Switching Bait as a Method to Improve Freshwater Turtle
Capture and Recapture Success with Hoop Net Traps
Ivana Mali, Donald J. Brown, Melissa C. Jones, and Michael R.J. Forstner
Southeastern Naturalist, Volume 11, Issue 2 (2012): 311–318
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2012 SOUTHEASTERN NATURALIST 11(2):311–318
Switching Bait as a Method to Improve Freshwater Turtle
Capture and Recapture Success with Hoop Net Traps
Ivana Mali1,*, Donald J. Brown1, Melissa C. Jones1, and Michael R.J. Forstner1
Abstract - We surveyed freshwater turtles at sites in the Lower Rio Grande Valley and
Bastrop Lost Pines ecoregions of Texas annually since 2008 and 2009, respectively,
and found that captures and recaptures per unit effort (CPUE and RPUE, respectively)
decreased annually. In 2011, we tested whether or not switching the type of bait used to
attract turtles affected CPUE and RPUE. Under the assumption that bait preferences affect
capture probabilities both among and within species, we hypothesized that switching
bait would increase CPUE in 2011 by attracting individuals not captured in previous
years. We also hypothesized that low recapture success in previous years was due to
an olfactory-induced trap-shy response. We tested this hypothesis by determining if
RPUE increased when we switched the type of bait used to attract turtles. We found that
switching from fish-based bait to red meat significantly increased CPUE, but not RPUE,
for Trachemys scripta elegans (Red-eared Slider). We also found weak evidence that
Apalone spinifera emoryi (Texas Spiny Softshell) preferred red meat over fish-based bait.
The results of this study indicate that switching bait can be an effective way to maximize
CPUE across multiple years when monitoring freshwater turtles using baited hoop nets.
However, switching bait did not affect RPUE, which indicates that the apparent trap-shy
behavior of turtles in our study areas is not driven by an olfactory-induced response to
the type of bait used.
Capture-recapture sampling is one of the most widely used techniques for
monitoring demographic components of wildlife populations (Nichols 1992). A
major assumption of this method is that all individuals in a population at the time
of sampling have the same probability of capture (Carothers 1979, Koper and
Brooks 1998). Post-capture changes in animal behavior can bias demographic
estimates (Carothers 1979, Feldhamer and Maycroft 1992, Nichols et al. 1984).
These behavioral changes are commonly referred to as “trap-happy” responses
(i.e., probability of recapture increases relative to probability of initial capture
[Chao et al. 2004, Deforce et al. 2004]) and “trap-shy” responses (i.e., probability
of recapture decreases relative to probability of initial capture [Brocke 1972,
In addition to potential biases introduced through post-capture behavioral
changes, sampling tools can inherently select for certain segments or individuals
in a population. For instance, the two most common sampling tools for freshwater
turtles are hoop nets and basking traps (Koper and Brooks 1998, Ream
and Ream 1966), and hoop nets have been shown to be inherently male-biased
(Ream and Ream 1966). Despite this, hoop nets are probably the most commonly
1Department of Biology, Texas State University-San Marcos, 601 University Drive, San
Marcos, TX 78666. *Corresponding author - firstname.lastname@example.org.
312 Southeastern Naturalist Vol. 11, No. 2
used sampling method for freshwater turtles (Davis 1982, Lagler 1943, Thomas
et al. 2008). Hoop nets are typically baited, with the type of bait chosen based
on species-specific preferences (Ernst 1965, Jensen 1998, Thomas et al. 2008).
Bait is usually placed in closed containers with numerous holes to allow scent
dispersal while eliminating the potential for bait consumption (Lagler 1943, Nall
and Thomas 2009).
We are aware of four studies that examined the efficiency of different bait
types used for hoop-net sampling of freshwater turtles (Ernst 1965, Jensen 1998,
Thomas et al. 2008, Voorhees et al. 1991). Ernst (1965) found that turtles were
most attracted to sardines among six types of bait. Voorhees et al. (1991) used
seventeen different types of bait and found that bait with jelly-like fluid (fresh
mussel, canned creamed corn, and canned sardines) was the most successful in
capturing nine species of freshwater turtles. Jensen (1998) found different bait
preferences for Macrochelys temminckii Troost (Alligator Snapping Turtle) and
Trachemys scripta elegans Wied-Neuwied (Red-eared Slider), with Alligator
Snapping Turtles preferring fresh fish and Red-eared Sliders preferring fresh
chicken entrails. Thomas et al. (2008) found that freshwater turtles preferred
frozen fish and canned mackerel over creamed corn. In addition, Deforce et al.
(2004) noted a “trap-happy” behavior of Phrynops gibbus Schweigger (Gibba
Turtle) towards hoop nets baited with chicken meat.
We have surveyed freshwater turtles annually in the Lower Rio Grande Valley
(LRGV) ecoregion of Texas since 2008 and in the Bastrop Lost Pines ecoregion
of Texas since 2009. We found that captures and recaptures per unit effort (CPUE
and RPUE, respectively) decreased annually (Fig. 1). Unfortunately, allowing
for long periods of time between re-sampling (ca. 1 year) did not mitigate this
Figure 1. Mean capture per unit effort (CPUE, ––●––) and recapture per unit effort
(RPUE, – –▲– –) of Trachemys scripta elegans (Red-eared Slider) at ponds (n = 4) in
the Lower Rio Grande Valley (LRGV) of Texas trapped annually since 2008 using hoop
nets baited with sardines.
2012 I. Mali, D.J. Brown, M.C. Jones, and M.R.J. Forstner 313
perceived trap aversion. In 2011, we tested whether or not switching the type of
bait used to attract turtles affected CPUE and RPUE. Under the assumption that
bait preferences affect capture probabilities both among and within species, we
hypothesized that switching bait would increase CPUE in 2011 by attracting individuals
not captured in previous years. We also hypothesized that low RPUE in
previous years was due to an olfactory-induced trap-shy response. We tested this
hypothesis by determining if RPUE increased when we switched the type of bait
used to attract turtles.
We conducted this study using 15 ponds that were surveyed for multiple consecutive
years as part of a statewide assessment of freshwater turtle populations in
Texas. Eleven ponds were located in the LRGV in south Texas (Cameron, Hidalgo,
and Willacy counties) and contained Red-eared Sliders and Apalone spinifera emoryi
Agassiz (Texas Spiny Softshell). Four ponds were located in the Bastrop Lost
Pines ecoregion in central Texas (Bastrop County) and contained Red-eared Sliders
and Chelydra serpentina L. (Common Snapping Turtle). Additional information on
the study areas can be found in Brown et al. (2011a, b, c).
Of the 15 ponds used in this investigation, we trapped six annually from 2008
to 2011 (including two that were not trapped in 2010), seven since 2009, and two
since 2010. We trapped all ponds during the summer months when the turtles
were likely to be most active (Thomas et al. 1999). We used 76.2-cm-diameter
single-opening, single-throated, widemouth hoop nets with a 2.54-cm mesh
size and four hoops per net (Memphis Net and Twine Co., Memphis, TN). We
stretched the nets open using two wooden posts connected to the first and last
hoop. We attempted to keep the locations within ponds and total area trapped
consistent among years.
We used exclusively fish-based bait (primarily canned sardines) between 2008
and 2010 and exclusively red meat from beef brisket in 2011. The ponds that were
initially trapped in 2010 were exclusively used for comparison between 2011 and
the first year the sites were trapped using sardines and excluded from other analysis.
In all years, we replaced the bait every two days. Annual trapping intensity
varied among years and among sites, depending on study goals in a given year
(see Brown et al. 2011a, b). In 2011, we completed 40 trap days (either 40 traps
set for one day or 20 traps set for two days) at each site except one, where we
completed 20 trap days (10 traps set for two days). Although we acknowledge
that annual differences in trap days could bias our CPUE comparisons, we found
in previous research that CPUE in these study areas was comparable if more than
10 trap days were completed (Brown et al. 2011b), which was also the case for
all sites and years in this study.
We individually marked hardshell turtles using the numbering system of
Cagle (1939) and a portable rotary tool (Dremel, Racine, WI). We marked softshell
turtles by engraving individual numbers on the posterior end of the carapace
314 Southeastern Naturalist Vol. 11, No. 2
using the same rotary tool. We determined sex using secondary sexual characteristics
(Conant and Collins 1998, Gibbons and Lovich 1990).
We used paired randomization tests with 10,000 iterations for all comparisons
of CPUE and RPUE between years. The P-values in these tests represent the proportion
of trials resulting in capture differences as great as or greater than those
obtained (Sokal and Rohlf 1995). Thus, a small P-value means that it is unlikely
our results were obtained by random chance given the inherent distribution of the
data. For each species, we analyzed only those sites that corresponded with their
geographic distribution. Red-eared Sliders were found in both study areas, Texas
Spiny Softshells were found only in the LRGV, and Common Snapping Turtles
were found only in the Bastrop Lost Pines ecoregion, with the exception of a single
individual representing a Hidalgo County record in 2009 (Dickerson et al. 2009).
For Red-eared Sliders, we first tested if there was a significant decrease in
CPUE between the first year and the last year the site was trapped with sardines
(n = 13). This analysis was used to determine whether or not CPUE decreased
over time. We then tested if CPUE differed between 2011 and the first year the
site was trapped (n = 15), and whether CPUE differed between 2011 and the last
year the site was trapped prior to 2011 (n = 13). These analyses were used to
determine if switching bait affected CPUE. If CPUE in 2011 increased relative
to the first year the sites were trapped, that would indicate a species-level bait
preference for the new type of bait. However, if CPUE decreased significantly
in 2011 there would not be strong evidence for species-level bait preference for
the original bait, given the negative impact of trap-shy behavior on CPUE in
subsequent years. If CPUE in 2011 increased relative to the last year the sites
were trapped, but not relative to the first year the sites were trapped, that would
indicate that bait preference varied intraspecifically. For the first and last comparisons,
we excluded two sites because they were first trapped in 2010.
For Red-eared Sliders, we also determined if RPUE significantly decreased
in consecutive years prior to 2011. A significant decrease would indicate trapshy
behavior. For this analysis, we used the study sites that were trapped every
year since 2008 (n = 4). We then determined if Red-eared Slider RPUE differed
between 2011 and the prior year the site was trapped (n = 12). If RPUE increased
in 2011, that would indicate that turtles become trap-shy due to a negative olfactory
response associated with hoop nets. For this analysis we excluded the two
sites that were initially trapped in 2010, as well as one site that was first trapped
in 2009, because no Red-eared Sliders were captured. For Texas Spiny Softshells
(n = 11) and Common Snapping Turtles (n = 4), we only tested CPUE due to
small sample sizes and low RPUE.
We inferred statistical significance at α = 0.05. However, because of the
relatively small sample sizes, we considered α = 0.1 to indicate a result that was
nearly significant and thus potentially biologically meaningful. We conducted
statistical analyses using program R 2.7.2 (The R Foundation for Statistical Computing,
Vienna, Austria). We calculated CPUE and RPUE using the following
CPUE = # captures / # trap days
RPUE = (# recaptures / # marked individuals from previous years) / # trap days
2012 I. Mali, D.J. Brown, M.C. Jones, and M.R.J. Forstner 315
For this study, one trap day was defined as one trap in the water for 24 hours. Note
that RPUE explicitly accounted for differences in number of marked individuals
at the beginning of each year.
Mean CPUE was 0.31 the first year sites were trapped and 0.09 the last year
sites were trapped using sardines; this decrease was significant (P < 0.001, n =
13; Fig. 2, Table 1). Mean CPUE was 0.19 in 2011 and 0.28 the first year each
site was trapped. Although mean CPUE decreased, the difference between the
two years was not significant (P = 0.12; n = 15). However, we found that CPUE in
2011 increased relative to the previous year the sites were trapped (mean = 0.21
and 0.09, respectively; P < 0.001, n = 13). Mean RPUE was 0.0007 in 2009 and
0.0001 in 2010; this decrease was significant (P < 0.05). Mean RPUE was 0.0016
in 2011 and 0.0015 the previous year the sites were trapped; this difference was
not significant (P = 0.44).
Texas Spiny Softshell and Common Snapping Turtles
For Texas Spiny Softshells, mean CPUE was 0.04 in 2011 and 0.01 the first year
each site was trapped; this increase was nearly significant (P = 0.07; Table 1). For
Common Snapping Turtles, mean CPUE was 0.03 in 2011 and 0.06 the first year
each site was trapped; this decrease was also nearly significant (P = 0.09).
Figure 2. Mean capture per unit effort (CPUE, ––●––) and recapture per unit effort
(RPUE, – –▲– –) of Trachemys scripta elegans (Red-eared Slider) at ponds in the
Lower Rio Grande Valley (LRGV) and Bastrop Lost Pines ecoregions of Texas during
the first and last year sites were trapped with hoop nets using fish as bait, and in 2011,
when sites were trapped with hoop nets using red meat as bait.
316 Southeastern Naturalist Vol. 11, No. 2
Table 1. Captures-per-unit-effort (CPUE) and recaptures-per-unit effort (RPUE) for Trachemys
scripta elegans (Red-eared Slider) and Apalone spinifera emoryi (Texas Spiny Softshell) at 11
ponds in the Lower Rio Grande Valley (LRGV) of Texas and Trachemys scripta elegans (Red-eared
Slider) and Chelydra serpentina (Common Snapping Turtle) at four ponds in the Bastrop Lost
Pines ecoregion of Texas. We trapped turtles using baited hoop nets in the summer months from
2008 through 2011 in the LRGV and 2009 through 2011 in the Bastrop Lost Pines ecoregion. We
used fish-based bait (primarily canned sardines) from 2008 to 2010. We used red meat in 2011 to
determine if switching the type of bait used affected CPUE and RPUE. Note that not all sites were
included in each statistical analysis, and thus not all mean values are identical to those given in the
Captures-per-unit-effort (CPUE)A Recaptures-per-unit-effort (RPUE)B
Study area 2008 2009 2010 2011 2009 2010 2011
LRGV 0.218 0.066 0.062 0.250 0.0001 0 0
LRGV 0.391 0.054 0.075 0.200 0 0.0002 0
LRGV 1.440 0.460 0.150 0.250 0.0006 0.0002 0.0004
LRGV 0.450 0.120 0.025 0.375 0.0600 0.0250 0
LRGV 0.036 0.005 - 0.200 0 - 0
LRGV 0.055 0.226 - 0.475 0 - 0.0020
LRGV - 0.720 0.225 0.175 - 0.0004 0.0005
LRGV - 0.160 0.062 0.100 - 0.0015 0
LRGV - 0.280 0.200 0.350 - 0 0
LRGV - 0.154 0.038 0.300 - 0 0.0019
LRGV - - 0.125 0 - - 0
Lost Pines - 0 0.038 0.025 - - 0
Lost Pines - 0.114 0.038 0 - 0.0003 0
Lost Pines - 0.020 0.038 0.075 - 0.0125 0.0083
Lost Pines - - 0.050 0.075 - - 0
Mean 0.432 0.183 0.086 0.190 0.0101 0.0040 0.0009
Texas Spiny Shell
LRGV 0.081 0.033 0.025 0.050 0 0 0
LRGV 0.023 0.005 0 0.025 0 0 0
LRGV 0 0 0 0 - - -
LRGV 0 0.060 0.025 0 - 0 0
LRGV 0 0 - 0 0 - 0
LRGV 0 0 - 0 - - -
LRGV - 0.020 0.100 0.200 - 0 0
LRGV - 0 0 0 - - -
LRGV - 0 0.038 0.150 - - 0
LRGV - 0 0.013 0 - - 0
LRGV - - 0.038 0 - - 0
Mean 0.017 0.012 0.038 0.039 0 0 0
Common Snapping Turtle
Lost Pines - 0.024 0 0.025 - 0 0
Lost Pines - 0.029 0 0 - 0 0
Lost Pines - 0.143 0.013 0.075 - 0 0.100
Lost Pines - - 0.038 0 - - 0
Mean - 0.065 0.013 0.025 - 0 0.025
ACPUE = # captures/ # trap days
B RPUE = (# recaptures/ # marked individuals from previous years) / # trap days
2012 I. Mali, D.J. Brown, M.C. Jones, and M.R.J. Forstner 317
The results of this study indicate that switching bait can be an effective way
to maintain high capture-rates in long-term freshwater turtle investigations using
baited hoop nets. The increase in CPUE for Red-eared Sliders in 2011 relative
to the previous year the site was trapped indicates that within-species bait preferences
influence capture probabilities. Thus, maintaining baiting consistency
when using CPUE as a metric for comparing relative abundance differences
among sites could be important. In terms of species-level responses, we did not
find significant bait preferences for any of the species. However, there was weak
evidence that Texas Spiny Softshells preferred red meat over fish. Our study indicated
that Red-eared Sliders develop a trap-shy response to baited hoop nets,
but the trap-shy response did not appear to be olfactory-induced. Thus, it may be
that turtles in our study areas develop a negative visual association with the hoop
nets. If so, switching the type of trap used could increase RPUE, and we intend
to test this hypothesis in the future.
In conclusion, the integration of capture-recapture methods for freshwater turtles
using baited hoop nets remains challenging in our study areas. Unfortunately,
it is only possible to census ponds (i.e., obtain N) if they are pumped dry and turtles
are noodled, which in most situations is both logistically impractical and prohibitively
expensive. Previous investigators have suggested that the optimal way to
maximize CPUE and RPUE is to use multiple sampling tools (Koper and Brooks
1998, Ream and Ream 1966). Unfortunately, different sampling tools have different
inherent biases associated with them, and thus using a combination of sampling
tools could introduce additional uncertainty in capture-recapture estimates.
We thank J.R. Dixon, D. Wallace, B. DeVolld, B. Dickerson, A. Schultz, J. Flores, J.
Tokarz, J. Duvall, J. Duvall-Jisha, R. Holihan, and J. Gaertner for assistance in checking
traps and data collection. We also thank M. Pons, Jr. and the Nature Conservancy of
Texas for allowing us to reside at Southmost Preserve and use the preserve for this study.
Thanks to the Boy Scouts of America, Texas Parks and Wildlife Department, and private
agencies and landowners for allowing us to trap turtles on their properties. This study
was funded by the Texas Parks and Wildlife Department through the State Wildlife Grant
program and Texas State University-San Marcos through a research scholarship awarded
to D.J. Brown. We conducted this research under Texas Parks and Wildlife Department
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