Seasonal encounter rate, life stages and main threats to the loggerhead sea turtle (Caretta caretta) in the Aeolian Archipelago (southern Thyrrenian Sea)

Abstract

Abstract Annual and seasonal encounter rates, life stages and the main threats to loggerhead sea turtles (Caretta caretta) in the Aeolian Archipelago (southern Italy) were studied. Dedicated boat surveys resulted in 258 surface observations and 138 captures of healthy (n = 309), ailing (n = 66) and dead (n = 21) turtles from 2009 to 2013. Loggerheads were encountered at the sea water surface while resting (87%) or feeding on pelagic prey (13%). The loggerhead encounter rate (observations per km surveyed) was higher during the autumn, suggesting the presence of potential foraging/overwintering habitats in the area. The mean (± SD) curved carapace length (CCL) was 48.8 ± 10.7 cm, with 65% of the individuals ranging from 40–70 cm in size. Smaller turtles were encountered more frequently during the spring months, probably as a result of the abundant pelagic prey within the coastal area. During the colder season, larger turtles were more common in the afternoon than in the morning, suggesting that the time needed for rewarming might increase with turtle size. Ingestion of anthropogenic debris was reported in 48.5% of the rescued turtles. Individual mortality was mainly related to longline fishing (70.6%), with debris entanglements/ingestion frequently associated with these records. Longline bycatch and boat collisions were higher in summer, whereas debris ingestion was highest in spring. Different threats might affect particular life stages because the longline bycatch was more frequent for larger turtles, whereas boat collisions were more frequent with smaller individuals. Migratory patterns, habitat characteristics and seasonal changes in sea temperature and currents might influence the seasonal occurrence of loggerhead turtles in this area. These results increase the current ecological knowledge of the factors driving loggerhead turtle life and are important for implementing management plans for its conservation in the Mediterranean Sea.