Bottom trawl
Since first studies the bottom trawl was considered among different fishing gears as one of major sources of impact on sea turtle populations, especially because it interacts with the larger size classes, thus having the greatest effect on reproductive potential and on population trends. The bathymetry of different areas determines the distribution and abundance of marine turtles. The greatest density of specimens in the demersal phase is found in shallow waters (<100 m). In fact C. caretta do not dive deep and the maximum recorded depth is 110 m. Thus most captures by trawl are observed in shallow waters, at a depth less than 50 m or even 20 m deep. Usually Mediterranean continental shelf, which constitutes the feeding habitats for adults turtles, is really narrow in most areas. Nevertheless in some place such as the Gulf of Gabés, the northern Adriatic Sea, the South Turkey and Egypt the continental shelf is large and turtles in demersal phase are used to spend winter time in these areas.
Mediterranean bottom trawlers are estimated to bycatch over 30.000 specimens per year, with a mortality of 25%, mainly in Italy, Tunisia, Croatia, Greece, Turkey, Egypt, and Libya.
Data available for Italian waters suggest a catch of at least 8.500 specimens per year. Some authors have estimated more than 40.000 capture events, considering that the same turtle can be caught more than one time. North Adriatic Sea with its shallow waters and rich benthic communities is considered as one of the most important feeding habitat in the whole Mediterranean, mainly for the population nesting in Greece. The occurrence of turtles in the north Adriatic is probably due to its relatively shallow waters (<100 m) where turtles in demersal phase are more likely to concentrate. For this area it is possible to estimate annual catch of over than 4.000 specimens, Italian side, at least 2.500 turtles per year are estimated to be captured by the Croatian bottom trawlers (although this picture was obtained thorough fishermen’s interviews, thus it could be underestimated).
Most of loggerhead turtle caught are still alive when captured and rejected to sea; some authors reported an observed mortality of about 9.4% and a potential delayed mortality really high.
Bottom trawl activity mainly impacts turtles in demersal phase since they prefer the shallow waters. Incidental catch of C. caretta probably occur during towing operations when turtles are foraging on the bottom. When captured by a trawl net, loggerhead turtles may drown, becoming first comatose and eventually dying. Mortality by trawling is due to forced apnoea, thus the longer or faster tows are responsible for higher bycatch rates. Therefore, towing time is one of the main factors affecting the mortality rate but, especially in the bottom trawl, additional factors probably occur: impacts with the gear, bumps with catch components such as wood and stones etc.
