Seminar: Dr. Elizabeth A. Babcock (Associate Professor, University of Miami)
Title: Sea turtle bycatch in shrimp trawls: a conservation success story?
Speaker: Dr. Elizabeth A. Babcock (Associate Professor, University of Miami)
Abstract: The incidental capture, or bycatch, of sea turtles in shrimp trawl fishery is a major source of mortality and a significant management concern. Measures to reduce bycatch have been implemented, such as the required use of turtle excluder devices (TEDs) beginning in the early 1990s. The NOAA Shrimp Trawl Observer Program assigns observers to shrimp otter trawl vessels in both the Gulf of Mexico (GOM) and the waters off the U.S. east coast within the jurisdiction of the South Atlantic Fishery Management Council (SATL), and vessel participation has been mandatory since 2007. We applied integrated Bayesian models to the observer data to estimate sea turtle bycatch. We also estimated mortality, defined as the total number of sea turtles that were caught in shrimp trawls and died at the time of capture. The total bycatch mortality was estimated by multiplying the probability of mortality for turtles caught in shrimp trawl nets by the total bycatch estimated from a linear model of catch per unit effort (CPUE) per strata (area, season, depth zone, time period) multiplied by the total effort in each stratum. Total bycatch mortality was estimated separately for the GOM and the SATL, and for standard shrimp otter trawl nets versus “try” nets, which are small nets fishers deploy in front of the primary nets to test catch rates. About 30% of sea turtles caught in standard nets were dead, while less than 1% of sea turtles caught in try nets were dead. Thus, although many Kemp’s ridley, loggerhead, and green sea turtles were caught in try nets in both regions, few of them were killed. For example, in the GOM in 2015, we estimated 95% credible intervals of 54-256 Kemp’s ridley, 173-495 loggerhead and 22-114 green sea turtles caught in try nets, but only 0-7 Kemp’s ridley, 0-17 loggerheads, and 0-3 green sea turtles were estimated to be killed. On the other hand, for standard nets in the GOM, we estimated 95% credible intervals of 63-369 Kemp’s ridley, 18-105 loggerhead and 75-226 green sea turtles captured, corresponding to mortality of 19-130 Kemp’s ridley, 5-36 loggerhead and 22-81 green sea turtles killed. Total bycatch mortality of Kemp’s ridley and loggerhead sea turtles in standard nets decreased from 2007 to 2015 in the GOM, but green sea turtle bycatch stayed constant. There was no trend over time in the SATL from 2007 to 2016, and total mortality in standard nets in 2016 was on the order of 5-111 Kemp’s ridley turtles, 9-139 loggerhead turtles, 2-86 green sea turtles, and 13-168 of unknown/other species of turtles. These estimates of total bycatch mortality from the observer data are much lower than values estimated in other studies, such as those that incorporated nest counts and strandings data to estimate the number of turtles that must have been killed in fisheries to explain observed trends in sea turtle abundance. The current study shows that sea turtle mortality in shrimp trawls appears to be much lower than previously thought, perhaps because of the effectiveness of TEDs. However, it is also possible that post escape mortality is higher than expected, or that there is high mortality of turtles captured in coastal fisheries that are not well sampled by the federal observer program."