A "BRD" in the Hand Worthy of Four in the Trap: Validation of Optimal Bycatch Reduction Device (BRD) Size to Maximize Blue Crab Callinectes sapidus Entry and Diamondback Terrapin Malaclemys terrapin Exclusion Through Theoretical Modeling and Application

Wire-mesh traps are extensively used to harvest blue crabs Callinectes sapidus but also capture over 40 other species. Numerous studies have evaluated bycatch reduction devices (BRDs) to reduce the incidental capture of one species, the diamondback terrapin Malaclemys terrapin, since 1992. Despite decades of research, widespread adoption of BRDs as a conservation action has not occurred, which partly stems from seemingly conflicting results from a multitude of BRD designs along the U.S. Atlantic and Gulf of Mexico coasts. In this study, we developed a simple scoring system to assess the probability of trap entry by legal-sized (12.7-cm carapace width; n=736) blue crabs as well as trap exclusion of diamondback terrapins (n=985) based on morphometric measurements for both species. Probability of entry or exclusion for each legal-sized blue crab and diamondback terrapin was assessed for each of 8,196 theoretical BRD sizes encompassing the range of BRD widths and heights actually tested since 1992. Predictive relationships for trap entry or exclusion for each theoretical BRD size were evaluated as a function of three geometric BRD attributes: BRD area (cm(2)), diagonal BRD opening (cm), and the ratio of BRD width to height. Comparison of observed (35 data sets) versus predicted (entry or exclusion probabilities for BRDs) catches for both species revealed that BRDs have performed as predicted for legal-sized blue crab catch (slope=0.99; >81,000 crabs) but have captured considerably fewer diamondback terrapins than predicted based on BRD size alone (slope=0.28; 1,430 diamondback terrapins). Analyses also substantiated superior (i.e., >30% better) diamondback terrapin exclusion potential for a BRD we designed that measured 5.1-6.4cm (height)x7.3cm (width). Increasing the width of this BRD by 0.4cm would also achieve a 99% capture probability for legal-sized blue crabs, comparable to other widely tested and marketed BRD designs.