TRANSACTIONS OF THE KANSAS
ACADEMY OF SCIENCE
Vol. 109, no. 3/4
p. 131-138 (2006)
An evaluation of single-pass versus multiple-pass backpack
electrofishing to estimate trends in species abundance and richness
in prairie streams
KATIE N. BERTRAND1, KEITH B. GIDO1, AND CHRISTOPHER S. GUY2,3
1. Division of Biology, Kansas State University, 232 Ackert Hall, Manhattan, KS
66506 Corresponding author (bertrand@ksu.edu)
2. U.S. Geological Survey, Biological Resources Division, Kansas Cooperative Fish
and Wildlife Research Unit, Division of Biology, Kansas State University, 205
Leasure Hall, Manhattan, KS 66506
3. Present address: U.S. Geological Survey, Biological Resources Division, Montana
Cooperative Fishery Research Unit, Department of Ecology, Montana State
University, Bozeman, MT 59717
Backpack electrofishing is a common method used to compare total species richness
and relative abundance of stream fishes across space and time. However, as with any
sampling method, it is important to evaluate the sampling effort necessary to capture
patterns of variation in fish assemblage structure across samples. Thus, we evaluated
the efficacy of single-pass versus multiple-pass backpack electrofishing for minnows
and darters in intermittent prairie streams. We found that in 14 of 19 three-pass
electrofishing samples, we detected all species during the first pass. The samples where
we missed species on the first pass were in pools with six to nine species, suggesting a
single-pass sample worked best for pools with lower species richness. We also found
that both the raw abundance (i.e., catch rates) and rank abundance of four common
species based on the first pass is highly concordant with the second and third passes.
Nevertheless, differences in capture efficiency varied by species and density. In
particular, our ability to deplete a species from a stream pool was highly variable when
fish densities were low, and for Phoxinus erythrogaster, it was variable across all
densities. Overall, our data suggest single-pass electrofishing can be used to detect
spatial and temporal trends in abundance and species richness given standardized
effort, but may not be representative of absolute population densities.
Keywords: backpack electrofishing, intermittent prairie streams, minnows, darters,
sampling efficiency
INTRODUCTION
Standardized methods to measure species
abundances and occurrences are necessary to
evaluate spatial and temporal variation in fish
community structure. However, obtaining
these estimates requires substantial sampling
effort, and comparisons across sites and time
are confounded by differences in physical
habitat and differing susceptibility of species
to capture (e.g. Bohlin and Sundstrom 1977;
Zalewski 1983; Angermeier and Smogor 1995;
Meador et al. 2003). For example, multiplepass depletion electrofishing is commonly
used to estimate population density of stream
fish populations by extrapolating the rate of
decline in catch per unit effort (C/f) over three
or more passes to estimate the population size
of a species in a closed habitat (Zippin 1956,
1958; Ricker 1975; White et al. 1982). The
advantage of this approach is that it generates
a population estimate that is independent of
habitat and susceptibility of a species to
capture.
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Bertrand, Gido and Guy
Many monitoring programs are limited by
time and resources or are aimed at evaluating
temporal trends in populations, and thus,
favor sampling protocols that require less
effort than multiple-pass depletion sampling
(e.g., Meador et al. 2003). Single-pass
backpack electrofishing may be a viable
alternative if sufficient information about the
fish assemblage is gathered from a single
pass. The accuracy of this approach will
likely depend on habitat and species
characteristics (Kruse et al. 1998; Meador et
al. 2003; Peterson et al. 2004). For example,
Meador et al. (2003) found that cyprinids and
centrarchids were underrepresented relative to
other species by single-pass backpack
electrofishing in a study of 10 U.S. river
basins. Peterson et al. (2004) also found that
multiple-pass removal population estimates of
Western cutthroat trout and bull trout were
significantly underestimated. In contrast,
other studies (Jowett and Richardson 1996,
Joy and Death 2002) reported no significant
differences in relative capture probability
among species in 38 New Zealand rivers.
Capture efficiency of a species may also be
biased by its abundance (Peterson et al. 2004).
Pusey et al. (1998) noted that many species
present at low densities were not captured
using a single pass electrofishing sample in
small to medium-sized streams in
Queensland, Australia.
relationship between single-pass and
multiple-pass population estimates for stream
salmonids (e.g. Lobon-Cervia and Utrilla
1993; Jones and Stockwell 1995; Decker et al.
1999; Kruse et al. 1998). Our goal was to
evaluate the concordance in abundance and
species richness of stream pools in an
intermittent prairie stream sampled with
single-, two- or three-pass electrofishing.
Discrepancies in the above studies indicate
the need to evaluate the effectiveness of
single-pass backpack electrofishing before it
is adopted as a standard sampling protocol.
There are several cases in the literature
comparing species relative abundances from a
single pass with those obtained after multiple
passes. As an example, Simonson and Lyons
(1995) reported a significant correlation in
abundance estimates between contiguous
stations in nine streams in southern
Wisconsin sampled with single- or multiplepass electrofishing. Additionally, several
studies have reported a significant
SITE DESCRIPTION AND LOCATION
The fish assemblage of Kings Creek on the
Konza Prairie Biological Station has been
sampled quarterly since May 1995 using
single-pass backpack electrofishing as part of
the National Science Foundation funded
Long-Term Ecological Research (LTER)
program. To evaluate the efficacy of singlepass backpack electrofishing, 35 stream pools
were sampled using three passes to evaluate
how much information was gained after a
single pass, versus multiple passes. We also
examined the effects of stream habitat
characteristics on the efficiency of capturing
minnows and darters in prairie streams. We
addressed three questions regarding singlepass backpack electrofishing:
1) How many species are missed in a single
pass?
2) Is there a significant correlation in species
abundances across passes?
3) Do different species vary in their
susceptibility to capture?
Kings Creek drains 1059 ha in the Flint Hills
region of northeastern Kansas and has a series
of natural spring headwaters, ephemeral
reaches, and a perennial downstream reach
(Grey et al. 1998; Grey and Dodds 1998).
Riparian vegetation is dominated by prairie
grasses and shrubs in the headwaters and gallery
forest in the lower reaches. Stream gradient
ranges from 0.017 m/m in the gallery forest to
0.038 m/m in the headwaters. Mean annual
discharge was 7.06 L/s, and mean specific
conductance in Kings Creek was 485 µS/cm.
Transactions of the Kansas Academy of Science 109(3/4), 2006
METHODS
Fishes were collected with a Smith-Root POW
DH-381, pulsed DC current-generating
backpack shocker set to a frequency of 60Hz
with 6ms pulse duration and an output of 100
volts. Stream pools were sampled in an
upstream direction by sweeping the anode
from bank to bank and attempting to net all
fish. Fishes were collected exclusively in
stream pools. Area of pools was estimated by
multiplying mean pool width (measured at
three equally-spaced transects along the
length of each study pool) by pool length.
Dominant substrate types were quantified
according to the Wentworth scale (Cummins
1962) at five points (points were equallyspaced between the stream banks) along these
same three transects (N=15 points per pool).
Prior to sampling, 19 pools were blocked at
the inflow and outflow with 5-mm bar mesh
block nets. Twelve pools were sampled
without block nets because they were bordered
upstream and downstream by shallow riffles,
which can act as natural barriers to movement
(e.g., Simonson and Lyons 1995). Nine of the
pools were sampled with one netter, and the
remaining pools were sampled with two
netters. Although catch rate with a single
netter was about half that of pools sampled
with two netters, this difference was not
statistically significant because of the high
variability in catch rates among samples (i.e.,
single-netter samples were conducted
primarily in the ephemeral reach where fish
densities are inherently lower). Nevertheless,
we included both the one and two netter
samples in our analyses because even when a
single netter was present, that effort was
replicated across the subsequent passes.
To quantify the degree to which a single-pass
backpack electrofishing sample accurately
represented species richness in these pools,
we compared the total number of species
caught after the first pass with the total
number of species caught after the second and
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third passes. We limited this analysis to 19
pools because only common species were
identified and enumerated in the other 12
sample pools.
Catch rate was calculated as the number of
individuals per m2. Although we could have
standardized by time shocked on each pass we
decided area sampled was a more appropriate
estimation of effort. Moreover, there was a
significant correlation between pool area and
seconds shocked (R2 = 0.553, P < 0.001). To
test the association between catch rate on the
first and second or first and third passes, we
used Pearson correlations. Prior to these
analyses, data were log-transformed to reduce
heteroscedasticity. In addition, we used
Spearman rank correlation to test the
relationship in rank abundances of species
across study pools. To evaluate if a singlepass electrofishing sample was able to
significantly deplete a population in a stream
pool, we tested the difference in mean catch
rate of each species between the first and
second, and the first and third passes using
paired t-tests. Because our ability to deplete a
population was likely associated with density,
we also examined trends in the proportion of
total catch captured during the first pass as a
function of catch rate.
RESULTS
Mean width of all study pools was 3.45 m (SD
= 0.84 m), mean depth was 0.20 m (SD =
0.07 m) and mean pool length was 34.8 m
(SD = 19.66 m). Pool area ranged from 23.3
m2 to 391.8 m2. Substrate was predominantly
cobble, pebble, and gravel.
A single-pass electrofishing sample of Kings
Creek pools yielded most of the species
captured after three passes; only 5 of 19 pools
had additional species captured after the first
pass (three of these incidents were pools
sampled with only one netter; Table 1). In
three of these cases, all but one species was
captured on the first pass, and in two cases,
134
Bertrand, Gido and Guy
Table 1. Cumulative number of species
captured after the first pass backpack
electrofishing compared to the total number of
species captured after the second and third pass
for 19 pools in Kings Creek. Number of netters
present during sampling is indicated in the
second column, and all 19 pools were blocked
at their upstream and downstream ends.
Figure 1. Relationship between log catch rate
(individuals / m2) on the first pass and log
catch rate on the second and third passes for
four fishes from Kings Creek. Pearson
correlations are shown for first versus second
pass (open circles, solid least-squares line)
and first versus third pass (open squares,
dash-dot least-squares line). The dotted line
represents the 1:1 relationship.
The catch rate (individuals / m2) on the first
pass was significantly correlated with that of
the second and third passes for Campostoma
anomalum, Phoxinus erythrogaster, Semotilus
atromaculatus, and Etheostoma spectabile
(all P-values < 0.001; Fig. 1). Likewise, pool
rankings based on catch rate were
significantly correlated between the first and
second and between the first and third passes
for those four species (all P-values < 0.001;
Fig. 2).
all but two species were captured on the first
pass. In pools with four or fewer species, all
species present were captured on the first
pass.
Mean catch rates on the first pass were
typically 13% to 46% greater than on the
second and 48% to 69% greater than on the
third pass for C. anomalum, P. erythrogaster,
and S. atromaculatus (Table 2). For E.
spectabile, there was not a significant
reduction in mean catch rate between the first
and second passes, but there was a significant
reduction (mean 70%) between the first and
Transactions of the Kansas Academy of Science 109(3/4), 2006
135
Figure 2. Relationship between ranked
abundance (individuals / m2) of species
captured on the first and second and first and
third passes for four fishes from Kings Creek.
Spearman rank correlations are shown for first
versus second pass (open circles, solid leastsquares line) and first versus third pass (open
squares, dash-dot least-squares line). The
dotted line represents the 1:1 relationship.
Figure 3. Relationship between percent of the
total number of individuals / m2 captured after
three electrofishing passes and the number of
individuals / m2 caught on the first pass.
Points above the dashed lines represent
samples where a disproportionate number of
individuals were removed on the first pass
relative to other passes, and these were used
as indicators of sampling effectiveness.
third passes. Averaged across the four
species, the first pass comprised 52% of the
total catch rate after three passes (Fig. 3). On
average, over half of the total three-pass catch
was captured during the first pass for C.
anomalum, S. atromaculatus, and E.
spectabile, and for all pools with densities
greater than one individual / m2, the percent
captured on the first pass was greater than
33% of the total catch from three passes. For
P. erythrogaster, the total number of
individuals were captured on the first pass
averaged 46%, but the percent of total
individuals captured on the first pass was
often < 33% across a range of densities.
estimating fish species richness. Other
investigators have reported that a large
proportion of the species at a site are captured
with a single-pass electrofishing survey.
Meador et al. (2003) found that first pass
sampling accounted for 81% to 100% of twopass species richness in North American
streams. Patton (1998) demonstrated that a
single electrofishing pass resulted in the
capture of at least 90% of the species in nine
Wyoming streams. However, Angermeier and
Smogor (1995) cautioned that richness
estimates based on small sampling efforts are
more likely to omit species. Our study
suggests that single-pass electrofishing
adequately represents species richness in
pools where four or fewer species are present,
but underrepresented species richness in
several pools containing between six and nine
species.
DISCUSSION
The low diversity and small size of Kings
Creek contributed to the effectiveness of using
single-pass backpack electrofishing for
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Bertrand, Gido and Guy
Table 2. Paired t-tests of catch rate (individuals / m2) among successive
passes of backpack electrofishing.
We showed that a single pass of backpack
electrofishing is typically representative of the
abundance of common fishes in Kings Creek.
Assuming constant effort across successive
passes, if the catch rate is high in a given pool
on the first pass, the catch rate will also be
high in that same pool on a second or third
pass. Thus, a single-pass backpack
electrofishing sample captures spatial, and
presumably temporal, variation in abundance
stream pools and should be adequate to
characterize long-term trends in assemblage
structure. Traditionally, most evaluations of
sampling gear have been made for sport
fishes. More recently, Meador et al. (2003)
evaluated the effectiveness of single-pass
electrofishing to detect the occurrence of
stream fishes, but didn’t evaluate how well a
single pass represented abundance. Thus, our
study provides additional information about
the efficacy of single-pass backpack
electrofishing for minnows and darters in
small, intermittent prairie streams.
Even though we could successfully
characterize variation and richness among
study pools, this may not translate into the
ability to use depletion samples for population
estimates or to infer differences in
proportional abundance of species within
pools because sampling effectiveness varied
by species. For example, in some pools, we
caught as many or more individuals on the
second or third pass as we did on the first
pass, suggesting we were unable to deplete
that population (Fig. 3). In these cases, the
standard methods of population estimation
using depletion sampling (e.g., Zippin 1956)
would not work. However, this was only a
problem for P. erythrogaster at all densities
and for all species at very low densities. It
appeared that P. erythrogaster tended to avoid
the electric field by moving laterally towards
shore and downstream, and were most
effectively captured when trapped in a narrow
channel below the block net or riffle. In
contrast, the other three species tended to
either escape into cover (e.g., S.
atromaculatus), or flee toward the stream
bottom (e.g., C. anomolum and E. spectabile)
where they were more effectively trapped in
the electric field. Based on these data and
Transactions of the Kansas Academy of Science 109(3/4), 2006
observations, it appears that P. erythrogaster
can be underrepresented in electrofishing
samples. Moreover, at low densities (i.e., <
20 individuals per pool), catch rates of all
species were quite variable and more subject
to random sampling error. Thus, we
recommend caution when evaluating
differences in abundance among pools with
fish densities < 1/m2.
Possibly the most important limitation to
single-pass backpack electrofishing is the
need to standardize procedures across samples
(i.e., same number of netters, equal effort per
unit area and consistent use of barriers to
block movement out of pools) because
variation in these procedures affects
electrofishing catch rate (Simonson and Lyons
1995; Reynolds 1996). In our analyses, most
of the variability in catch rates among pools
was captured in a single pass. This was likely
because of high sampling efficiency in
shallow, narrow stream pools.
Single-pass backpack electrofishing generates
valuable information about assemblage
structure in small prairie streams without the
effort and expense of multiple-pass sampling.
This avoids the biases associated with
multiple passes such as increased turbidity in
the water column and decreased susceptibility
of fishes to capture beyond the first pass (e.g.
Cross and Stott 1975; Mahon 1980; Zalewski
and Cowx 1989). This is particularly useful
for monitoring projects that may have budget
constraints or limited personnel. Although
our findings may be applied to other small,
high-gradient streams with similar species
composition, extrapolating these catch rate
relationships to another system is not
recommended without verification.
ACKNOWLEDGEMENTS
We thank the Kansas State University Aquatic
Journal Club for critical review. This
research was supported by NSF grants to K.
Gido (DEB-0416126), Konza Prairie Long
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Term Ecological Research, and Research
Experience for Undergraduates supplements
that supported Hope Phillips and Angela
Lickteig. Partial support was also provided by
the Kansas NSF EPSCoR program. The
Konza Prairie Biological Station, which is
owned by the Nature Conservancy and
managed by the Division of Biology at Kansas
State University provided facilities and
equipment.
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