from fishery independent
MD dredge surveys is estimated to range from 1.07 to
2.49 oysters per m2in 2002, as estimated using the total
population size extrapolated over an estimated area
of high and low quality habitat available in the Bay. It
should be noted that due to the highly gregarious
nature of the oyster, some areas have much higher
oyster density and there are large areas that contain
low densities or no oysters. In terms of biomass, it is
reasonable to assume that the dry tissue weight of an
average oyster is 1 g, so the total biomass would be
between 1,6 x 105 and 5.5 x 10s kg. These estimates
include all small (<76 mm) and market (>76 mm)
oysters, but do not include spat. Estimates of the
standing stock of market oysters in 2002 were 279, 245
bushels for Maryland. Using fishery independent and
dependent surveys, from 1994 to 2002 range from 0.160
to 0.555 x 10s oysters, with a mean of 0.414 x 10'' oysters
in Maryland. Intensive patent tong surveys in the
James River, Virginia between 1993 and 2000 indicate
a fairly stable population of 4.41 and 6.30 x 10soysters
with a current standing stock of approximately 365,000
bushels. Depending on location market oysters vary
from <1 % to 42% of the standing stock by number. By
comparing of dredge and patent tong data for the
James River for the 1993-2000 period conversion
functions have been calculated that allow extrapolation
of dredge data for larger spatial areas in other rivers
and regions in the Virginia portion of the Bay. The
grand total estimates for the Virginia productive
bottoms by this method vary from 5.31 x 10,J to 6.00 x
1051 oysters and are very sensitive to the conversion
function used. Presently productive oyster grounds in
Virginia are well below those surveyed by Baylor. The
current calculations use only the modest values for
high and moderate potential bottom area, as described
by Iiaven et al. (1981); together they constitute only
24.85% of the total Baylor area within the Virginia
portion of the Bay. Sub-market (<76mm) size oysters
constitute a mean of 79% of the total (spat not included)
for the period 1993-2001 in Virginia populations, a
much higher percentage than in Maryland.
Consequently biomass estimates for oysters in
Virginia, which vary from 2.16 x 106 and 2,45 x 10s kg,
are comparable to those for Maryland even though
Virginia has an estimated higher population.
There is concern for stocks in both states.
Young of the year recruitment, commonly termed
spatfall, is assessed in both states by fall dredge
surveys. Index values represent cumulative
recruitment to the benthos and post metamorphic
survival to approximately 25mm maximum
dimension. In addition, Virginia employs shell string
surveys wherein oyster shells, drilled, and strung on
wires, are deployed for weekly intervals throughout
the summer and early fall months at fixed stations in
the subestuaries of the Virginia portion of the Bay that
historically demonstrate regular recruitment. Oyster
larvae settle and metamorphose onto the shells, and
weekly replacement of the shellstrings allows
description of the quantities settling within specific

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68
Ovster Assessment
time periods in addition to cumulative estimates for
the entire year. Unlike the spat index value shell string
values indicate potential for benthic recruitment but
do not include a post metamorphie mortality
component. The Maryland Spat Index follows
recruitment at a series of 43 sentinel stations for a
period of over 60 years. Recent data illustrate the high
inter annual variability in recruitment (Fig. 2). The
majority of years since 1986,show values in excess of
the long-term median. Indeed six years are above the
75% quartile; however, the past three years are
generally between the 25% quartile and median. The
comparable Virginia Spat index is given for the past
16 years for 5 stations in the James River and shows
slightly higher median and quartile values than the
Maryland long term record. Indices within the past
decade have generally remained between the 25% and
75% quartile. Both Maryland and Virginia enjoyed high
spat indices in 1991, but the 1997 high value in
Maryland was not reflected in Virginia where the
lowest index in the 16-year record was recorded.
The diseases, MSX (Haplospridium nelsoni) and
Dermo (Perkinsus mnrinus) remain as threats to oyster
populations in both states, and a substantial portion
of natural mortality is directly attributable to one or
both diseases. VIMS has maintained a disease
monitoring program for MSX since 1960. A general
trend of increasing maximum prevalence is observed
over the 40 year period with intervals of decreased
activity in the early 1970s, 1980s, and 1990s. Recent
years have been characterized by very high maximal
values, so much so that 2001 monitoring was
terminated in September with 91% cumulative
mortality of the test population. Maryland, which
generally observes lower salinity than Virginia across
its 43 sentinel sites, has recorded considerably lower
MSX prevalence since 1991. This has, however, notably
increased above the average in the 2000-2002 period.
The cumulative impacts of disease on long term
population trends are evident in recent analysis of stock
assessment in the Maryland region of the Bay. In the
low salinity (<12 ppt) zone populations are moderate
and stable in the face of limited disease pressure (MSX
is rare and Perkinsus only in low intensity). In the mid
salinity zone (12-14 ppt) a declining population from
19S5 through 1994 was followed by a modest recovery
in 1995-1999. By contrast no recovery has been
observed in the high salinity zone (>14 ppt) where MSX
in enzootic and Perkinsus causes consistently high
mortality. The Virginia oyster resource is all
predominantly in this region.
Current data describe only limited
demographics of the populations and do not include
broad surveys of oyster condition index that describe
meat content for standard size oysters as a proxy for
physiological condition. With the definition of sentinel
sites for long term monitoring towards the 2010 goal,
there is increasing need for better description of oyster
physiological condition and condition index data.
Current demographics allow estimation of size and
possibly age-dependent natural mortality (M). In stable
populations, knowledge of M can allow prediction of
year strength recruitment in older year classes with
the option to effect area management wherein sub-
market size class strength, after debiting for M, can
facilitate prediction of the harvestable stock (F, fishery
mortality) for the subsequent year. However, extensive
analysis of historical survey data for sub-market and
market oysters, in concert with fishery landings,
suggest that sub-market abundance is not a statistically
robust predictor of fishery landings in subsequent
years. In Virginia, consistent heavy disease pressure
has effectively reduced F to near zero values, while M
is both spatially and temporally variable depending
on salinity and temperature as promoters of disease
pressure. The high proportion of sub market sizes in
extant populations illustrates the potentially high size
and age specific mortality in oysters approaching
market size and underscores the limited value of sub-
market sizes as a fishery predictor in Virginia
500
	7S'3i Quartile (historic)
	Median (historic)
...... 15% Quartile (historic)
•= 4n"
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1986 1988 1990 1992 1994 1996 1998 2000 2002
1986 1988 1990 1992 1994 19% 1998 2000 2002
Year	Year
Figure 2. Spat indices based on annual dredge data for Maryland (A) and Virginia (B)

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Oyster Assessment
69
populations. The only stable populations in Virginia,
that is populations with minimal disease mortality, are
in the low salinity sanctuaries of the upper James River.
Estimates for mean annual M values for 1, 2, 3 and 4
year olds are 0.16,0.19,0.55 and 0.14 respectively based
on transformation of size demography to age
demography using unpublished growth data and
annual survey data. Estimates can also be made based
on ratios of recent mortality, with empty valves or
"boxes" as a proxy, to live oysters. Such estimates are
slightly problematic because the longevity of intact
boxes, as mentioned earlier, is unknown and probably
less than one year. This could result in underestimation
of M. Robust estimates of M by size class, age class,
area and time frame are critical to resource
management and are worthy of further study. Current
survey records provide a considerable database for the
initial stages of this investigation, although
supplemental field studies on "box" degradation rates
are warranted. Historical data provide a rich
environment for examination of underlying population
dynamics, although limitations in data collection
protocols must be considered.
Management and research recommendations
emerging from the project data
Future management would benefit from
definition of biological reference points, including
over-fishing. A broader goal should include
developing a Total Allowable Catch (TAC) for
Maryland alone, Maryland and Virginia together as
part of a bi-state fishery, Virginia alone, and defined
management zones based on salinity as a proxy for
disease pressure. Once the above have been completed,
evaluation of alternative reference points might
provide new options for fishery management. Finally,
recognizing the unique value of shell as a habitat
limiting resource for oysters, a cost - benefit analysis
of various methods of use of the shell resource is
suggested. The suggested analysis will provide a
framework for evaluation of introduction of oysters
with lowered disease susceptibility on the genetic
composition of the population in the Bay, with the long
impact on estimation of M in the presence of continued
disease challenge. Long-term data provide the basis
to develop population models, examine possible stock
recruit relationships, and examine the sensitivity of
management options to variation in M and F. Finally,
long term rebuilding of the resource requires
continuing knowledge of habitat condition. A
promising avenue to update this information on a bay
wide basis is to compare spatially referenced patent
tong demographic data against high-resolution
acoustic surveys in support of total population
estimates.
LITERATURE CITED
Baird, D. and K.E. Ulanowicz. 1989. The seasonal
dynamics of the Chesapeake Bay ecosystem.
Ecological Monographs 59:329-364
Calvo, L. and E. Burreson. 2002. Status of the major
oyster diseases in Virginia 2001. Annual monitoring
report. Virginia Institute of Marine Science, Gloucester
Point, Virginia, 23062. VMR 2002-1.
Haven, D. 5., J. P. Whitcomb and P. Kendall. 1981.The
present and potential productivity of the Baylor
Grounds in Virginia. Va. Inst. Mar. Sci., Spec. Rep.
Appl. Mar. Sci. Ocean. Eng. No. 243:1-154.

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