At sea mortality estimates of loggerhead turtle (Caretta caretta) in Southwestern Atlantic Ocean

Sea turtles are vulnerable to various threats, with incidental capture in fisheries being a main concern. Limited knowledge exists regarding sea turtle mortality at sea, and stranding events serve as an indicator of such mortality. However, stranding data only represent a fraction of the turtles killed in the ocean. The southern coast of Brazil is an important foraging area for loggerhead turtles. This study aimed to estimate loggerhead turtle mortality at sea in southern Brazil using stranding data. Between 2012 and 2016, a total of 70 incidentally caught and dead turtles in pair trawl fisheries were tagged and released back into the sea, with the recording of date and geographic position. Biweekly beach surveys were conducted in an effort to retrieve these carcasses. Bayesian logistic regression models were employed to assess the influence of meteoceanographic and geographic variables on the stranding probabilities of tagged turtles. Considering the total number of stranded loggerhead turtles in the study area and the predicted stranding probabilities, the annual mortality of the species at sea was estimated. Season, distance from the shore, and currents were identified as variables influencing the stranding probability. The stranding probability was 19.72 % for the warm period and 12.96 % for the cold period. A total of 1897 loggerhead turtles stranded between 2012 and 2016. The estimated mortality during this period was 14,377 loggerhead turtles killed. This high mortality can have a substantial impact on the population, thus conservation measures that effectively protect sea turtles are in urgent need.