Potential bycatch impact on distinct sea turtle populations is dependent on fishing ground rather than gear type in the Mediterranean Sea

Bycatch is one of the main threats affecting marine megafauna worldwide, not only because of its prevalence, but also because the impact of high levels of bycatch in small oceanic regions may spread over whole oceans due to the complex dispersal patterns of bycaught species. Here, we use intrinsic and genetic markers to understand the impact of bycatch on the Atlantic and Mediterranean populations of the loggerhead turtle sharing the same foraging grounds in the western Mediterranean Sea. Turtles of Atlantic origin settle on the continental shelf later and at a larger size than turtles of Mediterranean origin and hence have been suggested to be more vulnerable to pelagic fishing gears, whereas those of Mediterranean origin would be more vulnerable to neritic ones. To assess whether this hypothesis holds true, we compared the genetic make-up of turtle bycatch from drifting longlines and bottom trawl/trammel nets in three different regions (eastern mainland Spain, southern Balearic Islands and southern Italy). A total of 176 incidentally caught turtles were considered, and size and habitat use, as revealed by stable isotopes, were incorporated to the analysis. No genetic, size or isotopic differences were found between turtles caught with drifting longlines and bottom trawl/trammel nets within any of the three regions. However, genetic, size and isotopic differences were detected among regions, regardless of the fishing gear. Thus, the population make-up of loggerhead bycatch depends on the area where the fishing operations are conducted, but not on the fishing gear used. Accordingly, the actual impact of loggerhead bycatch in the Mediterranean Sea will depend not only on the total number of turtles taken, but also on the geographic distribution of the fishing effort.