Selected abstracts from published papers and
conference presentations
Use of line transect methods to estimate abundance of benthic stream fishes
William E. Ensign, Paul L. Angermeier, and C. Andrew Dolloff
Abstract: Visual estimation of stream salmonid abundance using strip transect sighting
models has become commonplace. Application of visual estimation to other stream fishes,
particularly benthic forms, has been limited. Examination of the distribution of sighting
distances for the Roanoke darter (Percina roanoka), Roanoke logperch (P. rex), and black
jumprock (Moxostoma cervinum) indicates that strip transect sighting models that assume
probability of sighting remaining constant out to the limits of observer visibility are not
appropriate for these benthic species. Our datasets indicate that distance sampling models
that assume decreased sighting probability with increasing distance of the target from the
observer provide a reasonable alternative to strip transect sighting models. There was a
strong positive correlation between abundance estimates calculated using two alternative
distance sampling models, as well as between the distance sampling model estimates and an
estimate of abundance obtained with a backpack electroshocker.
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Factors influencing stream fish recovery following a large-scale disturbance
William E. Ensign, Kevin N. Leftwich, Paul L. Angermeier and C. Andrew Dolloff
Abstract.-We examined fish distribution and abundance in erosional habitat units in South
Fork Roanoke River, Virginia, following a fish kill by using a reachwide sampling approach for
3 species and a representative-reach sampling approach for 10 species. Qualitative
(presence-absence) and quantitative (relative abundance) estimates of distribution and
abundance provided consistent measures of fish recovery for 2 of 3 species, at the
reachwide scale and 8 of 10 species at the representative-reach scale. Combining results
across scales and estimator types showed that distributions and abundances of 5 of 11
species in the reach affected by the kill were similar to those observed in unaffected
upstream and downstream reaches 8-11 months following the perturbation. Differences in
distribution and abundance between the affected reach and unaffected reaches indicate
that 4 of 11 species had not fully recovered during the same time period; results were
equivocal for 2 other species. We attribute differences in recovery rates between these
two groups to differences in parental investment in offspring. Species exhibiting rapid
recovery either engage in extensive spawning site preparation or guard the spawning site
following egg deposition and fertilization; species that had not recovered in the year
following the kill show limited spawning site preparation and do not guard the spawning site.
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Summer food limitation reduces brook and rainbow trout biomass in a southern
Appalachian stream
William E. Ensign, Richard J. Strange and Stephen E. Moore
Abstract: - We assessed salmonid production in summer in a second- and third-order
montane stream in eastern Tennessee in 1987. We sampled three stream sections, one
containing exclusively brook trout Salvelinus fontinalis, one containing exclusively rainbow
trout Oncorhynchus mykiss, and a third with a mixture of both species. Population
estimates for July and October 1987 revealed reductions in biomass over the 4-month
period that ranged from 31 to 49%, whereas population densities declined by 32-46% during
the same period. Total net production during the 4-month period was low and ranged from
0.38 to 0.45 g/ml. From June through September, the mean number of prey items per
stomach ranged from 4.2 to 29.9, whereas the mean relative weight of stomach contents
ranged from 0. 12 to 1. 34 mg dry weight per gram wet weight of fish flesh. There was a
strong positive correlation between the mean relative weight of stomach contents and the
mean condition factor for a given date. Calculated energy intake was below the level
necessary to provide energy for maintenance metabolism.
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Food resource competition in Southern Appalachian brook and rainbow trout
William E. Ensign, James W. Habera and Richard J. Strange
Food habits of sympatric and allopatric populations of adult brook trout (Salvelinus
fontinalis) and rainbow trout (Oncorhynchus mykiss) from 4 streams in the Great Smoky
Mountains National Park were compared to determine if competition for food resources
might explain the exclusion of the former by the latter in Southern Appalachian streams.
When diets of brook trout in sympatry with rainbow trout were compared with diets of
brook trout in allopatry, there was little significant change in prey composition. Based on
this information, exploitation competition for food resoources does not seem to play a role
in the invasion of rainbow trout into areas of Southern Appalachian streams occupied by
brook trout.
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The importance of proximal habitat units in explaining benthic riffle fish densities
William E. Ensign (Presenter), Paul L. Angermeier and C. Andrew Dolloff
In this study we determined the value of riffle area and proximal habitat units in explaining
variation in benthic riffle fish densities. Densities of Roanoke darter, Roanoke logperch,
and black jumprock were estimated in 45 riffle/run units in Roanoke River, VA using
underwater observation. We estimated surface area of target riffles and of pool and riffle
units beginning within 250 m of the upstream and downstream edge of the target unit. In
addition, measures of water depth, current velocity, and substrate coarseness were
obtained from the first pool and riffle immediately upstream and downstream of the target
unit. Significant univariate correlations between fish density in target riffles and at least
one measure of proximal habitat were obtained for all three species. Multiple regression
analysis incorporating those measures provided statistically significant models relating fish
density to adjacent habitat unit characteristics. Although there was no relationship
between riffle size and fish density, there were differences in the explanatory power of
adjacent unit variables among small, medium and large riffles for all three species. Our
results are consistent with a model linking habitat use patterns to the life history
attributes of the three species.
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Biases of sampling gear and spatial resolution on estimates of microhabitat selection by
stream fishes
William E. Ensign and Paul L. Angermeier
1. Understanding the habitats selected and avoided by stream fishes requires collection of
unbiased data on habitat use and availability. In this study, we assess the relative
importance of fright bias and bias associated with the resolution of data collection on
perceived microhabitats selected by three species of riffle-dwelling stream fish. 2. To
assess fright bias, we compared data on fish positions collected by electroshocking and
underwater observation within large (> 40m2) quadrats. When compared to underwater
observation, electroshocking samples indicated use of either shallower water, faster water
a or a combination of shallower and faster water for all three species, a pattern consistent
with cover-seeking behavior (i.e. fright bias). 3. To assess resolution bias, we compared fish
position data collected using underwater observation from point locations and from large
quadrats. Quadrat-based underwater observations failed to indicate utilization of velocity
refuges by two of the three species when compared to point-based samples. Conversely,
point-based samples did not adequately represent use of the higher-velocity mesohabitats
in which these velocity refuges were embedded. 4. Differences among data sets indicated
both fright bias and resolution bias occurred. While resolution bias can be addressed by
carefully matching the research question to the appropriate resolution for data collection,
fright bias associated with the electroshocking technique we employed is more difficult to
address. We recommend the use of underwater observation for collection of habitat
selection data whenever feasible.
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Effects of fright bias on sampling efficiency of stream fish assemblages.
William E. Ensign, Richard J. Neves and Alan J. Temple
Relative efficiencies of three different fish sampling techniques (pre-positioned area
shockers, single pass electroshocking, and a combined shocking-seining method) with
differing potential to induce fright bias were compared in riffle, run and pool habitats in
Roanoke River, Virginia. Twenty-three species were captured by the three techniques. Of
the three techniques, pre-positioned area shockers had the highest species accumulation
rate in each of the three habitats sampled. The single pass electroshocking method had
higher species accumulation rates than the combined technique in pools and runs, whereas
the performance of the two techniques was similar in riffles. Pre-positioned area shockers
also captured a higher proportion of larger, more mobile individuals than the other two
techniques. Based on our results, pre-positioned area shockers were the least biased of the
three techniques for sampling riverine fish.
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Diel habitat use by fishes in a Blue Ridge stream, Georgia
William E. Ensign and Edward E. Leonard
We determined diel habitat use by fishes in a Blue Ridge stream in Georgia using direct
underwater observation on two occasions in September 1999. During both day and night
sampling periods, individuals in the family Centrarchidae were most commonly found in
shallow (< 20 cm depth) and deep (>= 20 cm depth) areas with low water velocities (< 10 cm·s1 average velocity). During the day, representatives of Cyprinidae were most common in
deep areas with higher water velocities (> 10 cm·s-1 average velocity) but shifted to shallow
areas with low water velocities during the night. Differences in diel habitat use by the two
families are best explained by differences in predator avoidance strategies.
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Diet of banded sculpins from Bartow County streams
Calley J. Brewer and William E. Ensign
Biological monitoring is an important method for assessing the degree of environmental
degradation of aquatic systems. One part of biological monitoring is identifying indicator
taxa of fish due to their sensitivity to environmental stresses and toxins. A major trend
that can be identified with omnivorous fish is that since they are opportunistic feeders
they can survive on the resources available. In this study, I assessed the stomach contents
of 45 banded sculpin, Cottus carolinae, from excellent/good quality streams and fair/poor
quality streams. The majority of the stomach contents were aquatic invertebrates, while
some contained fish, crustaceans and terrestrial organisms. In streams of excellent/good
quality there was a greater diversity of organisms. Only in these good quality streams were
Coleoptera and Odonata found. The fair/poor streams showed less species diversity, more
occurrence of empty stomachs and a lower percent occurrence of the organisms that were
identified. The information from this study could lead to developing new methods for
monitoring biological communities.
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Fish Community Structure along an Ecoregion Boundary
William E. Ensign and Joseph M. Dirnberger
The use of biological community structure as an indicator of the health and integrity of
freshwater aquatic systems is now commonplace. Current applications (i.e. variations on
Karr’s IBI and the EPA’s Rapid Bioassessment Procedure) suggest that reference sites and
metric criteria developed within ecoregions or physiographic provinces provide the best
measure of anthropogenic impact. However, there has been limited analysis of watersheds
that span ecoregions or physiographies. In this study we present data on fish community
structure from 20 stream sites in the Etowah River watershed (Bartow County, Georgia)
draining the Ridge and Valley, Blue Ridge, and Piedmont ecoregions. Hierarchical cluster
analysis (average linkage method using Euclidean distances) of sites using the 20 most
common species and 20 most abundant species produced site classifications consistent with
ecoregion boundaries. In contrast, cluster analysis of sites based on reproductive guilds,
trophic guilds and a combination of reproductive and trophic guilds produced classifications
consistent with the degree of anthropogenic impact observed at each site. Our results
indicate that use of metrics or reference conditions specific to ecoregions for sites along
ecoregion boundaries could lead to erroneous assessments of the biological integrity of
boundary sites.
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Comparing abiotic and biotic parameters when assessing streams within a geologically
diverse area (Bartow County, GA)
Joseph M. Dirnberger, William Ensign, Heather Sutton, and Donald McGarey
During the summer of 2000, we sampled twenty-two stream sites in Bartow County
(Georgia) for fish, macro-invertebrates, water quality, habitat condition, and bacterial
indicators as part of a county-wide watershed assessment. Three major geological regions
occur within the county (to the west rocks are mainly limestone, to the north shales and
sandstones, and to the east and southeast harder metamorphic rocks). Because the county
is so geologically diverse yet all sites are within the same drainage basin, this data set
provides a rare opportunity to examine the influence of watershed geology within a
relatively small geographic area on parameters and metrics traditionally used to assess
anthropogenic impacts.
Principal component analysis (PCA) and other comparisons indicate that most biotic
and abiotic parameters over all sites are related to one another in ways expected among
sites that vary due to anthropogenic impacts. Multimetrics developed for fish and for
invertebrates reflected trends along the primary PCA 'water quality' factor (turbidity,
suspended solids, and BOD), whereas site geology did not reflect trends across this factor.
As expected dissolved ions (as well as alkalinity and pH) were strongly related to watershed
geology, but other less obvious water quality parameters such as nitrate were also
associated with geologic location.
Most individual metrics that are traditionally used to assess invertebrate and fish
communities did not appear to be influenced by geology, but rather by anthropogenic
habitat and water changes. In addition, differences in taxonomic composition of
invertebrate communities were associated with watershed geology. Any given invertebrate
community within the western limestone region was most similar (based on similarity indices
of species composition) to other communities within this same region. Communities within
the other two regions were not similar to western communities, but not distinctly different
from each other. Because water chemistry between streams of western limestone and
streams of northern shale-sandstone are more similar, geological influences on invertebrate
species distribution may reflect physical stream characteristics (such as streambed
morphology and substrate characteristics) rather than water chemistry.
While underlying geology strongly affected many water quality parameters, biotic
measures were relatively independent of these water quality effects, indicating biotic
parameters are reliable in assessing degradation in a geologically diverse basin.
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Fish Diversity in Bartow County, Georgia
William E. Ensign
As part of a countywide watershed assessment during Summer 2000, fish diversity and
abundance were quantitatively assessed at 22 sites in 14 separate watersheds in Bartow
County, Georgia. Watershed area above the sites ranged from 585 to 47,555 hectares. A
total of 54 species of fish from 14 families were obtained from the 22 sites.
Representatives of Cyprinidae, Centrarchidae and Percidae accounted for the majority of
diversity with 16, 12 and 8 species, respectively. Species richness at individual sites ranged
from 7 to 26. No species was found at all 22 sites, but Hypentelium etowanum and Lepomis
auritus were both found at 21 sites and Cottus carolinae, Percina nigrofasciata and
Campostoma oligolepis were found at 20 sites. Seven species were found at only one site
and six species occurred at only two sites. Regression of watershed area on species richness
explained 24% of the variance in the species richness data. Residual variance in the
regression relationship is accounted for, in part, by differences in habitat quality among
sites. Sites suffering from environmental perturbation typically had lower species richness,
given watershed area, than sites that were less perturbed.
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Investigating hydrologic alteration as a mechanism of fish assemblage shifts in
urbanizing streams
A.H. Roy, M.C. Freeman, B.J. Freeman, S.J. Wenger, W.E. Ensign, and J.L. Meyer
A primary mechanism of urban effects on stream ecosystems is via altered hydrology;
increases in impervious cover result in rapid delivery of stormwater to streams creating
flashy flows and reduced baseflows. Although it is presumed that these changes in flow
regimes can alter fish assemblages, there is little empirical evidence demonstrating
relations between fishes and hydrologic alteration. Further, we do not know what aspects
of the hydrograph are particularly important in driving fish assemblage structure and what
fishes are most affected by these hydrologic alterations in small streams. We measured
hydrologic patterns and sampled fish assemblages in 30 small streams (8--20 km2) in the
Etowah River basin (Georgia, USA) that were stratified by percent impervious cover (<10%,
10-20%, >20%) and the estimated degree of hydrologic alteration (based on synoptic
measurements of baseflow yield). Hydrologic variables were derived from stage gauges at
each study site for one year (January 2003--2004). Increased % impervious cover was
positively correlated with the frequency, magnitude, and duration of storm events above
50%, 75% and 100% mean stage of the 0.5-year recurrence interval flood during most
seasons. Other measures of flashiness such as the rate of the rising and falling limb of the
hydrograph were also positively related to impervious cover, except during high flow spring
months when soil saturation likely masked any land cover effects. Increased duration of low
flows associated with impervious cover only occurred during the autumn low flow period, and
this corresponded to significant increases in richness of lentic tolerant species. Altered
stormflows in summer and autumn were related to decreased richness of endemic,
cosmopolitan, and sensitive fish species, and decreased abundance of lentic tolerant species.
Species that were predicted to be sensitive to urbanization based on life history and other
attributes were also related to stormflow variables after accounting for variation
associated with % fines in riffles. Overall, hydrologic variables explained 20-66% of the
variation in fish assemblage richness and abundance. The linkages between hydrologic
alteration and stream fishes were potentially complicated by contrasting effects of
elevated flows on sediment delivery and scour, and mediating effects of stream gradient.
However, stormwater management practices that promote natural hydrologic patterns are
likely to reduce the negative effects of impervious cover on stream fish assemblages.
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