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

Abstract

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 (cm2), 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 5.1–6.4 cm (height) × 7.3 cm (width). Increasing the width of this BRD by 0.4 cm would also achieve a 99% capture probability for legal‐sized blue crabs, comparable to other widely tested and marketed BRD designs.

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

“…The oval BRDs we designed do not change how terrapins are excluded but offer an alternative to rectangular BRDs that are simpler to install and—because their shape mimics the funnel opening—may be more palatable to commercial and recreational crabbers hesitant to install any device that might reduce the funnel opening. Arendt et al (2018) showed mathematically that optimization of crab catch and terrapin exclusion can be achieved with a smaller BRD design that excludes terrapins based on shell width. Although that design is indeed effective (Arendt et al 2018; Grubbs et al 2018) and is included as a BRD option in North Carolina crabbing regulations, additional testing will be needed to determine whether regulators and crabbers will be responsive to a funnel opening even more reduced in size than other BRD designs.…”

“…Arendt et al (2018) showed mathematically that optimization of crab catch and terrapin exclusion can be achieved with a smaller BRD design that excludes terrapins based on shell width. Although that design is indeed effective (Arendt et al 2018;Grubbs et al 2018) and is included as a BRD option in North Carolina crabbing regulations, additional testing will be needed to determine whether regulators and crabbers will be responsive to a funnel opening even more reduced in size than other BRD designs. We previously conducted an informal review of BRD studies and found that some were inconsistent with baiting traps or did not catch enough terrapins to assess BRD effectiveness (refer to https://story maps.arcgis.com/stori es/3555d 14f5c a2442 e960a 955d0 0f68044).…”

“…This result suggests that tests of additional bycatch reduction methods that exclude terrapins based on shell height are needed beyond traditional rectangular BRDs. Arendt et al (2018) designed a BRD that excludes terrapins based on shell width, but to date, BRD regulations in blue crab fisheries from six states refer exclusively to rectangular BRDs. With that current regulatory focus, we designed two oval-shaped BRDs and conducted field tests to determine whether oval BRDs are more effective at excluding terrapins and maintaining crab catch than standard rectangular BRDs and control treatments.…”

“…The oval design might allow crabs easier entry into traps, as rectangular BRDs could create "traffic flow" issues and impede entry (Grubbs et al 2018). Further, both crabs and terrapins may exhibit different behavioral responses to BRD size and shape (Arendt et al 2018). Wilson et al (2022) conducted lab tests comparing terrapin exclusion in a trap fitted with oval BRDs (9.4 × 5.3 cm), a control trap without BRDs, and a trap with compressed funnels and found that both oval BRDs and narrow funnels significantly reduced terrapin entry.…”