A Comparison of Sampling Gears for Capturing Juvenile Silver Carp in River–Floodplain Ecosystems

Abstract

Effective management and monitoring programs require confidence regarding basic biological sampling. Gear comparisons are often required to determine the most effective techniques. Such is the case for populations of invasive Asian carps Hypophthalmichthys spp., which have recently occurred in large numbers throughout sections of the Mississippi River basin. We tested five gears (mini-fyke nets, beach seine, purse seine, pulsed-DC electrofishing, and gill net) that targeted juvenile (age 0) Silver Carp H. molitrix at sites along the Illinois River during 2014 and 2015 to determine the most effective ones for age-0 Silver Carp. We considered the most cost-effective gear to be the one that provided the largest catch at a minimal expenditure of labor. Mini-fyke nets were the most effective at collecting large numbers of age-0 Silver Carp, followed in decreasing order by beach seines, pulsed-DC electrofishing, purse seines, and gill nets. The smallest Silver Carp were caught in beach seines and the largest were caught in gill nets, and there was considerable variation in size distributions among gears. However, when we considered cost-effectiveness in terms of labor hours for each gear, both beach seines and mini-fyke nets had similar and overlapping labor expenditures. Gill nets and purse seines were not cost-effective, as they required more labor and had lower overall catch rates.
Received May 23, 2016; accepted September 19, 2016

Full text

MANAGEMENT BRIEF
A Comparison of Sampling Gears for Capturing Juvenile
Silver Carp in River–Floodplain Ecosystems
Scott F. Collins,*Matthew J. Diana, Steven E. Butler, and David H. Wahl
Illinois Natural History Survey, Kaskaskia Biological Station, 1235 County Road 1000N, Sullivan,
Illinois 61951, USA
Abstract
Effective management and monitoring programs require con-
fidence regarding basic biological sampling. Gear comparisons
are often required to determine the most effective techniques.
Such is the case for populations of invasive Asian carps
Hypophthalmichthys spp., which have recently occurred in large
numbers throughout sections of the Mississippi River basin. We
tested five gears (mini-fyke nets, beach seine, purse seine,
pulsed-DC electrofishing, and gill net) that targeted juvenile
(age 0) Silver Carp H. molitrix at sites along the Illinois River
during 2014 and 2015 to determine the most effective ones for
age-0 Silver Carp. We considered the most cost-effective gear to
be the one that provided the largest catch at a minimal expendi-
ture of labor. Mini-fyke nets were the most effective at collecting
large numbers of age-0 Silver Carp, followed in decreasing order
by beach seines, pulsed-DC electrofishing, purse seines, and gill
nets. The smallest Silver Carp were caught in beach seines and
the largest were caught in gill nets, and there was considerable
variation in size distributions among gears. However, when we
considered cost-effectiveness in terms of labor hours for each
gear, both beach seines and mini-fyke nets had similar and over-
lapping labor expenditures. Gill nets and purse seines were not
cost-effective, as they required more labor and had lower overall
catch rates.
Effective management and monitoring programs require con-
fidence in basic biological sampling. Fisheries scientists typically
rely on a range of differing active and passive gears to quantify
and describe fish assemblages. Because of biases inherent to the
design and deployment of fisheries gears (e.g., species collection,
habitat limitations, gear evasion, and deployment durations),
some gear types may be ill-suited in specificapplications
(Lyons 1986; Weaver et al. 1993; Bayley and Austen 2002;
Breen and Ruetz 2006;Lapointeetal.2006; Hubert et al.
2012). Species traits, such as their mobility (sedentary versus
mobile), ontogenetic shifts in habitat associations, and patchy
distributions caused by schooling, can affect catches. Because of
these factors, evaluating the effectiveness of multiple gears is
often required to ensure confidence that sampling reflects the
numbers of organisms in nature and minimizes nondetections.
Detecting and monitoring populations of invasive Asian
carps, namely Silver Carp Hypophthalmichthys molitrix and
Bighead Carp H. nobilis, exemplify this basic knowledge gap.
Presently, fisheries scientists are working to describe the abun-
dances, distributions, and movements of these fishes at various
life stages throughout the Mississippi River basin (e.g.,
DeGrandchamp et al. 2008; Sass et al. 2010; Collins et al.
2015;Cooke2016; Norman and Whitledge 2015). Important
gains have been made for adults; however, progress for juveniles
has been slower, in part because of variable reproduction among
years (DeGrandchamp et al. 2007; Irons et al. 2011). Thus, there
have been few opportunities to test the effectiveness of gears
targeting these early life stages or to track cohorts through time
and space. Indeed, early detection of reproduction of these inva-
ders provides vital information for the management of these
species. Moreover, identifying effective gears allows scientists
to begin describing the dynamics of early life stages, including
building recruitment indices and identifying the factors that
influence interannual variation. During 2014 and 2015, high
reproduction of Silver Carp in the Illinois River resulted in
these fish being vulnerable to sampling gears, allowing for com-
parisons of catch rates between gears (ACRCC 2016).
To address which sampling gears are most effective under
differing seasons and river stages, we compared fisheries gears
at sites along the Illinois River to determine the most effective
sampling approach for collecting juvenile (age 0) Silver Carp.
We de fined effectiveness as the ability of the gear type to
capture greater numbers of fish. Our objectives were to
*Corresponding author: collscot@illinois.edu
Received May 23, 2016; accepted September 19, 2016
94
North American Journal of Fisheries Management 37:94–100, 2017
© American Fisheries Society 2017
ISSN: 0275-5947 print / 1548-8675 online
DOI: 10.1080/02755947.2016.1240121

compare juvenile Silver Carp catches and the cost-effective-
ness of mini-fyke nets, beach seines, purse seines, pulsed-DC
electrofishing, and gill nets at multiple main-stem and contig-
uous backwater lake habitats along the Illinois River.
METHODS
Study design and sampling regime.—Comparisons of
sampling gear for juvenile Silver Carp were conducted within
the LaGrange pool of the Illinois River, which is known to have
high abundances of Asian carps (Sass et al. 2010;Collinsetal.
2015; ACRCC 2016). Sampling gears were evaluated at two
main-stem sites along the Illinois River and two contiguous
backwater lakes. Sampling sites were located near Beardstown,
Illinois (river kilometer [rkm] 133.6), which was paired with Lily
Lake backwater (rkm 133.7), and near Havana, Illinois (rkm
192.1), which was paired with Matanzas Lake backwater (rkm
186.7). These locations along the Illinois River are frequently
used for monitoring Asian carp populations and testing
experimental gears (e.g., Collins et al. 2015; ACRCC 2016).
Sampling events occurred twice annually in summer (July–
August) and fall (late September–October) of 2014 and 2015
after the detection of larval and postlarval Silver Carp during
monitoring efforts by multiple agencies. Sampling of paired sites
(i.e., Illinois River main stem and a nearby contiguous backwater
lake) typically occurred over a 1-week period. All gear types
were deployed concurrently at each site and deployment of gears
were interspersed so that no particular gear type blocked or
inhibited another gear. Likewise, gears were deployed in their
intended environments following standard approaches. Because
hydrology varied, so did the location of the shoreline. Yet, gears
specific to shoreline sampling were conducted at the shoreline
and those specific to deeper waters were deployed in a similar
and consistent manner. Differences in river stage during our
2-year evaluation were assessed using data from a gauging
station located near Meredosia, Illinois (U.S. Geological
Survey gauging station 05585500; Figure 1).
Description of sampling gears.—Five sampling gears were
selected for experimental evaluation. Gears sampled either
shoreline habitats (0–5mfromthebank)ordeeperwaters
(typically 10–15 m from the shoreline) of channel margins at all
sampling sites. Littoral shorelineareasweresampledusingmini-
fyke nets and beach seines; gill nets and purse seines were used to
sample deeper water areas, and electrofishing was used to sample
areas along the nearshore–offshore continua (Gutreuter et al.
1995). Mini-fyke nets (4.5- ×0.6-m lead, 0.6- ×1.2-m trap,
3-mm mesh; eight net-nights per site habitat per sample event)
have smaller mesh sizes than traditional fyke nets and are
deployed by extending the lead from shore and deploying the
trap in 0.3–1mdepth.Beachseines(10mlength,3-mmmesh;
four hauls per site habitat per sample event) also sampled shoreline
habitats in areas free from rocks or woody habitat. Beach seines
were pulled parallel to the shoreline over a 10-m distance and
retrieved to shore. Sampling with pulsed-DC electrofishing (250
V, 8 –10 A, 60 Hz, varied pulse width) consisted of four 15-min
transects per site habitat per sample event (Gutreuter et al. 1995).
Each transect was parallel to and within 5 m of the shoreline
and followed an in-and-out pattern that integrated the nearshore-
FIGURE 1. Continuous measurement of stage height of the Illinois River from 2014 to 2015 at U.S. Geological Survey gauging station 05585500, Meredosia,
Illinois.
MANAGEMENT BRIEF 95

to-offshore continuum (Gutreuter et al. 1995). Small-mesh purse
seines (122 ×3.05 m, with 2.5-cm mesh; four hauls per site
habitat per sample event) were deployed in deeper water
habitats in order to encircle any fish in the open water. One
end of the purse was buoyed, then the boat was driven in a
large circle while releasing the seine, returning to the end. The
purse line was then cinched at the bottom and then pulled into
the boat (Hayes et al. 2012). Finally, floating experimental gill
nets (45.8 m long ×3.05 m deep, consisting of 1.9-, 2.5-, 3.2-,
3.8-, and 5.1-cm mesh panels; four 4-h sets per site habitat per
sample event) were deployed in open-water habitats. All
captured fish were enumerated, and subsamples of each
species in each gear (n=30persitevisit)weremeasured
(mm TL).
Analyses.—For consistency, we limited analyses of juvenile
Silver Carp catches to age-0 individuals (<200 mm TL). No
age-1 Silver Carp were observed in 2014, and only small
numbers were captured in 2015. Previous analysis of Silver
Carp growth in the Illinois River (Stuck et al. 2015) as well as
examination of seasonal, size-frequency distributions from
multiple years of sampling (S. E. Butler, unpublished data)
indicated that age-1 Silver Carp in this system are
considerably larger than 200 mm by the summer sampling
period but that age-0 individuals do not surpass this size by
fall. We compared catches to evaluate the gears with the
highest total and mean numbers of age-0 Silver Carp
collected. Raw catch numbers were used due to difficulty in
scaling catches into rates in similar units across gears. For
instance, catches of purse seines are measured on a per haul
basis, and sampling a single replicate can take minutes. In
contrast, mini-fyke net catches also represent numbers within
a net but collect over a 24-h period. The scaling from minutes
to a full day would be mathematically feasible; however, it
could introduce biases, and metrics would not be reflective of
what is sampled in the field by fisheries scientists. Such
inherent differences are unique to each gear.
Mean catches were evaluated within a gear type to assess
the effect of sample period (i.e., summer and fall of each year)
and differences between the main-stem Illinois River and
connected backwater lake habitats using two-way ANOVA.
Differences in catch rates among habitats during sample per-
iods were assessed via the interaction of sample period and
habitat. Additionally, we tested for differences in lengths of
age-0 Silver Carp collected between gears using two-way
ANOVA. For this analysis, we compared lengths across
gears as well as evaluated the interaction of gear and habitat.
Data were log
10
transformed to meet the necessary distribu-
tional assumptions.
Because hydrology varied over time, we further compared
catches of age-0 Silver Carp over time to native shiners (Emerald
Shiner Notropis atherinoides,SpottailShinerN. hudsonius)and
centrarchids (Bluegill Lepomis macrochirus,Orangespotted
Sunfish L. humilis,BlackCrappiePomoxis nigromaculatus)in
sampling gears where these taxa are frequently collected. We
reasoned that if a reduced catch of age-0 Silver Carp corre-
sponded with a reduced catch of native fishes, hydrologic con-
ditions rendered gears inefficient. However, if age-0 Silver Carp
catch changed but catch of native species remained similar, the
pattern suggests that gears are effective, but age-0 Silver Carp
were not present in these sampling locations.
Cost effectiveness of sampling gears.—Accounting for
differences in both catch rates and effort (e.g., crew size, labor
hours) required to deploy (and recover when appropriate)
sampling gears provides a means of identifying the most
cost-effective monitoring strategy. Here, we considered the most
cost-effective gear to be the one that provided the largest catch at a
minimal expenditure of labor. Because Silver Carp catch rates
were low and inconsistent during 2015 (see Results), we used
2014 catch rates to determine effect sizes between gears. Crew
sizes (two or three individuals) and effort varied for each gear type.
The effort required to deploy or collect (i.e., remove fish) and
redeploy mini-fyke nets was estimated to be between 5 and 9 min.
Because mini-fyke nets were deployed overnight, requiring two
trips, effort was doubled to 10 and 18 min. Individual beach-seine
hauls varied between 5 and 7 min. Purse-seine hauls ranged from
10 to 15 min. Pulsed-DC electrofishing sampled 15-min transects.
For electrofishing, we added an additional 20 min to account for
preparing the boat (e.g., setup of electrical equipment, installation
of boom arms). Experimental effect sizes (i.e., differences in
overall catch rates between gears) were used to determine how
many replicates of a gear would be required to achieve similar
numbers relative to the gear with the greatest average catch rate.
Daily labor estimates in hours, LD
i
,werecalculatedas
LDi¼Gi"Ei
60
!"
"Ci;
where G
i
is the number of gears of type ithat were deployed,
E
i
is the daily effort required to use the gear, recorded in
minutes and divided by 60 to convert to hours, and C
i
is the
field crew size. Other factors such as travel times between
sites and handling–processing times of fish were not included,
as these vary by protocol, agency, and monitoring sites.
RESULTS
River stage over the 2-year experiment differed between
sampling periods (July 1–September 30) of each year
(Figure 1). River stage averaged 2.91 ±1.38 m (mean ± SD)
in 2014 and 4.02 ± 2.93 m in 2015 from July to September.
Flooding in July of 2015 resulted in river stages of 7.77 ±0.75
m, which delayed our sampling. River stage decreased to 2.56
± 0.94 m during the summer 2015 sample period (August
10–19) and 2.06 ± 0.38 m during the fall 2015 sample period
(September 20–30).
Age-0 Silver Carp comprised 37% of all fishes sampled
(N= 40,390) across all gears, sites, and habitats. Total
numbers of Silver Carp varied substantially between years
96 COLLINS ET AL.

and gear types. A total of 40,285 (99%) juvenile Silver
Carp were collected in 2014 and only 105 (<1 %) indivi-
duals in 2015 across all sites, habitats, and gears (Table 1).
Total catch of age-0 Silver Carp was greatest for mini-fyke
nets, followed in decreasing order by beach seining, purse
seining, DC boat electrofishing, and gill netting (Tabl e 1 ).
Gears that collected the greatest mean numbers of age-0
Silver Carp (i.e., mini-fyke nets and beach seines) were asso-
ciated with shoreline sampling that occurred at shallower depths
(Tab le 2). Mini-fyke nets, which collected the most Silver Carp,
were deployed at an average depth of 0.7 ± 0.4 m (mean ± SD)
over the course of the study. Average catches of juvenile Silver
Carp in mini-fyke nets were lowest during summer 2015 (Time:
F
3, 15
=5.19,P=0.02)butdidnotdifferbetweenthemain-stem
river and backwater lake habitats (Habitat: F
1, 15
=2.49,
P= 0.15), nor were there any interactions between factors
(Time ×Habitat: F
3, 15
= 2.04, P= 0.18). Catch rates of Silver
Carp in 2014 averaged 1,012 ± 2,489 fish per night in the main-
stem Illinois River and 50.1 ± 146.2 fish per night in backwater
lakes (Table 2 ). However, during 2015, mean catch rates were
substantially lower. In contrast, catch rates in mini-fyke nets did
not differ through time for centrarchids (F
3, 15
= 2.12, P= 0.17)
and shiners (F
3, 15
= 3.40, P= 0.07).
Average catches in beach-seine hauls varied across sample
periods (Time: F
3, 15
= 3.77, P= 0.05), and the greatest
numbers of age-0 Silver Carp were collected in summer
TABLE 1. Total numbers of Silver Carp collected by gear in backwater lakes and main-stem river habitats of the Illinois River in 2014 and 2015. DC-EF = pulsed-DC
electrofishing.
Habitat Site Mini-fyke net Beach seine Purse seine DC-EF Gill net
2014
Backwater lake Lily Lake 330 2,210 261 43 0
Matanzas Lake 1,276 3 200 0 0
Illinois River Havana 29,330 253 5 8 0
Beardstown 3,061 2,935 13 357 0
Total 33,997 5,401 479 408 0
2015
Backwater lake Lily Lake 0 0 0 0 8
Matanzas Lake 0 0 0 0 0
Illinois River Havana 53 1 1 38 0
Beardstown 4 0 0 0 0
Total 57 1 1 38 8
Grand total 34,054 5,402 480 446 8
TABLE 2. Catches (mean ± SD) in various gears of juvenile Silver Carp, centrarchids, and shiners in backwater lake (BW) and main-stem river (MS) habitats
of the Illinois River during 2014 and 2015. DC-EF = pulsed-DC electrofishing.
Year Habitat Mini-fyke net Beach seine Purse seine DC-EF Gill net
Silver Carp
2014 BW 50 ± 146 138 ± 538 28 ± 68 2.6 ± 4.1 0 ± 0
MS 1,012 ± 2,489 199 ± 491 1.12 ± 3.3 22 ± 83 0 ± 0
2015 BW 0±0 0±0 0±0 0 ± 0 0.5 ± 2
MS 1.7 ± 4.0 0.06 ± 0.2 0.06 ± 0.02 2 ± 4 0±0
Centrarchids
2014 BW 24 ± 20 12 ± 12 0.18 ± 0.2 11 ± 16 0.3 ± 0.4
MS 92 ± 127 18 ± 20 0±0 0.8 ± 1.0 0.1 ± 0.1
2015 BW 71 ± 42 5.8 ± 5.8 5.8 ± 5.8 21 ± 14 0.6 ± 0.3
MS 88 ± 110 5.4 ± 8.1 5.4 ± 8.1 1.4 ± 1.1 0.3 ± 0.5
Shiners
2014 BW 3.4 ± 5.9 13 ± 14 2.4 ± 4.1 1.1 ± 0.7 0 ± 0
MS 231 ± 420 44 ± 62 0.16 ± 0.1 4.1 ± 4.0 0 ± 0
2015 BW 41 ± 34 20 ± 13 20 ± 13 2.7 ± 3.3 0 ± 0
MS 569 ± 621 28 ± 24 28 ± 24 3.1 ± 2.1 0 ± 0
MANAGEMENT BRIEF 97

2014 and the fewest were captured in summer 2015. Beach-
seine hauls sampled mean depths of 0.79 ± 0.21 m and caught
199 ± 491 fish per haul in the main channel and 138 ± 538 fish
per haul in backwater lakes (Habitat: F
1, 15
= 0.12, P= 0.73),
and no interactions were detected (Time ×Habitat: F
3, 15
=
0.13, P= 0.93). Catch per haul of centrarchids (F
3, 15
= 5.82,
P= 0.03) and shiners (F
3, 15
= 5.80, P= 0.02) varied through
time, and lower catches occurred during fall sampling events
than during summer periods. For instance, centrarchid catch
per haul ranged from 9.1 to 18.2 during summers 2014 and
2015 and were 0.2–0.5 during fall. For shiners, catch per haul
averaged 2.0 in fall of 2014, which was greatly lower than in
all other sampling events (catch per haul range: 15.1–47.7).
Catch rates of age-0 Silver Carp were typically lower in
gears that targeted deeper water habitats farther from the
shoreline. In terms of catch rates, purse seines and DC elec-
trofishing produced similar numbers (Table 2), but effective-
ness varied by habitat. Purse-seine hauls were conducted at
mean depths of 2.2 ± 1.02 m. Mean catch rates of purse seines
differed across sample periods (time: F
3, 15
= 161.04, P<
0.001) and between main-stem Illinois River and backwater
lakes (habitat: F
1, 15
= 76.43, P< 0.001), and had the highest
catches within backwater lakes during summer 2014 (time ×
habitat: F
3, 15
= 76.77, P< 0.001). Purse seines collected far
greater numbers of Silver Carp in backwater lakes than in
main-channel habitats, but only during summer 2014.
Because electrofishing samples fish along the nearshore–off-
shore continuum, depths ranged between 0.7 and 4.9 m.
Unlike purse seines, no differences were detected across sam-
ple periods (Time: F
3, 15
= 1.17, P= 0.37), between main-stem
river and floodplain lake habitats (Habitat: F
1, 15
= 1.56, P=
0.24), or in their interaction (Time ×Habitat: F
3, 15
= 0.98, P=
0.44). Gill nets were set at a mean depth of 2.3 ± 0.9 m.
Because so few Silver Carp, centrarchids, and shiners were
collected in gill nets statistical analysis for gill-net data was
not warranted.
Total lengths of age-0 Silver Carp varied across gear types
(Figure 2;F
4, 29
= 2.81, P= 0.04), but only between gill nets
and beach seines (Tukey’s honestly significantly different test:
P= 0.05), which collected the largest and smallest individuals,
respectively. In general, gill nets collected larger age-0 Silver
Carp. Lengths of Silver Carp did not differ between backwater
lake and main-stem river habitats (F
1, 29
= 0.23, P= 0.54).
Cost-effectiveness in terms of hours of labor varied widely
across gears (Table 3). Based on comparisons of overall mean
catch rates in 2014, mini-fyke nets collected the largest num-
bers of Silver Carp. A comparison of effect sizes relative to
mini-fyke-net catch rates indicated between three and four
hauls or transects would be required to achieve similar catches
with other gears (Table 3). Consequently, the labor required to
achieve these catches ranged from as low as 0.52 labor-hours
for beach seining to as high as 45 labor-hours for pulsed-DC
electrofishing. In general, when accounting for the labor
invested, mini-fyke nets and beach seines were similar and
had overlapping daily labor estimates. In contrast, using gears
deployed in deeper waters to achieve similar catches would
require considerably greater levels of effort. Because gill nets
often produced no fish per set, cost-effectiveness could not be
estimated (i.e., multiplying effect sizes by zero).
DISCUSSION
Findings from our experimental gear evaluation indicated
that mini-fyke nets were the most effective at collecting large
numbers of age-0 Silver Carp, followed in decreasing order by
beach seines, pulsed-DC electrofishing, purse seines, and gill
nets. However, when we considered effort for each gear type
to achieve similar catches, both beach seining and mini-fyke
nets had similar and overlapping labor expenditures. Our
inferences are limited to Silver Carp, as they comprised
more than 99% of the juvenile Asian carps collected during
the 2 years. Mini-fyke nets captured by far the highest num-
bers of age-0 Silver Carp. Beach seines also captured large
FIGURE 2. Total lengths (mm) of age-0 Silver Carp captured in each sam-
pling gear in main-stem river (MS) and backwater lake (BW) habitats in the
Illinois River during 2014 and 2015. The vertical line within each box
represents the mean. The ends of the box represent the 25th (left) and 75th
(right) quantiles. Whisker lines represent the upper and lower data point
values, but do not include outliers. Jitter points indicate outliers.
TABLE 3. Estimates of daily hours of labor required to collect similar catches
of juvenile Silver Carp between sampling gears, based on the crew size, effort
per gear, and effect sizes. Due to limited and often zero catches of Silver Carp
per set, data from gill nets were not analyzed.
Gear
Crew
size
Minutes of
effort
Effect
size
Daily labor
hours
Mini-fyke net 2 10–18 1.0 0.3–0.6
Beach seine 2 5–7 3.1 0.5–0.7
Purse seine 2 10–15 35.5 11.8–17.7
DC electrofishing 3 35 41.7 45.1
98 COLLINS ET AL.

numbers of age-0 Silver Carp and may be useful for rapid
monitoring purposes when overnight gear sets are not practi-
cal. In terms of catches of Silver Carp, mini-fyke nets out-
performed beach seines in a manner consistent with other
evaluations for different species and aquatic environments
(Lyons 1986; Clark et al. 2007). Both gears primarily target
shallow-water habitats (<1 m), but mini-fyke nets collected a
broader range of lengths of juvenile Silver Carp than did
beach seining. Other evaluations indicate mini-fyke nets col-
lect a greater range of species as well (Clark et al. 2007),
which is relevant if management objectives are to determine
differences in growth and size structure during monitoring
efforts or to avoid nondetections of uncommon species. Both
mini-fyke nets and beach seines are well suited for sampling
littoral locations of the Illinois River and backwater lakes,
shallow creeks, and flooded off-channel habitats.
Cost-effectiveness in terms of daily labor hours was similar
between beach seine hauls and mini-fyke net sets. When
controlling for catch rates but accounting for labor, both
gears were relatively similar. Greater numbers of fish were
caught in mini-fyke nets, but this method was more time
consuming to deploy and collect the gear in comparison with
beach seine hauls, which required greater numbers of hauls
but could be done relatively quickly. Instances where multiple
trips to the same site are required would result in either gear
being appropriate. However, if overnight gear sets are not
desirable, a more cost-effective use of time and labor would
be to sample a location via beach seining more frequently.
Neither purse seines nor gill nets were cost-effective because
of the higher labor cost relative to the number of fish captured.
Despite having similar, though slightly higher time require-
ments, the overall reduction in catch rates was the primary
driver of observed differences.
We ob s e r v e d diverg e n c e between c a t c h r ates of age - 0 S i l ver
Carp and native fishes. Because catches of native fishes were
consistent across sample periods, our findings indicated that
mini-fyke nets were sampling other fishes but not Silver Carp.
The reasons why catch rates of Silver Carp were low during
summer 2015 remain unclear, but catches may have been impacted
by multiple processes. For instance, Silver Carp may have been
occupying different habitats following high water conditions that
occurred in summer 2015, whereas shiners and juvenile centrarch-
ids were constrained to shoreline environments. However, this
assumes that processes such as recruitment, growth, maturation,
out-migration, and survival of Silver Carp were similar between
years, none of which did we evaluate in this study. Efforts to track
and monitor Silver Carp during or following higher river stages
might require alternate means of sampling. For instance, although
pulsed-DC electrofishing was generally ineffective relative to
other gears during most sampling periods, it was slightly better
than mini-fyke nets in main-stem habitats during summer 2015.
Effective immobilization of fish via electrofishing is closely
linked to body size (e.g., Dolan and Miranda 2003), as larger
fish are easier to immobilize than are small ones because less
power is required (e.g., Anderson 1995). Although large num-
bers of juvenile Silver Carp were visually observed by field
personnel in close proximity to the operating electrofishing
boat on some site visits, this was not reflected in catches made
by this gear type. For perspective, pulsed-DC electrofishing
accounted for only 1% of all age-0 Silver Carp collected
during our 2-year experiment. Electrofishing settings followed
long-term resource monitoring program protocols (Gutreuter
et al. 1995), which may work well for large-bodied fishes;
however, it is apparent that these same settings were ineffec-
tive at sampling small-bodied fishes such as age-0 Asian carps
and that these settings for sampling juvenile Asian carps may
need further assessment.
Recommendations
Based on total and mean catches, we recommend the
deployment of mini-fyke nets for sampling age-0 Silver
Carp. However, this requires two trips: first to deploy the
net, then to collect the net and samples. Despite the added
effort, mini-fyke nets deployed overnight in shallow (<1 m)
littoral margins of the Illinois River provide the best chance to
collect large numbers of age-0 Silver Carp. Characteristics of
sampling locations such as slope and vegetation can affect
their ability to capture fish. Beach seines are also an inexpen-
sive sampling tool but have limited effectiveness where wood
structure, vegetation, or steep slope limit usage. Yet, beach
seines can be used to rapidly determine habitat use by fish and
to sample diets. Purse seines can be a useful tool for poten-
tially estimating fish densities; however, this gear is primarily
effective when deployed in off-channel and backwater lake
habitats where flows are limited. In contrast, pulsed-DC elec-
trofishing collected more age-0 Silver Carp in main-stem
environments, but additional work to determine optimal set-
tings for collecting juvenile Asian carps is required.
ACKNOWLEDGMENTS
This study was supported by the Great Lakes Restoration
Initiative, with funding administered through the Illinois
Department of Natural Resources (CAFWS-93). We thank K.
Irons and M. O’Hara of the Illinois Department of Natural
Resources for their assistance in coordinating this project. We
are grateful to the numerous individuals who have provided
field assistance to make this project possible, including the
graduate students and staff of the Kaskaskia and Sam Parr
Biological Stations, Illinois Natural History Survey, and the
University of Illinois.
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