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22001144 NORTHEASTERN NATURALIST V2o1l.( 12)1:,1 N2–o2. 21
The Influence of Recreational Crabbing Regulations on
Diamondback Terrapin By-catch
Amy J. Upperman1, Timothy M. Russell1, and Randolph M. Chambers1,*
Abstract - Malaclemys terrapin terrapin (Northern Diamondback Terrapin) is susceptible
to drowning in commercial-style pots used for the Callinectes sapidus (Blue Crab) fishery.
Regulations to reduce by-catch mortality vary by state. We compared three different regulatory
strategies with respect to crab catch and their relative effectiveness at reducing terrapin
by-catch. To mimic their possible use by recreational crabbers, we grouped and fished
together ten unbaited crab pots with no by-catch reduction devices (BRDs), ten with large
BRDs, and ten with small BRDs in a tidal creek in southeastern Virginia. Over 24 sampling
days, the total legal crab catch (crabs ≥ 12.7 cm) in pots with no BRDs (29.9 ± 10.0 SD
crabs pot-1) and large BRDs (27.9 ± 6.2 crabs pot-1) was significantly greater than catch in
pots with small BRDs (14.0 ± 5.8 crabs pot-1). Legal crabs varied in average size from 14.1
± 1.2 cm, 14.0 ± 1.2 cm, and 13.7 ± 1.0 cm from pots with no BRDs, large BRDs, and small
BRDs, respectively, but these differences were not significant. Of a by-catch of 71 terrapins,
69 were from pots with no BRDs, 2 from pots with large BRDs, and none from pots with
small BRDs. The potential mortality of terrapins in pots without BRDs would have reduced
the population in this tidal creek by 42% in just 24 days. Based on these results, regulations
requiring the use of large BRDs come closest to the objective of reducing by-catch
mortality of terrapins without a large effect on crab capture in Virginia. Both recreational
crabbing and commercial crabbing with no BRDs on pots in terrapin habitat can contribute
to declines in local terrapin populations.
Introduction
Malaclemys terrapin terrapin (Schoepff) (Northern Diamondback Terrapin)
lives in estuaries along the Atlantic coast of the US. As the only exclusively
brackish turtle species in North America, the terrapin is recognized as a potential
keystone predator in tidal marshes (Silliman and Bertness 2002), where terrapin
consumption of snails and other invertebrates (Butler et al. 2012, Tucker et al.
1995, Whitelaw and Zajac 2002) may contribute to top-down control of herbivore
populations grazing on marsh grasses.
Throughout its range, Northern Diamondback Terrapin habitat overlaps a large
commercial and recreational fishery for Callinectes sapidus Rathbun (Blue Crab).
In the northeast, the crab fishery is more intense in Virginia (VA), Maryland (MD),
Delaware (DE), New Jersey (NJ), and New York (NY) relative to Connecticut (CT),
Rhode Island (RI), and Massachusetts (MA), where crabbing with commercialstyle
crab pots is not allowed. Numerous studies have documented the mortality
of juvenile and adult terrapins as by-catch in crab pots (Bishop 1983, Dorcas et al.
1Keck Environmental Lab, College of William and Mary, Williamsburg, VA 23187.
*Corresponding author - rmcham@wm.edu.
Manuscript Editor: Todd Rimkus
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A.J. Upperman, T.M. Russell, and R.M. Chambers
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2007, Gibbons et al. 2001, Grosse et al. 2009, Roosenburg 2004, Wood 1997). The
mortality of adult female terrapins is particularly worrisome since turtle population
models indicate that survival of adult females is more critical to population maintenance
than, for example, reproductive output and recruitment of hatchlings (Heppell
et al. 1999, Mitro 2003).
In response to the demonstrated negative impacts of the crab fishery on Diamondback
Terrapins, regulations to reduce by-catch mortality have been enacted in
some states (Table 1). The regulations require the use of by-catch reduction devices
(BRDs), though the sizes of the BRDs vary by state. Further, the regulations target
either within-state geographic areas where commercial and recreational crabbing
overlap with terrapin habitat (NJ, NY) or recreational crabbers specifically (MD,
DE). No BRD regulations are in place in VA. Whereas in most states the majority
of commercial crabbers set pots in open-water portions of estuaries, bait and check
them daily, and do not overlap significantly with terrapin habitat (Hart and Crowder
2011), recreational crabbers typically set their pots in nearshore waters around private
docks and may not bait and check pots on a daily basis. Because of their usual
nearshore location in terrapin habitat and the demonstrated by-catch of terrapins
even in unbaited pots (Hoyle and Gibbons 2000, Morris et al. 2011), recreational
crab pots have been a regulatory focus.
Experimental results from studies testing the relative merits of different BRD
regulatory strategies on both crab catch and terrapin by-catch have been equivocal
(Hart and Crowder 2011, Roosenburg 2004). Some studies conclude the use of
BRDs reduces terrapin mortality without affecting crab catch significantly (Butler
and Heinrich 2007, Morris et al. 2011, Rook et al. 2010, Roosenburg and Green
Table 1. Review of state regulations for by-catch reduction devices (BRDs) on commercial-style crab
pots, giving BRD size, whether regulations apply to commercial and recreational crabbing, and notes
on the specific locations where or user groups to whom the regulations apply. Com. = commercial,
rec. = recreational
State BRD Size Com. Rec. Notes
New YorkA 5 cm x 15.2 cm (2” x 6”) Yes Yes Large BRD: Only in waters listed
by NY Department of Environmental
Conservation
New JerseyB 5 cm x 15.2 cm (2” x 6”) Yes Yes Large BRD: Only in waters less than 150
ft from shoreline to shoreline
at mean low water
DelawareC 4.5 cm x 12 cm (1-3/4” x 4-3/4”) No Yes Small BRD: Recreational crabbing
only
MarylandD 4.5 cm x 12 cm (1-3/4” x 4-3/4”) No Yes Small BRD: Recreational crabbing
only
VirginiaE N/A N/A N/A No BRD: Not required on any
pots
Ahttp://www.dec.ny.gov/regs/4011.html
Bhttp://www.state.nj.us/dep/fgw/pdf/2012/comregs12.pdf
Chttp://www.dnrec.delaware.gov/fw/Fisheries/Documents/rec%20crab%20pot%20summary.pdf
Dhttp://www.dnr.state.md.us/fisheries/regulations/crabpot-trd/crabpot.asp
Ehttp://www.mrc.state.va.us/regulations/recfish&crabrules.shtm
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2014 Vol. 21, No. 1
2000), but another observed a significant decrease in crab catch (Cole and Helser
2001). Some of these studies have been completed in different estuaries and often
have used different-sized BRDs, thereby making comparisons between them difficult.
Roosenburg (2004) suggested that geographic variation in crab and terrapin
size could influence the results of BRD studies and create the need for different
state-by-state regulations.
For the present study, our objective was to test the influence of different
regulatory strategies employed by different states on the capture of crabs and the
by-catch of terrapins. Our goal was to determine the relative impacts of the use of
no BRDs with the use of two sizes of BRDs affixed to pots for recreational crabbing.
We completed a grouped comparison of the capture of crabs and by-catch
in pots employing these three different regulatory strategies and use the results to
make recommendations for BRD implementation in VA.
Field Site Description
The study was completed in Felgates Creek (37.2667°N, 76.5850°W), a tributary
to the York River sub-estuary of Chesapeake Bay in southeastern VA (Fig. 1).
Felgates Creek lies entirely within the boundary of the York River Naval Weapons
Station military installation, and thus public access has been restricted for
decades. As a result, no commercial or recreational crabbing occurs in Felgates
Creek. Vegetation in the 105-ha wetland-creek complex is dominated by Spartina
alterniflora (Saltmarsh Cordgrass), and the surrounding upland forest is
mostly pine and oak. The tidal range is 1.3 m. The most significant predators of
Diamondback Terrapins in Felgates Creek include Procyon lotor (L.) (Raccoon)
that depredate terrapin nests, and Haliaeetus leucocephalus (L.) (Bald Eagle) and
Lontra canadensis (Schreber) (North American River Otter) that prey on juvenile
and smaller adult turtles (R.M. Chambers, pers. observ.; Clark 1982). The salinity
varied from 24 ppt to 12 ppt across ten sampling sites chosen to reflect a potential
range in environmental conditions throughout which terrapins were known to occur,
with three sites located near the mouth of Felgates Creek into the York River,
four sites located near a bridge 1.5 km upstream from the mouth, and three sites
located 2.1 km upstream from the mouth (Fig. 1).
Methods
We completed data collection over 24 non-consecutive sampling days between
6 June and 26 July 2012. At each of the 10 sampling sites, we placed three commercial-
style crab pots (60 cm x 60 cm x 60 cm)—one from each experimental
treatment group—approximately 5 m apart. “No BRD” pots were not fitted with
any BRDs. “Large BRD” pots were fitted with 5-cm x 15.2-cm plastic, rectangular
BRDs affixed to each of the four funnel openings to the pots. “Small BRD” pots
were fitted with 4.5-cm x 12-cm plastic BRDs of the same material. The pots were
not fitted with cull rings, which are 6-cm circular openings in the sides of pots used
by commercial crabbers to allow smaller crabs to escape capture. We modified all
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2014
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30 pots with chicken wire “chimneys” (height = 120 cm) attached to an opening
cut into the top corner of each crab pot, allowing terrapins access to the surface if
captured in the crab pots. Chimneys were held in place by an elastic cord attached
to a wooden stake driven into the creek bed. We assumed any effect of chimneys or
stakes was equal among all three treatment groups.
To determine crab catch and by-catch in untended pots (mimicking pots checked
infrequently by recreational crabbers and derelict pots lost by commercial crabbers),
we deliberately did not bait the pots. On each sampling day, sampling of crabs
and by-catch was completed 20–28 hours after setting the pots. The number and
sex of both legal (point-to-point carapace width greater than 12.7 cm) and sublegal
crabs were recorded in each pot per site, and all crabs were released. We identified
Figure 1. Map of Felgates Creek in southeastern Virginia showing the general clustering of
sampling sites near the mouth (1–3), around the bridge (4–7), and farther upstream (8–10).
Light gray lines are roads; open water and tidal marsh habitats are shown in solid gray fill
and diagonal hatching, respectively.
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2014 Vol. 21, No. 1
the sex and measured the carapace width and shell height of any terrapins captured
as by-catch. Using a file, we notched a unique numeric code on the marginal scutes
of unmarked terrapins and released them. We identified all fish by-catch to species
prior to release.
We calculated the mean ± standard deviation of legal crab captures and terrapin
by-catch from pots without BRDs and compared them among the three locations
in Felgates Creek using ANOVA with Tukey HSD post-hoc comparisons. We also
used ANOVA with post-hoc comparisons to evaluate the average daily number and
average size of legal crabs among the three BRD treatments. For all statistical analyses,
the significance level was set at α = 0.05. Finally, we compared the number
of terrapins captured in Felgates Creek during the study period with the estimated
population size in the creek from Morris et al. (2011).
Results
For this study using unbaited crab pots, we captured a total of 717 Blue Crabs,
over half of which were legal size (≥12.7 cm; Table 2). Crab catch was dominated
by males (94% of total). The by-catch included a total of 71 Diamondback Terrapin
captures and 40 fish captures comprising Leiostomus xanthurus Lacépède (Spot
Croaker), Micropogonias undulatus (L.) (Atlantic Croaker), Dorosoma cepedianum
(Lesueur) (Gizzard Shad), and Sciaenops ocellatus (L.) (Red Drum).
The daily average catch per unit effort (CPUE) of legal-size crabs and terrapins
in pots without BRDs at the mouth, bridge, and upstream sites varied
significantly (ANOVA: F = 11.05, df = 2, P < 0.001). Significantly more legalsize
crabs were captured at the mouth of the creek relative to bridge and upstream
locations (post-hoc comparisons: P < 0.005; Fig. 2). No significant difference
was found between the crab CPUE at the bridge and upstream sites (P = 0.277).
The daily average CPUE of terrapins among sites was significantly different
(ANOVA: F = 6.38, df = 2, P = 0.002) and was greater at the bridge location relative
to the mouth and upstream locations (post-hoc comparison: P < 0.05; Fig. 2).
The CPUE of terrapins between mouth and upstream locations was not significantly
different (P = 0.729).
Over the entire study, the average total number of legal crabs captured per pot
varied significantly by BRD regulation (ANOVA: F = 12.97, df = 2, P < 0.001;
Fig. 3). Significantly fewer crabs were captured in pots fitted with small BRDs relative
to pots fitted with either large BRDs or no BRDs (post-hoc comparisons: P less than
0.001). Although fewer crabs were captured per pot fitted with large BRDs relative
Table 2. Summary from 24 sampling dates of crab and by-catch from unbaited pots with no BRDs and
those fitted with small and large BRDs.
Catch Small BRD Large BRD No BRD Total
Total crabs 139 279 299 717
Legal crabs 67 138 168 373
Terrapins 0 2 69 71
Fish 5 10 25 40
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Figure 2. Catch per unit effort (CPUE) for crabs and terrapins from pots without BRDs
placed in locations near the mouth of Felgates Creek, around the bridge, and farther upstream.
Capital and lowercase letters refer to the results of post-hoc comparisons of crab
catch and terrapin by-catch, respectively, among sampling locations.
Figure 3. Comparison of average total legal crab catch and average crab size from pots
conforming to different BRD regulations (n = 10 pots for each BRD group). Capital and
lowercase letters refer to the results of post-hoc comparisons among pot types with respect
to crab catch and crab size, respectively.
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2014 Vol. 21, No. 1
to pots with no BRDs (29.9 ± 10 vs. 27.9 ± 6.2), the dif ference was not significant
(post-hoc comparison: P = 0.828). The largest crabs were captured in pots fitted
with no BRDs (14.1 ± 1.2 cm), and the smallest crabs were captured in pots fitted
with small BRDs (13.7 ± 1.0 cm), but the differences in legal crab size among
BRD regulations were not significant (ANOVA: F = 2.47, df = 2, P = 0.086).
We captured 23 male terrapins with three additional recaptures, and 33 female
terrapins with 12 additional recaptures. For the terrapin captures, we plotted carapace
width and shell height relative to the opening in pots constrained by BRD
regulation (Fig. 4). Only two of the captured terrapins were small enough to fit
through the small BRD opening (shell height < 4.5 cm), neither of which was
captured in a pot fitted with small BRDs. Although 14 terrapins could have passed
through the openings into pots fitted with large BRDs, only two terrapins were
captured in large BRD pots. Twelve of the 14 terrapins small enough to fit into
large BRD pots were adult males, which—owing to sexual size dimorphism—are
generally smaller than adult females.
Morris et al. (2011) used the Schnabel method at Felgates Creek in 2009 to estimate
a terrapin population size of 133. We had 54 original terrapin captures in pots
without BRDs (plus 15 recaptures); if these terrapins had drowned in the pots, the
loss would have been 42% of the estimated population size.
Figure 4. Comparison of captured terrapin dimensions (carapace width and shell depth) with
the size of the opening in crab pots created by small BRDs, large BRDs and no BRDs. Terrapins
that plot within the dimensions of each BRD are able to enter a crab pot fitted with that
BRD, but were not necessarily actually caught in pots fitted wit h that size BRD (see text).
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Discussion
By-catch reduction devices on commercial-style crab pots are effective at reducing
the capture of terrapins and fish (Table 2). Given the different regulatory
strategies regarding BRD implementation by state, our results indicate crabbing
success and the risk of by-catch must also vary by state. In MD and DE, recreational
crabbing requires the use of small BRDs that appear to be effective in
excluding most terrapins and fish. Crab catch in the current study in VA, however,
indicated total capture and legal crab size are reduced in pots fitted with
small BRDs. Our pots were not baited, so it is possible that baiting could attract
more and larger crabs, thereby eliminating any effect of the small BRDs on crab
capture. The larger BRDs required by NJ and NY in areas where recreational and
commercial crabbing overlap with terrapin habitat also reduced terrapin by-catch
significantly but had less of an effect on crab catch and size (Fig. 3), as has been
found in prior studies (Hart and Crowder 2011). Both BRD sizes excluded most
adult terrapins relative to no BRDs at all.
For unbaited pots without BRDs, we measured an average daily CPUE as high
as 0.48 terrapins at one location in Felgates Creek (Fig. 2). This is a very high rate
of terrapin by-catch relative to most prior studies (Hart and Crowder 2011, Roosenburg
2004). However, the spatial pattern of crab capture did not match terrapin
capture (Fig. 2), suggesting that terrapin and crab distributions and/or activities are
independent of each other. Larger and more numerous crabs appear to be concentrated
closer to the open water of the York River, and terrapins tend to concentrate
in sub-tidal areas adjacent to expansive marshes (Fig. 1). Our work demonstrates
that even unbaited pots can trap both crabs and terrapins, sometimes in significant
numbers. Morris et al. (2011) found that crab capture tended to be highest in baited
pots, whereas terrapin capture was independent of pot baiting. Grosse et al. (2009)
reported the drowning of over 90 terrapins in a single untended crab pot. Together,
these results demonstrate that untended and unbaited pots can attract, trap, and
drown terrapins.
In the absence of BRDs, the estimated loss of 42% of the terrapin population in
Felgates Creek would have occurred in just 24 days of crabbing with 10 pots. Females
comprised 33 of the 54 captured terrapins, and although BRDs exclude most
adult females, pots without BRDs do not. The potential loss of so many reproductive
females from the population would reduce terrapin recruitment dramatically.
Because terrapins have limited home ranges (Sheridan et al. 2010), the chronic
pressure of crabbing without BRDs has the potential to significantly reduce terrapin
population size and alter population demographics (Dorcas et al . 2007).
BRD regulations in MD, DE, NJ, and NY seek to protect terrapins from drowning
in crab pots. To date, however, no studies have been completed that document
terrapin population recovery or expansion into areas where BRDs are now being
used. Disturbingly, Radzio et al. (2013) estimated less than 35% compliance with
BRD regulations by recreational crabbers in MD. In the ongoing absence of BRD
regulations in Virginia, 100% of all recreational crabbing will continue to exert
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2014 Vol. 21, No. 1
negative pressure on terrapin populations. Recreational crab pots are placed nearshore,
typically set in the water next to docks of shoreline homeowners who are
allowed to fish two pots per person per residence. These pots often are not checked
and/or baited daily and thus pose a risk to terrapins because of their nearshore location
in terrapin habitat and irregular operation. Lost recreational and commercial
pots without BRDs also would pose a risk, as the pots would continue to trap terrapins
even without baiting.
The number of docks in tidal waters of Virginia has been estimated at over
20,000 (R.G. Isdell, College of William and Mary, Williamsburg, VA, pers. comm.).
If every resident with a dock set and operated just one crab pot, then terrapin mortality
bay-wide would likely be very significant, even with a low rate of daily capture.
Over a 100-day summer season, for example, with a relatively low average daily
CPUE of 0.05 terrapins (Fig. 2), those pots would drown some 100,000 terrapins
annually in Chesapeake Bay. From our results, if those same pots were fitted with
large BRDs, the recreational crabbers would experience an 18% reduction in crab
catch, but by-catch mortality of terrapins would decrease by 97 %.
Although not a direct focus of our study, a substantial amount of commercial
crabbing in addition to recreational crabbing is completed in Diamondback Terrapin
habitat. Except for NJ and NY, these commercial pots set in tidal creeks are
not regulated for BRDs and thus have the potential to impact terrapin populations
substantially. Commercial crabbers fish millions of pots daily throughout estuaries
of the Atlantic and Gulf Coasts; in some states, many of those pots are placed in
shallow tidal creeks. Based on the number and placement of pots in terrapin habitat,
the negative effects of commercial crab operations without BRDs could well be
just as devastating as recreational crabbing, and in some states may be worse (M.E.
Dorcas, Davidson College, Davidson, NC, pers. comm.).
BRD regulations in northeastern states where crabbing and Northern Diamondback
Terrapin habitat overlap can be effective in reducing by-catch without affecting
crab catch. From the results of the present study in Virginia, the large BRDs
(5 cm x 15.2 cm) reduce terrapin by-catch dramatically without a large reduction in
crab catch. Whether targeting recreational crabbers specifically or terrapin habitat
more generally, BRD regulations lead to reduced terrapin mortality. Because terrapins
also occur in more southern estuaries, coastal states from Virginia to Texas
should consider crabbing regulations and the use of BRDs as part of their fisheries
management plans.
Acknowledgments
Terrapin research completed under VA DGIF Permit #045090 to RMC and College of
William and Mary protocol IACUC-2012-04-30-7883-rmcham. A.J. Upperman received
funding from the Robert Noyce Teacher Scholarship Program at the College of William of
Mary School of Education. Thanks to Captain Crow and the Environmental Division at the
Yorktown Naval Weapons Station for access and logistical support. Thanks to C. Sornborger
for providing large BRDs.
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