Using spatial and behavioral data to evaluate the seasonal bycatch risk of diamondback terrapins Malaclemys terrapin in crab pots

Mortality of marine vertebrates due to incidental entanglement in fishing gear is of global concern. Trends in diamondback terrapin Malaclemys terrapin abundance and demography suggest that bycatch mortality associated with the blue crab Callinectes sapidus fishery may be contributing to population declines and demographic shifts of this estuarine turtle. Designing effective regulations to minimize terrapin-crabbing interaction requires information on the spatial ecology and seasonal behavior of terrapins. Our goals for this study were (1) to identify spatial and temporal patterns in the distribution of terrapins and crab pots and (2) to determine bycatch risk based on seasonal shifts in terrapin behavior and the degree of overlap between terrapins and crab pots. We documented crab pot locations and monitored the movements and activity patterns of 29 terrapins via radio telemetry in southeastern North Carolina from June 2008 to May 2009. To assess spatial overlap and resulting bycatch risk (BR), we calculated seasonal distributions and densities of crab pots and terrapins and incorporated them into a spatial overlap index model (SOI) modified to include the seasonal aquatic behavior of terrapins. Spatial overlap is greatest in warm months when terrapins are swimming in the same shallow, near-shore habitat as blue crabs. When the seasonal and semi-aquatic behavior of terrapins is incorporated into the spatial model, BR is reduced. This behaviorally modified SOI model may be applied to other geographic areas to identify areas and times of bycatch risk between fisheries and non-target species.