J OURNAL OF C RUSTACEAN B IOLOGY, 34(1), 54-60, 2014
A COMPARISON OF TWO GEARS FOR QUANTIFYING ABUNDANCE OF
LOTIC-DWELLING CRAYFISH
Kristi Williams 1,∗ , Shannon K. Brewer 2,∗∗ , and Mark R. Ellersieck 3
1 302
Anheuser Busch Natural Resources Building, Department of Fisheries and Wildlife Sciences,
University of Missouri, Columbia, MO 65201, USA
2 U.S. Geological Survey, Oklahoma Cooperative Fish and Wildlife Research Unit, 007 Agriculture Hall,
Oklahoma State University, Stillwater, OK 74078, USA
3 Agriculture Experiment Station, Department of Statistics, University of Missouri, Columbia, MO 65211, USA
ABSTRACT
Crayfish (saddlebacked crayfish, Orconectes medius) catch was compared using a kick seine applied two different ways with a 1-m2
quadrat sampler (with known efficiency and bias in riffles) from three small streams in the Missouri Ozarks. Triplicate samples (one of
each technique) were taken from two creeks and one headwater stream (n = 69 sites) over a two-year period. General linear mixed models
showed the number of crayfish collected using the quadrat sampler was greater than the number collected using either of the two seine
techniques. However, there was no significant interaction with gear suggesting year, stream size, and channel unit type did not relate to
different catches of crayfish by gear type. Variation in catch among gears was similar, as was the proportion of young-of-year individuals
across samples taken with different gears or techniques. Negative binomial linear regression provided the appropriate relation between the
gears which allows correction factors to be applied, if necessary, to relate catches by the kick seine to those of the quadrat sampler. The
kick seine appears to be a reasonable substitute to the quadrat sampler in these shallow streams, with the advantage of ease of use and
shorter time required per sample.
K EY W ORDS: capture efficiency, Orconectes medius, quadrat sampler, saddlebacked crayfish, seine
DOI: 10.1163/1937240X-00002212
I NTRODUCTION
The increasing interest in researching the ecology and
monitoring the status of freshwater crayfishes is likely due
to an evolving appreciation of their key ecological role in
many lotic systems (Momot, 1995), their vulnerability to
extirpation from habitat destruction (Taylor et al., 2007), and
the damage invasive crayfish species can cause (Lodge et
al., 2000). Studies involving crayfish ecology often require
quantitative estimates of population numbers, and although
numerous methods for sampling populations are commonly
used, most have serious biases and unknown efficiencies. It
follows that confidence in results and conclusions requires
confidence in the sampling approach.
Various active and passive sampling methods to study
crayfish have been employed for several decades. Methods
include baited traps, hand netting or hand collecting, visual
assessments using SCUBA, snorkeling or bankside above
stream observation, electrofishing, and enclosure nets and
seines (see Haag et al., 2013 for a complete assessment).
The usefulness of results from these methods is often
questionable because of suspected, but unquantified biases,
very low efficiencies that increase variation, and utility in
only special situations.
∗ Present
An enclosed quadrat sampler has been extensively used,
and its characteristics examined, for collecting both crayfish
(Rabeni, 1985; DiStefano et al., 2003) and benthic fishes
(Fisher, 1987; Peterson and Rabeni, 2001). This sampler
has been evaluated for its sampling variance and sampling
precision for crayfish under a variety of conditions found
in Ozark streams (DiStefano et al., 2003). Further work by
Larson et al. (2008) determined actual efficiencies (% of
individuals present that are collected) in riffle channel units
of an Ozark stream. Because of these efforts, we believe this
technique to be the best way currently available to estimate
crayfish densities and abundances in small to medium-sized
streams.
Whereas the quadrat sampler has proven effective in
many situations, it is not without problems. The quadrat is
heavy, cumbersome, and labor intensive. Most importantly
it is time consuming. Use in Ozark streams has shown
two operators can reasonably collect 16 quadrat samples
per day (two samples per h) depending on the number of
crayfish collected for processing and if additional abiotic
measurements are taken at each site (DiStefano, 2000). This
rate of sampling discourages its use in situations requiring
substantial numbers of samples to be taken over a short
period of time.
address; Arkansas Department of Environmental Quality, 5201 Northshore Drive, North Little Rock, AR 72118, USA.
author; e-mail: [email protected]
∗∗ Corresponding
© The Crustacean Society, 2014. Published by Brill NV, Leiden
DOI:10.1163/1937240X-00002212
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WILLIAMS ET AL.: GEARS FOR LOTIC-DWELLING CRAYFISH STUDIES
Recently, a small kick seine has gained popularity in
the Ozark region of Missouri and Arkansas (Flinders and
Magoulick, 2003), and in other areas of the U.S. (Mather
and Stein, 1993) for determining crayfish population characteristics. The kick seine is lightweight, easily constructed
and used, and inexpensive. Most importantly, we have found
sampling time is generally 10-15 min per sample (approximately 4 samples per h) depending on habitat conditions and
the number of crayfish that need to be processed. This means
use of the kick seine could result in 32 samples taken in one
day and that estimate generally agrees with the findings of
others (Allert et al., 2008).
However, no analyses of the characteristics of the kick
seine for collecting crayfish have been conducted. The
objective of this study was to test the performance of the
kick seine as compared to the quadrat sampler whose biases
and efficiencies are known under some common stream
situations. We assumed that the efficiency of the quadrat
sampler would be similar in runs and pools of small, shallow
streams but realize that the ability to seal the sampler to the
bottom (likely better in these habitats than riffles) will affect
efficiency. If the kick seine captures comparable numbers
of crayfish as the quadrat sampler under varying conditions,
or if measurable biases exist and can be accounted for, the
seine may be a less labor intensive substitute for the quadrat
sampler.
M ATERIALS AND M ETHODS
Study Area
Three streams were chosen for crayfish sampling, each located in the
Ozark Highlands, Missouri, USA. The Ozark Highlands ecoregion consists
largely of karst topography with relatively clear streams that carry low
suspended sediment loads, have well-developed riffle-pool complexes, and
have significant spring influences (Nigh and Schroeder, 2002). One stream
was a small (mean channel width 2.3 m), shallow (mean depth 0.12 m)
unnamed headwater stream (1st order, Strahler, 1957) with an average
velocity of 0.20 m · s−1 . The other two streams sampled were Lost and
Courtois creeks, both fourth-order streams. Courtois Creek had similar
available velocity conditions (mean velocity 0.20 m · s−1 ) but as expected,
was wider (mean width 13.2 m) and deeper (mean depth 0.33 m) than
the headwater stream. Depth and velocity profiles were not taken at Lost
Creek but the channel width was similar to Courtois Creek (mean width
11.2 m). Particle sizes indicate dominant and subdominant substrate types
were primarily pebble and cobble (modified Wentworth scale, Gordon et al.,
1993) in all of the streams sampled (Williams, 2006). The maximum depth
sampled was 0.40 m in the headwater stream and 0.55 m in Courtois Creek.
In each stream, riffle, run, and pool channel units (discrete morphological
habitat features formed by high flows) were delineated based on gradient,
velocity, and depth. Riffles had higher gradients than the surrounding areas,
increased velocities, and shallow depths. Runs were transition areas with
intermediate depths and velocities whereas, pools were depositional areas
with limited velocity and were generally deeper than surrounding habitat
(Rabeni and Jacobson, 1993 for a review of channel unit classification in
Ozark streams).
Sampling Methods
Sample sites were chosen in each channel unit to minimize variation due
to microhabitat differences among samples. Criteria for sample sites were:
a channel unit area that would allow three 1-m2 samples to be taken while
leaving at least a 1 m2 undisturbed between each sample, a similar dominant
substrate type, and little variation in depth and velocity. Samples were
taken in triplicate, one quadrat and two seines, from the same channel
units with the underlying assumption being each sample was representative
of the crayfish densities within that channel unit. During the first year of
sampling, the order of gear used was not randomized and the seine samples
were always taken first. During the second year of sampling, gear order
was randomized at each site in case more crayfish were sampled via the
first gear. Samples were taken beginning downstream and working in an
upstream direction.
Crayfish were sampled in autumn 2002 and summer 2003 from riffles,
runs, and pools of three streams using two gear types (quadrat sampler and
seine) and one variation of technique (seine variation). The 1-m2 quadrat
sampler has a known efficiency for sampling lotic crayfish in riffle habitats
(Larson et al., 2008). The sampler is 0.5 m high and is covered on each
side by 3-mm netting and has a 1.2 m bag seine attached to the downstream
end (see diagram in Larson et al., 2008). Sampling procedures for using
the quadrat sampler were described by DiStefano et al. (2003). Briefly, the
sampler was placed in the substrate and the side nets were sealed to the
streambed to prevent crayfish escape. The substrate within the sampler was
disturbed for approximately 3-5 minutes, and large substrate was removed
from the interior portion of the sampler to a depth of 15 cm. Water and
debris were swept into the downstream seine and examined for crayfish
at the end of the sample. The kick seine was compared to the quadrat
sampler using two different techniques from qualified field personnel. Both
techniques required two people for operation and both seines had 3-mm
netting and a 22.7-kg lead line attached to the bottom. With the first
technique (hereafter referred to as seine 1), one individual held the seine
handles about 1 m apart while the second person secured the lead line to
the streambed using cobble or larger substrates. The second person would
then use a side-to-side kicking action over an estimated 1-m2 area upstream
of the seine. The area was disturbed once, large substrates were removed,
and then a second pass was made over the 1-m2 area. The second technique
(hereafter referred to as seine 2) used a seine of the exact same dimensions
as seine 1 but used slight variations in the technique. The seine was held
with the top of the handles together and near the water’s surface to create
more of a bag in the back of the seine and less effort was spent creating a
seal with the leadline and the streambed. The area was disturbed as with the
other technique, but instead used a front-to-back kicking motion and swept
water into the seine while kicking, rather than making two passes. Personnel
operating each of the techniques were alternated at each sampling event
to negate any influence related to sampler and operator bias. Regardless
of technique, adult and juvenile crayfish were enumerated on the stream
bank following each sample, held in buckets and returned after to the
stream following the completion of sampling in that channel unit. Lengthfrequency histograms were used to delineate a carapace length threshold for
young-of-year versus adult crayfish (Rabeni, 1992).
Analyses
Analyses were restricted to captures of saddlebacked crayfish Orconectes
medius (Faxon, 1884) because the species represented 95% and 97% of the
total number of individuals captured in year 1 and year 2, respectively.
Prior to analyses, some variables were combined into a single metric
so multiple models could be created to account for missing data from
one sample year (pools were not sampled from one stream during one
year) and to include the influence of stream size even though only one
headwater stream was sampled. Stream sampled and sampling year were
combined to form a single variable called ‘environment.’ The first general
linear model excluded pool channel units because they were not sampled
from one stream during one sample year. By omitting these data, we could
analyze for possible influences related to sample year and stream size. For
the second model, a subset of data from the ‘environment’ variable (2002
data from Courtois Creek) was omitted because pools were not sampled
during that period. Removing these data and then creating a second model
allowed us to assess the influence of all three channel units sampled (riffle,
run, and pool). We were primarily interested in the interactive effects to
determine if stream size or channel unit sampled influenced densities of
crayfish captured by each gear or technique. In addition, we were interested
in examining possible interactive effects between environment and gear to
assess whether lack of randomization of gear and technique during the first
year of sampling had any relationship to the observed results.
Two general linear mixed models were created to assess the influence
of channel unit, stream size, year sampled, and gear on crayfish catch.
Our primary interest was assessing the significance of gear and any
interactions with gear (channel unit, stream size, year sampled). The results
of these models guided the second portion of the analyses (regressions)
by identifying environmental conditions that would interact with gear
thereby creating a situation where regressions would need to be created
for varying environmental conditions, e.g., in headwater streams versus
creeks. Data were analyzed as two split plots in space using the ‘Proc
56
JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014
Glimmix’ procedure in SAS (SAS, 2000) to account for possible spatial
autocorrelation. Both models used a log link and a Poisson distribution.
Plots of each subset of data indicated either a Poisson or negative binomial
distribution would be appropriate but Poisson seemed to fit these data
subsets slightly better. One was added to all of the adult crayfish densities
because the log of zero was missing. The first model contained the main
effects of environment (year and stream) and the subplot contained the
effects of channel unit (riffles and runs) and gear (two seining methods and
the quadrat sampler) and all possible combinations with the main effect
environment. The second model contained the main effects of environment,
but this time with year by stream combinations by omitting 2002 data from
sampled creeks because pools were not sampled from creeks during that
year. The subplot contained the effects of channel unit (riffle, run, and
pool), and gear (two seining methods and the quadrat sampler) and all
possible combinations with the main effect environment. For both models,
we used the Tukey-Kramer honestly significant difference post-hoc test to
determine where significant differences occurred given the overall model
was significant.
Two negative binomial linear regressions were created to examine the
relationship between the number of crayfish collected with the quadrat
sampler compared to the number of crayfish collected with each seining
technique (seine 1 and seine 2) to allow for adjustments to be made to seine
samples taken under similar environmental conditions (small headwater
streams and creeks). Because there were no significant interactions between
gear and other fixed effects in the general linear mixed models, data were
combined to include all of the quadrat and seine samples into one dataset.
Two samples were omitted from the analyses because no quadrat sample
was taken with the two seine samples. We examined the data for outliers
and checked the observations to ensure data were entered correctly. The
dataset included 69 observations for each gear (quadrat, seine 1 and seine 2)
because the gears were combined at each site (one of each seine and one
quadrat sample were taken at each of 69 sites, n = 39 in 2002 and
n = 30 in 2003). Descriptive statistics were calculated for each gear to
determine if over-dispersion was indicated by these data (variance > mean;
Osgood, 2000). In addition, a dispersion parameter was estimated for each
model (Proc Genmod; SAS, 2000) to assess model fit. If dispersion is >0
(by examining 95% confidence intervals around the dispersion estimate),
then a negative binomial distribution is appropriate rather than a Poisson
distribution. The negative binomial distribution is a generalization of the
Poisson distribution but includes an additional parameter allowing the
variance to exceed the mean (Osgood, 2000). Given fit was appropriate,
negative binomial regressions were fit to the data using a negative binomial
distribution and log link (Proc Genmod; SAS, 2000). The count of crayfish
collected using the quadrat sampler was the dependent variable in each
model since some estimate of efficiency is known for the quadrat sampler
(i.e., riffles). Models were considered significant at α 0.05.
We assumed that correcting data from one gear to another would be most
relevant if the precision among the gears or techniques (seine 1 and seine 2)
were comparable. We calculated the coefficients of variation in crayfish
catch for each gear and technique so comparisons of precisions could be
made given the samples represent different methods.
Adjustments for young-of-year crayfish could not be made because
the efficiency of the quadrat sampler in collecting juveniles is unknown.
However, we compared the ratio of young-of-year to total crayfish collected
for individual streams in each of two years to determine if size selectivity
was evident between gears or techniques. If differences exist, the choice of
gear might then relate more to project objective rather than simply ease of
collection.
R ESULTS
Influence of Year and Habitat
In the first model, the number of adult crayfish sampled
was significantly related to the main effects of environment
(F4,156 = 13.69, P < 0.01), channel unit (F1,156 = 7.64,
P < 0.01) and gear (F2,156 = 6.92, P < 0.01). In
addition, the expected interactive effect of environment ∗
channel unit was also significant indicating the number of
crayfish sampled in a particular stream and year depended
on the channel unit sampled (riffle or run) (F4,156 = 7.11,
P < 0.01). Densities of crayfish collected using the quadrat
Fig. 1. Mean crayfish densities (±95% confidence limits) associated with
two gears (quadrat and seine) and two seining techniques (seine 1 and
seine 2). The top panel represents data included in Model 1 and the bottom
panel represents data included in Model 2.
sampler were greater than when using either of the two
seining techniques (seine 1: t156 = −3.19, P < 0.01;
seine 2: t156 = −2.96, P < 0.01; Fig. 1). Generally, the
quadrat sampler collected approximately 20% more crayfish
than either of the two seining techniques. No significant
interactions were detected with gear indicating stream size
(headwater or creek) and year did not alter the number of
crayfish collected. These results suggest lack of randomizing
gear type in year one had no significant effect on the
outcome and there was no influence on catch by gear
between the stream sizes we sampled.
In the second model created, the number of adult crayfish
sampled was significantly related to the main effects of
environment (F3,113 = 30.97, P < 0.01) and gear (F2,113 =
6.92, P < 0.01). As we expected and similar to model 1, the
interactive effect of environment ∗ channel unit (F6,113 =
7.11, P < 0.01) was also significant indicating the number
of crayfish sampled in a particular stream and year depended
on the channel unit sampled (riffle, run, or pool). Again, the
quadrat sampler collected more crayfish than either of the
two seining techniques (seine 1: t113 = −2.44, P = 0.04;
57
WILLIAMS ET AL.: GEARS FOR LOTIC-DWELLING CRAYFISH STUDIES
Table 1. Maximum likelihood parameter estimates for two negative binomial distribution regression models and untransformed data for each seining
technique (no significance is assumed with the models created using the actual data due to distributional assumptions).
Parameter
df
Estimate
SE
Wald 95% confidence limits
χ2
P > χ2
<0.01
<0.01
Model 1: negative binomial model (seine 1)
Intercept
1
1.58
Seine 1
1
0.07
Dispersion
1
0.50
0.15
0.02
0.11
1.29
0.03
0.32
1.87
0.10
0.77
116.10
15.60
Model 1: actual data (seine 1)
Intercept
1
Seine 1
1
4.64
0.63
1.12
0.12
2.45
0.40
6.83
0.86
17.24
28.49
Model 2: negative binomial model (seine 2)
Intercept
1
1.56
Seine 2
1
0.07
Dispersion
1
0.45
0.14
0.02
0.11
1.29
0.04
0.29
1.83
0.11
0.71
129.78
20.25
Model 2: actual data (seine 2)
Intercept
1
Seine 2
1
1.03
0.11
2.21
0.51
6.27
0.95
16.79
41.04
4.24
0.73
seine 2: t113 = −3.47, P < 0.01; Fig. 1). As with model 1,
no interactive effects occurred with gear suggesting channel
unit (including sampled pools) did not relate to different
catches of crayfish by gear.
Correction Factors
The variance associated with number of crayfish collected
with each gear type was greater than the means suggesting
over-dispersion was present and that a negative binomial
model would be appropriate. The 95% confidence intervals
of the dispersion parameter for each model were greater than
zero providing further support for use of a negative binomial
distribution (Table 1). The first model indicated a significant
linear relationship between the number of crayfish collected
via the quadrat sampler compared to the number of crayfish
collected via seine method 1 (Table 1, Fig. 2). The intercept
of the first model represents the negative binomial regression
<0.01
<0.01
estimate when the count of crayfish collected using seine 1
is zero. In this case, when the seine count = 0, the log of
the expected quadrat count = 1.58 (Table 1). Seine method
1 was significant to the model and the maximum likelihood
parameter estimate indicates that if we increase our seine 1
sample by one crayfish, the difference in the logs of expected
counts would increase by 0.06, while holding the intercept
constant. The actual data values (non-transformed data with
a normal distribution) are reported in Table 1 and should
approximate the mean (but should not be used to interpret
significance). As we would expect given that there were
no differences between crayfish catches between the two
seine techniques, the second model results were similar to
the first model. The second model indicated a significant
linear relationship between crayfish collected via the quadrat
sampler and crayfish collected using seine 2 (Table 1). The
parameter estimates for the intercept and seine 2 (1.56 and
0.07, respectively) are very similar to those presented above
for seine 1.
Precision
A comparison of variation among the three techniques
indicates a greater sampling precision (as measured by the
coefficient of variation) for the quadrat compared to the
seines (Table 2), but the 9% or 13% difference between gears
is likely not biologically meaningful.
Size Selectivity
Graphs comparing gears and techniques (Fig. 3) reveal no
consistent differences in the catch of different ages, i.e.,
sizes, of crayfish. Although year 2 results show slightly
Table 2. A comparison of sampling precision of collections of adult
crayfish between the seine techniques and quadrat sampler.
Fig. 2. Scatterplot of crayfish densities obtained using the quadrat sampler
versus densities estimated using the seines (one of two techniques). Black
circles indicate approximate densities using seine one whereas open circles
represent approximate densities obtained using seine two. Data presented
are the 69 observations presented in the regression analyses.
Method
n
Mean
SD
CV
Seine 1
Seine 2
Quadrat
69
69
69
7.0
6.6
9.0
6.4
6.2
9.0
91%
95%
82%
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014
Fig. 3. Percent of young-of-year crayfish that occurred in catch by sampling method from two streams that were sampled in 2002 and 2003. Gears and
techniques are represented by: S1: seine technique one, Q: quadrat sampler, and S2: seine technique two.
higher ratios of young-of-year, year 1 results are very similar
with a range of results of only 2% in the creek and 5% in the
headwater stream.
D ISCUSSION
Accurate estimates of crayfish population densities and
abundances are required to increase understanding and management of this important faunal group. The ideal crayfish
sampler would be efficient, have quantifiable biases, and be
time resourceful. We believe the 1-m2 quadrat sampler to
be the only stream crayfish sampler with known efficiencies
under some conditions (riffles) and with minimal size selection (see catch curve in Rabeni, 1992), except for very small
(young) individuals that may pass through the net mesh, and
very large individuals that occupy specialized habitats. Large
crayfish often use deeper pools with large boulders and limestone bluffs making their capture difficult (DiStefano et al.,
2003). However, the larger individuals in these streams contribute little to overall secondary production (Brewer et al.,
2009).
Although the quadrat sampler can be used to estimate
densities over a known area, it is relatively time consuming
and often logistically cumbersome to use. Field experience
(DiStefano et al., 2003) has shown a 2-person team can
complete about two quadrat samples per h. Larson et al.
(2008) indicate with power analysis that even at a power of
0.8, approximately 20 quadrat samples would be necessary
for confidence intervals (half width) to be within 40% of
the mean. A day and a half effort to produce a meaningful
density estimate for a crayfish population in a reach of
stream may not be realistic in many situations, depending
on the objective. This is especially true when gear need to
be carried over long distances to sample sites as the quadrat
sampler is heavy.
A two-person team can collect an average of >30 kickseine samples in a day, although this value will be reduced
under some habitat conditions or if large numbers of crayfish
are collected. Although the speed and ease of taking a
sample may be important in some situations, investigators
must be sure not to have a significant loss of sample
precision when using alternative gears. We investigated
precision by examining coefficients of variation, using
our side-by-side comparisons, and found the seines and
quadrat sampler to have comparable precisions, although
the quadrat was generally better. The quadrat sampler also
collected more crayfish, and could be deployed in some
habitats where the seine is likely to be very ineffective,
e.g., vegetated habitats or deeper pools. Furthermore, use
of the seine as reported in the current study has inherent
operator bias because the 1-m2 area sampled was estimated,
although recent modification have ameliorated this problem
by attaching a 1-m2 frame at the bottom of the seine (Allert
et al., 2012). Variation obtained from sampling benthic
59
WILLIAMS ET AL.: GEARS FOR LOTIC-DWELLING CRAYFISH STUDIES
stream organisms is consistently high (Downes et al., 1993)
and the fact that all of these samplers have SD < mean values
is an indication of acceptable levels of precision.
Size bias of many crayfish collection methods is well
known (Momot, 1967; Haag et al., 2013). Our examination
of size bias is not conclusive, but the catch ratio of young of
year to adult is certainly comparable between all methods.
Comparisons need to be made with caution because the
efficiency of catching very small crayfish with the quadrat
is unknown as all efficiency estimates have been made
using adults in select habitats (Larson et al., 2008). We do,
however, know that high crayfish mortalities occur during
the first year of life and temporal changes should be expected
(Brewer et al., 2009). Further, reduced sampling precision
should be anticipated if sampling during periods of crayfish
hatching (DiStefano et al., 2003). It is likely that no method
accurately portrays young-of-year densities, but this subject
is certainly in need of further investigation.
We conclude that while the quadrat sampler catches significantly more crayfish than does the kick seine, the kick
seine results are easily corrected to be comparable to the
quadrat, with little loss of precision under the conditions
tested in this study. Whereas correction factors likely do not
improve variation of density estimates, they do provide a
truer estimate of density and therefore useful if transitioning
from a historical sampling approach to a new approach in
a long-term study of the same system. The kick-seine technique appears to have appropriate characteristics to make it a
suitable sampler for crayfishes under some conditions, e.g.,
relatively shallow, non-vegetated habitats. However, we suggest additional testing is warranted to assess the efficiency
of either gear in pool or other slackwater habitats or areas of
increased habitat complexity, e.g., vegetated areas. The kick
seine comparison was conducted on small streams, with similar conditions of depth and substrate composition; however,
results might differ if obtained under less homogeneous conditions.
Projects involving crayfish populations have varying
objectives – e.g., research, determining population status, or
monitoring over space and time. If abundances or densities
of crayfish are central to the collection effort, investigators
or managers would be well served to further test characteristics of their sampler under conditions specific to the study
area. A pilot project examining efficiencies and biases such
as described by Peterson and Paukert (2009) would be relatively easy to accomplish and would give greater confidence
in study results. Results from this study show a simple approach to adjust catch from one gear to another given samples are taken from small, relatively shallow, pebble-cobble
streams with riffle-pool morphologies. The need to adjust
data in this study appears minimal but the approach is useful where greater differences may be evident. This ‘pairedsamples approach’ may also be taken when monitoring using one gear and then switching to another so historic data
may continue to be useful in conservation and management
decisions.
ACKNOWLEDGEMENTS
This research is a contribution of the Missouri Cooperative Fish and
Wildlife Research Unit (U.S. Geological Survey, Missouri Department of
Conservation, University of Missouri, and Wildlife Management Institute
cooperating) with collaboration from the Oklahoma Cooperative Fish and
Wildlife Research Unit. Any use of trade, firm, or product names is
for descriptive purposes and does not imply endorsement by the U.S.
Government. We thank Robert DiStefano for input on study design and
two anonymous reviewers for improving the quality of the manuscript.
The Missouri Department of Conservation provided field equipment.
Paul Horner, Jacob Westhoff, Jennifer Girondo, Eric Rahm, and Sabrina
Davenport provided valuable field assistance.
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R ECEIVED: 18 May 2013.
ACCEPTED: 6 November 2013.
AVAILABLE ONLINE: 13 December 2013.