Effect of Three Bycatch Reduction Devices on Diamondback TerrapinMalaclemys terrapinCapture and Blue CrabCallinectes sapidusHarvest in Delaware Bay

In recent years, reduction of the taking of nontarget species in fisheries worldwide has received considerable attention. Bycatch mortality in pot fisheries has led to severe declines in local populations of the diamondback terrapin Malaclemys terrapin along the U.S. East Coast. We conducted a 4-year study between 1997 and 2000 to investigate the impact of four bycatch reduction devices (BRDs), wire rectangular devices fitted into the entrance of crab traps, on diamondback terrapin bycatch mortality as well as its effect on the harvest of blue crabs Callinectes sapidus in the Delaware River estuary. A total of 52 sets consisting of five replicate blue crab pots for each BRD treatment (5.0 × 10.0, 5.0 × 12.0, 4.5 × 12.0, and 3.8 × 12.0 cm) and controls were deployed at random in 10 tidal tributaries and inland bays within the Delaware estuary. During this study 372 diamondback terrapins and 3,522 legal-size blue crabs were caught. The 5-cm × 12-cm BRD was the only treatment for which the reduction in overall diamondback terrapin catches (–12%) was not statistically significant. In crab pots fitted with the 5-cm × 10-cm BRD, we observed a statistically significant (P < 0.01) reduction in terrapin catches (–59%) versus control pots, with no statistical difference in blue crab catches. Only a nominal loss (–12%) of legal-size blue crab catches was observed in pots fitted with the 4.5-cm × 12.0-cm BRD, whereas the reduction in diamondback terrapin captures (38% male and 96% female) was significant (P < 0.01). No diamondback terrapins were captured in the smallest BRD (3.8 × 12.0 cm), but the losses of legal-size blue crab catches were substantial (26%). We therefore recommend the use of the 4.5-cm × 12.0-cm BRD with recreational crab pots as a conservation measure because this would effectively protect all subadult and reproductive female diamondback terrapins with minimal loss of legal blue crabs. We also stress the importance of using the BRDs in concert with other conservation measures, such as efforts directed at monitoring the effects of BRD implementation on bycatch mortality and preservation of nesting.