Incidental catch of marine turtles by Spanish bottom trawlers in the western Mediterranean

Abstract

A questionnaire-based survey of fishermen was conducted in the Valencia region (eastern Spain) to assess the levels of marine turtle bycatch. Interviews covered 41.3% of the bottom trawling fleet in the region and included 111 fishermen (one per boat). Results were combined with those from previous studies conducted in neighbouring regions using the same methodology to assess the global impact of bottom trawling on the western Mediterranean stock of the loggerhead turtle (Caretta caretta). Estimated turtle catch per unit effort (CPUE) is 0.09 turtles per vessel per month, according to the information reported by fishermen. As a consequence, the whole fleet of bottom trawlers operating off the Valencia region was involved annually in 238 turtle bycatch events (95%, CI: 173-304). Ivlev's electivity index revealed that bottom trawling bycatch was higher than expected according to fishing effort in the northern part of the study area, where the continental shelf widens. The fishermen's perception was that the local stock of loggerhead turtles is declining, and that drifting longlines and bottom trawling are, at least in part, responsible for this trend. According to previous studies as well as the current results, bottom trawlers operating off mainland Spain capture mainly loggerhead turtles from the rookeries in the eastern Mediterranean, with only a low percentage of turtles of Atlantic origin. Loggerhead turtle bycatch by bottom trawling deserves more attention in the Valencia region, particularly in its northern part. Conservation measures in the area are discussed.