Legal Crab Research Articles

Bycatch reduction devices exclude diamondback terrapins and maintain blue crab catch in two Virginia tidal creeks

AbstractObjectiveThe diamondback terrapin Malaclemys terrapin is frequently caught as bycatch in both commercial and recreational blue crab Callinectes sapidus fisheries throughout its range along the Atlantic and Gulf coasts of North America. Bycatch reduction devices (BRDs) are plastic or wire rectangles mounted in the entrance funnels of crab traps, designed to exclude terrapins while still allowing crabs to enter. In this study conducted in two tidal creeks in Virginia, USA, we assessed the effectiveness of four BRD designs, including two novel oval designs, against control traps with no BRDs.MethodsWe tested four replicates of each of the five trap treatments (1.75‐inch oval BRD, 2‐inch oval BRD, 1.75‐inch rectangular BRD, 2‐inch rectangular BRD, and a control trap without a BRD) in each tidal creek, for a total of 20 traps per creek and 40 traps total. We fitted traps with wire chimneys that extended above the high tide water line to allow trapped terrapins to surface for air. For eight weeks during summer 2021, we baited traps with Atlantic Menhaden Brevoortia tyrannus each day, beginning on Monday. Tuesday–Friday, we emptied traps of any animals inside, and recorded terrapin sex, terrapin carapace length, terrapin carapace width, and terrapin shell height, as well as blue crab carapace length.ResultWe found that all four BRD designs were highly effective at excluding terrapins and maintaining crab catch when compared to control traps. We found a significant difference (p = 0.003) in catch per unit effort (CPUE) of diamondback terrapins among treatments, with the control group (those without BRDs) having the highest CPUE (0.97 ± 0.18). All traps fitted with BRDs decreased terrapin capture significantly compared to the control group. Oval BRDs excluded more terrapins than their rectangular counterparts with the same height dimension, although these differences were not significant. We found no significant difference in CPUE of blue crabs among treatments (p = 0.392), or in the size of legal crabs caught in each treatment (p = 0.216).ConclusionThis study provides evidence of the effectiveness of both rectangular and oval‐shaped BRDs to exclude terrapins and maintain crab catch in Virginia waters, where BRDs are not currently required.

Experiments with By-Catch Reduction Devices to Exclude Diamondback Terrapins and Retain Blue Crabs

Experiments were completed in SE Virginia during June–July 2014 and 2015 to examine the responses of blue crabs (Callinectes sapidus) and diamondback terrapins (Malaclemys terrapin) to commercial-style crab pots modified in visual and other ways that might attract and retain crabs while excluding terrapins as by-catch. In a seawater tank, far fewer crabs entered crab pots fitted with red plastic by-catch reduction devices (BRDs), relative to pots without BRDs. Crab retention times, however, were significantly longer in pots fitted with red BRDs. In a second experiment, fewer terrapins entered crab pots with funnels painted red relative to black. From a field pilot study, the legal crab catch from pots with red BRDs was similar to pots without BRDs, and terrapin by-catch was reduced. Relative to those treatments, fewer crabs and more terrapins were captured in pots with orange BRDs and blue BRDs, and in pots with a magnetic field directed into the funnel openings. Based on these results, a final field trial yielded comparable crab catch from 15 pots without BRDs and 15 pots fitted with red plastic BRDs. Of a by-catch of 68 terrapins, 58 were from pots without BRDs. The structure and color of BRDs can exclude most terrapins; because crab retention rates are high, the net effect of BRDs on crab catch is relatively minor, even though fewer crabs may enter pots fitted with BRDs.

Retention Probability Varies with Cull Ring Size in Traps Fished for Blue Crab

AbstractThe blue crab Callinectes sapidus is a commercially important crustacean species fished along the Atlantic and Gulf of Mexico coastlines. Cull rings on traps are required to allow for egress of sublegal blue crabs in commercial and recreational fisheries in this region. We undertook a study to estimate contact selectivity (retention probability) of traps with different cull‐ring sizes fished for male crabs. We estimated retention probability using logistic models and catch data from 38.1‐mm square mesh traps, each affixed with one size of cull rings (58.7, 60.3, or 61.9 mm internal diameter), and fished along with control traps (no cull rings). Each experimental trap type displayed relatively steep retention around its l50, the carapace width at which 50% of individuals are selected. The l50 for traps with 58.7‐mm cull rings was less than the minimum legal crab size in North Carolina (127 mm carapace width) while the l50 for traps with 60.3‐mm or 61.9‐mm cull rings was greater than the minimum legal size. Mean catch rates of legal male crabs were not statistically different among the three experimental trap types; however, catch rates of sublegal males were significantly lower in traps with the two larger cull ring sizes. We conclude that traps with 61.9‐mm cull rings are most appropriate for the North Carolina trap fishery because they maintain catch rates of legal males but show the greatest reduction in catch rates of sublegal males. The use of larger cull‐ring sizes would reduce rates of fishing mortality resulting from excessive trap confinement, injury during commercial culling, or intentional harvest by fishers who do not cull their catch.Received April 29, 2015; accepted October 22, 2015

The Influence of Recreational Crabbing Regulations on Diamondback Terrapin By-Catch

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.

Size selection of red king crabs, Paralithodes camtschaticus, in traps with escape openings

A high number of red king crabs, Paralithodes camtschaticus, captured in the Norwegian commercial trap fishery have to be returned to the sea because they are females or sublegal males. In the present study, a comparative fishing trial was performed to investigate whether escape openings in traps lead to lower catches of small king crabs. Traps equipped with circular escape openings with an inner diameter wider than 160 mm retained legal crabs, while catches of sublegal male crabs became significantly reduced. Small female king crabs had a lower escape rate from traps with escape openings than small males.

Decline in size of male blue crabs (Callinectes sapidus) from 1968 to 2000 near Calvert Cliffs, Maryland

Maryland commercial landings of the blue crab (Callinectes sapidus) and catch per unit effort (CPUE) have remained fairly stable over the past 33 yr despite occasional large deviations from the long-term average. During this time, however, significant declines in the percent of legal male crabs and the mean size of legal males caught in fishery-independent surveys near Calvert Cliffs, Maryland have become apparent. Sublegal females and two of the three legal female classes (152–177 and >178 mm CW) showed no significant trends over this 33-yr period when examined by linear regression. Males showed significant trends for all size classes. Sublegal males increased from 24% of the male population during the first 5 yr of the study (1968–1972) to 71% during 1996–2000. All classes of legal males, however, exhibited downward trends. Males 127–151 mm CW decreased from 45% of the male population in the earliest period to 22% during the last 5 yr. Males 152–177 mm decreased from 27% during 1968–1972 to only 6% during 1996–2000, and males > 178 mm declined from 4% in the earliest period to 0.5% in the recent period. These size decreases for the most valuable portion of the blue crab population are further evidence of over-exploitation. The declining trends in male size indicate that growth overfishing is occurring as intense fishing pressure removes so many male crabs from the population as they reach legal size that few remain to molt to larger size. A 3-yr data set from the Patuxent River, where commercial use of crab pots is restricted and fishing pressure is lower, suggests that legal male crabs are able to attain larger size compared to an area where the pot fishery is intense. A recommendation could be made for reducing effort where the pot fishery is intense by means of time, gear, catch limits, and/or by increasing the minimum size of legal crabs to allow larger crabs to enter the fishery.

Reversal of Handedness, Growth, and Claw Stridulatory Patterns in the Stone Crab Menippe Mercenaria (Say) (Crustacea: Xanthidae)

ABSTRACT All 103 stone crabs reared from eggs of one left-handed and two right-handed female crabs were initially right-handed (side bearing crusher claw), but 88% of laboratory-reared juveniles whose crushers were removed reversed handedness on the first regenerative molt. Eight of 12 (67%) feral adult or pre-adult crabs reversed handedness on third or fourth molt following crusher loss, but only 5% (1 of 19) and 6% (1 of 16) reversed on the first and second molts, respectively. Reversal of handedness following claw loss by young juveniles is the most likely cause of the change from 100% initial right-handedness to 80% among older juveniles and sublegal (carapace width < 80 mm) adults in natural populations. The ratio is not altered in fished populations, probably because natural death occurs before most legal crabs can complete the number of molts (3 or more) required for adult claw reversal. No adult crab regained a normal stridulatory pattern on a regenerated claw, even after five postregenerative molts. Therefore, most, if not all, claws regenerated following initial claw harvest by the fishery should be recognizable as such in subsequent landings. Growth averaged 20% carapace width increase per molt for 94 laboratory molts of adult crabs; this was comparable to field growth increments in other studies.