v>EPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97333
Research and Development EPA-600/D-83-094 September 1983
ENVIRONMENTAL
RESEARCH BRIEF
Habitat Structure and Fish
Communities of Warmwater Streams
James R. Karr, Paul L. Angermeier, and Isaac J. Schlosser*
Introduction
The basic goal of clean water legislation passed in the last
two decades is restoration and maintenance of the chemi-
cal, physical, and biological integrity of the nation's waters
The mechanisms required to reach this goal are not entirely
obvious Early efforts concentrated on physical and/or
chemical pollution; however, a broader perspective is
required.1'2 Based on the authors' work over the past
decade, five major sets of variables that impact the
structure of stream communities (Figure 1) have been
identified.
Most research on the role of physical habitat characteristics
in the regulation of fish community structure has concen-
trated on cold water systems, with emphasis on salmonids.
The significance of physical habitat in warmwater streams
has been largely ignored by water resource planners. Even
studies that describe physical habitat of streams rarely
examine the cause and effect interactions of habitat
structure, availability of food resources, and other factors
that shape fish communities.
With this background in mind, a research program was
initiated to evaluate the role of physical habit at in regulating
the structure of fish communities in warmwater streams in
'Co-authors Karr and Angermeier are with the Department of Ecology,
Ethology, and Evolution, University of Illinois. Champaign, Illinois; Co-
author Schlosser is with the Department of Biology, University of North
Dakola, Grand Forks, North Dakota.
Figure 1. Primary variables that affect the structural and
functional integrity of an aquatic biota.
east central Illinois. The study combined an empirical
approach involving observations of fish in relatively natural
conditions with studies of stream areas subjected to
extensive modifications by human society. Finally, several
experimental field and laboratory studies were designed to
clarify aspects of fish community dynamics. This research
brief is a summary of several journal articles published and
in review. Complete citations for those articles can be found
at the end of this brief.
Fish Communities Along Physical Habitat
Gradients—Natural Gradients
Two habitat gradients (upstream to downstream and riffle
to pool) were investigated in Jordan Creek in Vermilion
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County, Illinois.3 Several major patterns were identified—
from the diversity of the fish community to the apparent
dominant processes regulating community organization
along each of these gradients (Table 1}.
The major attributes of physical habitat measured in this
study were habrtal diversity and habitat volume. Habitat
diversity is a complex integration of depth, current velocity,
and substrate attributes. Habitat volume is measured as
stream area times mean depth for a study region, Both
habitat diversity and habitat volume increased from up-
stream to downstream and riffle to pool habitats. Temporal
variation in habitat diversity wasgreater in upstream areas
and habitat volume tended to vary more over time in
upstream and riffle areas. Seasonal and year-to-year
variation in rainfall also caused variation in habitats,
especially volume.
Benthic insect density in Jordan Creek was high from
autumn (Oct-Nov) through spring (May-June). Following
emergence of adults in late spring, invertebrate densities
were low in summer in areas with riparian vegetation.
Insect availability, as indicated by drift samples, increased
along a gradient from silt-sand to gravel-rock substrates.
Potential food availability for top carnivore fish peaked in
late summer and early fall with increased numbers and
biomass of young-of-the-yearfish.
Although habitat diversity was significantly (p <.05)
correlated with fish species diversity, the relationship
between the two variables varied as a result of seasonal
migration by fishes. These seasonal migrations were tied to
changing flow conditions, food availability, and the search
for suitable spawning and nursery areas. As a result, the
utility of the habitat/ fish diversity relationship as a
predictive model varied seasonally. In addition, the preci-
sion of the relationship was lowest in more variable
upstream and riffle habitats.
Younger fish (age classes O-ll) were found primarily in
shallow, temporally variable areas upstream and in riffles.
Relative growth rates were highest during summer but they
did not increase (relative to spring) as much as expected
from seasonal increases in water temperature. Centrar-
chids had substantially higher growth rates than cyprinids
during early life stages. Net production for age O-ll fish was
highest in upstream and riffle areas because those areas
supported high densities of young, generalized insecti-
vores. Net production of insectivore-piscivores was highest
in downstream and pool habitats. Stream reaches with
large, stable pools and raceways produced fewer fish due to
shifts in age structure toward fewer, large individuals (age
III+) with slower relative growth rates. Temporal variation in
reproductive success and survival of younger age groups
(O-l) was associated with variation in peak flows. Finally,
Table 1.
Summary of Relative Characteristics of Habitat Structure and Fish Organization Along Two Physical
Gradients in a Headwater Stream
Relative Position on Gradient
Characteristics
Downstream or Pool
Environment
Upstream or Riffle
Environment
1. Habitat Structure
2. Fish Community Structure
a. Species richness
b. Age structure
c. Size composition
d. Dominant trophic group(s)
3. Fish Community Function
a. Net production
b. Absolute and relative
growth rates of age O-l
of the dominant trophic
group(s)
4. Seasonal and annual stability
of community attributes; i.e.,
species richness, trophic
structure, age structure,
and production
5. Hypothesized dominant
processes regulating
community organization
Deep, temporally stable
High
Old fish
Large fish
Insectivore-piscivores
Benthic insectivores
Low
High
High
Shallow, temporally variable
Low
Young fish
Small fish
Generalized insectivores
High
Low
Low
Competitive exclusion
and predation
Recolonization dynamics,
effects of gradual
changes in the physical
environment on competitive
interactions, and temporal
variation in reproductive
success
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variation in peak flow was a major factor determining
spatial and temporal variation in production.
Therefore, spatial shifts in physical conditions from shallow,
temporally variable areas (upstream and riffles) to deeper,
more stable areas (downstream and pools) result in
consistent spatial changes in community structure (Table
1). The ultimate mechanisms responsible for these changes
were not precisely documented, although they clearly vary
from headwater to downstream or riffle to pool. We
hypothesize that in shallow, unstable habitats, recoloniza-
tion dynamics, the effect of gradual changes in physical
conditions on competitive interactions, and temporal
variation in reproductive success are more important than
competitive exclusion and predation as determinants of
community organization (Table 1).
Fish Communities Along Physical Habitat
Gradients—Human Disturbance
We assessed the impact of channel straightening and
removal of riparian vegetation on trophic structure, repro-
ductive success, and growth rates of fishes in relatively
natural (Jordan Creek - JC) and modified (Big Ditch - BD)
headwater streams.4 Shallow habitats and organic sub-
strates (diatoms and/or filamentous algae) were more
common in BD (not shaded by riparian vegetation) than JC
(shaded) during low flow periods in summer. Insect
densities in JC were highest in late spring, declining to low
levels by late summer. Insect densities in BD were high
throughout summer.
Fish species in JC were predominantly benthic insectivores
and insectivore-piscivores, and trophic structure, age
structure, and biomass of the fish community were stable
between years and seasons. Recruits made up a small and
stable portion of community biomass and were primarily
insectivore-piscivores and generalized insectivores. Young-
er age classes occupied shallow riffle habitats.
In contrast, trophic structure and recruits in BD were
predominantly generalized insectivores, omnivores, and
herbivore-detritivores. Omnivores and herbivore-detriti-
vores were primarily mid-river species (quillback and
gizzard shad). Considerable seasonal and annual variation
in trophic structure, total biomass, and age structure
occurred in BD associated with annual fluctuations in flow
regime, abundance of organic substrates, and reproductive
success of mid-river species. Younger age classes had
higher summer growth rates in BD than JC.
The effects of alteration of headwater streams are evident
when placed in the context of the stream continuum
concept which suggests that interactions between physical
environment and the organic energy base result in a
relatively predictable pattern of lotic community structure
and function from headwaters to downstream areas. The
critical effect of stream alterations in the context of this
concept is that alterations create a shallow, temporally
variable physical environment typical of headwater areas
where most recruitment occurs. Yet, at the same time, the
alterations shift the energy base toward autotrophic
processes which are more typical of mid-river habitats. As a
result, mid-river omnivores and herbivore-detritivores domi-
nate recruitment in modified headwaters. Reduced avail-
ability of benthic invertebrates and altered habitat condi-
tions result in declining abundance of insectivores and
carnivores due to lowered reproductive success. The
authors conclude that land use and channelization activities
in headwater streams have played a major role in the shift
in recent decades of many large river communities in the
midwestern United States from dominance by insectivore
and insectivore-piscivore fishes to omnivores and herbi-
vore-detritivores.8
Physical Habitat and Fish
Assemblages in Divided Streams
Two 35-m sections of Jordan Creek were divided in half
longitudinally with 6mm mesh hardware cloth supported by
steel posts.6 On one side of each section, all cover features
(logs, limbs) were removed from in or near the water. On the
other side, a continuous series of similar objects was
secured along the stream. In July and September, samples
of the biomass of fish were 4.8 to 9.4 times as high in the
areas with structurally complex habitats (Table 2). Further,
larger fish, and especially top predators, tended to select
the structured habitat. In this case we know that water
quality was the same in the structured and unstructured
sides of the stream, yet the numbers of fish are markedly
different. These improved habitat conditions seem to
provide two things: habitat for small fish including a
diversity of substrates for food organisms and hiding places
Table 2.
Fish and Invertebrate Densities and Fish Biomass in Adjacent Sides of a Stream Split with 6mm Mesh
Hardware Cloth
July 1979
No Cover Cover
September 1979
No Cover Cover
Fish
Number of individuals
Number >120mm TL
Total biomass (gms)
Benthos
Number/0.1 m2
34
2
170
92
46
28
1606
383
4
2
284
39
73
17
1366
219
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(cover) from which large fish can prey on smaller species
This emphasizes the importance of habitat structure as a
determinant of biotic conditions in a stream.
Woody Debris in Warmwater Streams
The importance of woody debris to the structure and
function of a warmwater stream ecosystem was examined
by removing woody debris from a series of stream reaches
Experimental reaches were compared with unaltered
reaches over a 2-year period.7
At initiation of the experiments (June 1980), altered and
unaltered sites were similar with respect to depth profiles,
current regimes, and standing stock of organic litter (Figure
2). Water depth declined in all sites through the summer
and autumn due to lack of precipitation, but shifts toward
shallow depths were especially pronounced in altered sites
due to the filling of pools by unstable substrates (Figure 2)
In addition, the abundance of organic litter on the stream
bottom declined markedly in altered sites but remained
relatively constant in unaltered sites.6 Litter abundance
declined in altered sites due to burial by shifting substrate
and the absence of retention structures. In June 1981,
depth profiles, current regimes, and organic litter abun-
dances for unaltered sites were similar to those in June
1980. In altered sites, however, deep areas were not re-
established, currents were faster, and organic debris was
only one-third as abundant as the previous year (Figure 2).
Seasonal shifts in depth, current and litter abundance in
1 981 were similar to those observed in 1 980, though less
pronounced due to the uncommonly stable flows that
occurred through the summer of 1981. In reaches of Jordan
Creek with stable (rocky) substrates, removal of woody
debris had less impact on stream structure and function
than in reaches with unstable (silt-sand) substrates.
Fish were also monitored throughout the experimental
period to evaluate effects of habitat changes on the fish
community.7 In June 1 980, mean fish biomass was similar
(p >05) between groups of altered and unaltered sites
(Figure 3) but by October 1 981 mean biomass in unaltered
sites (1 833gm/35m) was significantly (p <.05) greater than
that found in altered sites. Declines in fish abundance
June 1980
October 1980
June 1981
1
Altered
Sites
Unaltered
S/tes
10- 20- 30 +
Depth cm
Litter
10- 20-
Depth cm
Litter
jo- 20- 30
Depth cm
L/tier
Figure 2. Depth frequency distribution and litter abundance in ,
Debris was removed in July 1980
naltered and altered (woody debris removed) reaches of Jordan Creek.
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Unaltered
Sites
Altered Sites
Artificial Cover
n = 3
No Cover
n= 6
3.5-1
-H 3.0 H
E
.0
3S g), their abundance being lower in altered
than unaltered sites. Distributions of small fish (<40 mm
total length) among sites were variable. Small cyprinids
tended to be most numerous in altered sites, while small
centrachids tended to be most numerous in unaltered sites.
These experiments illustrate the interactions of physical
and biological processes that occur in stream ecosystems.
Woody debris has several important functions in Jordan
Creek. It affects channel hydraulics and so maintains depth,
current, and substrate diversity, ft acts as a stable substrate
for retaining organic material and supporting macroinver-
tebrates. These functions, in addition to providing cover, are
essential to maintaining habitat quality for fish. In streams
with unstable substrates (silt, sand), woody debris may be
the most important attribute of physical habitat in determin-
ing ecosystem structure and function.
Effects of Fish Consumption
On Food Availability
Paired screen exclosures were erected to assess the impact
offish consumption on benthic invertebrate abundance and
size distribution.8 One exclosure (closed) excluded fish from
a section (0.54 m2) of stream bottom, while the other (open)
permitted fish to enter and exit freely. Exclosures were set
up for 4-week periods in upstream (silt-sand substrates)
and downstream (gravel-pebble substrates) reaches of
Jordan Creek. Closed exclosures from upstream sites
supported greater densities of invertebrates than open
enclosures (Table 3). These density differences were
largely due to the greater abundance of chironomid larvae
and copepods, both of which are important food items for
Jordan Creek fish. In addition, large invertebrates (at least
4.7mm long), which are geperally preferred by fish, were
more abundant in closed exclosures than open ones. Data
from downstream exclosures indicated that fish consump-
tion had no effect on invertebrate abundance or size
frequency. These results suggest that the potential for
competition for food among stream fishes is greatest in
upstream areas. If so, stream modifications such as
removal of riparian vegetation or woody debris, which may
have dramatic effects on invertebrate availability, can be
expected to have greatest impact on upstream fish popula-
tions. Competition for food among fish in upstream areas
may be a common phenomenon, particularly during sum-
mer. Thus, disturbances that further destabilize food
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resources may ultimately contribute to instability in fish
community structure.
Table 3.
Pair
Invertebrate Densities (No./0.0116m2) from
Paired Exclosures in Jordan Creek. Open
Exclosures were Accessible to Fish, While
Closed Ones were Not
Month
Open
1
2
3
4
5
6
June
June
June
September
September
September
72
11
11
77
51
140
108
93
34
84
191
210
Effects of Cover and
Current on Predation Rate
A series of experiments were conducted in a laboratory
stream to examine the influence of habitat variables on
predator-prey interactions among fishes.9 Several hypoth-
eses were tested: 1) fish seek cover to avoid predators; 2}
presence of cover decreases predation rates; 3) fish seek
cover to avoid current. Experiments included various
combinations (presence or absence) of prey, predators,
cover, and current.
Small fish (4 species) were attracted to cover (plastic plants)
in the absence of current, but not in the presence of current
(4-15 cm/s). In addition, small fish were more strongly
attracted to cover when predators (large fish) were absent
than when predators were present. The presence of
predators also inhibited activity (swimming) of small fish.
Small centrarchids exhibited stronger associations with
cover than did small cyprinids.
Presence of cover did not affect average predation rates
suffered by small fish in these experiments, though rates
were more variable in the presence of cover. However,
centrachids suffered higher mortalities in experiments
without current than in those with current.
In conclusion, these experiments indicate that small fish
may effectively alter their habitat use so as to avoid
predators. Furthermore, predation on fish by other fish may
be more important to community organization in non-
turbulent (i.e. lentic) environments than in turbulent (i.e.
lotic) ones.
Conclusion
Overall, the study results clearly show that factors limiting
biotic integrity in warmwater stream ecosystems are not
restricted to water quality. Indeed, physical habitat is a
major determinant of biotic integrity. The role of physical
habitat conditions includes direct effects on fish abun-
dances as well as indirect effects resulting from complex
interactions with channel hydraulics, availability of food
(both primary and secondary production), and susceptibility
to predators. The interactions of these and other variables
are exceedingly complex and require that water resource
planners consider factors in addition to water quality
(physical and chemical attributes) in efforts to restore and
maintain biotic integrity.
Closed Literature Cited
1. Karr, J. R. and P. R. Dudley. 1981. Ecological perspec-
tive on water quality goals. Environmental Manage-
ment. 5:55-68.
2, Karr, J. R. 1 981. Assessment of biotic integrity using
fish communities. Fisheries. 6:21-27.
3. Schlosser, I. J. 1982. Fish community structure and
function along two habitat gradients in a headwater
stream. Ecological Monographs. 52:395-414.
4. Schlosser, I. J. 1982. Trophic structure, reproductive
success, and growth rate of fishes in a natural and
modified headwater stream. Canadian Journal of
Fisheries and Aquatic Sciences. 39:968-978.
5. Karr, J. R., L. A. Toth, andG. D. Carman. 1983, Habitat
preservation for midwest stream fishes: principles and
guidelines. U.S. Environmental Protection Agency,
Corvallis, OR. EPA-600/3-83-006. 120 pp.
6. Angermeier, P. L. and J. R. Karr. 1983. Functions of
woody debris in a small warmwater stream. Trans. Am.
Fish. Soc. (in review).
7. Angermeier, P. L. 1983. Effects of depth and cover on
fish distributions in selected Illinois streams. Ecoi.
Monogr. (in review).
8. Angermeier, P. L. 1983. Effectsof fish consumption on
the abundance of benthic invertebrates in an Illinois
stream. Chapter in Ph.D. Dissertation, University of
Illinois at Urbana-Champaign.
9. Angermeier, P. L. 1983. Effects of current and preda-
tors on cover use by fish in a laboratory stream. Chapter
in Ph.D. Dissertation, University of Illinois at Urbana-
Champaign.
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