Using Observed Interactions between Sea Turtles and Commercial Bottom-Trawling Vessels to Evaluate the Conservation Value of Trawl Gear Modifications

Abstract The bycatch of sea turtles (order Testudines) in bottom-trawl fisheries is an important conservation issue currently being addressed by the U.S. National Marine Fisheries Service and its stakeholders. The agency is considering the implementation of new sea turtle conservation regulations in several mid-Atlantic trawl fisheries, including the expanded use of turtle excluder devices (TEDs). The characteristics of observed sea turtle bycatch are used to calculate four conservation metrics. The lowest-level metric is simple to calculate but not informative of population impacts because it does not adequately incorporate the magnitude and demographics of the affected population. The highest-level metric incorporates both magnitude and demographics and is therefore more difficult to calculate but also more informative. Five size categories of turtles were evaluated with respect to the protection afforded by various TED configurations, and the conservation value of 12 bycatch mitigation alternatives was estimated using each of the four conservation metrics. The most informative metric was adult-equivalent bycatch mortality. A TED with a large escape opening had the highest estimated conservation value for loggerheads Caretta caretta. Up to 66 more adult-equivalent loggerheads were estimated to be protected by the large opening than by the standard opening. A similar number could be protected by extending TED use north of 37°N. The number of adult-equivalent loggerheads estimated to be protected by standard and wide-bar spacing is nearly the same. The percentage of encountered turtles caught in trawls is not an adequate proxy for the number of adult-equivalent mortalities caused by the fishery. Evaluating sea turtle bycatch using adult-equivalent mortalities facilitates comparisons across disparate bycatch mitigation alternatives and provides a meaningful way to assess the efficacy of bycatch mitigation alternatives for the recovery of sea turtle populations.