Evidence for trophic differences between live and bycatch oceanic juvenile loggerhead sea turtles

The loggerhead sea turtle Caretta caretta is a vulnerable migratory species that spends its first years of life in the open sea. During this developmental phase, loggerheads can be found foraging in the epipelagic zone of the waters surrounding the Madeira Archipelago, providing a rare opportunity to gather information on the ecology of its oceanic developmental stage. In this study, we characterized the isotopic niche of these juveniles, using stable isotope analysis. We assessed two groups of turtles, turtles captured alive (n = 24) and turtles captured as bycatch on local longlines (n = 12), and explored whether animals caught in the local fishing gear represented a random sample of the population, or whether there is some evidence for a specialized foraging behaviour. We found that turtle bycatch had a significantly higher mean stable nitrogen isotope value in whole blood (δ15N = 8.5 ± 0.6‰ SD) compared to the group of turtles captured alive in the same period (δ15N = 7.6 ± 0.5‰ SD), indicating that they had a different diet. While there was a tendency for turtle bycatch to be slightly larger, we found no effect of body size on δ15N values. We propose a distinct foraging behaviour strategy hypothesis, with a group of turtles being more susceptible to interactions with fisheries and thus having a higher mortality risk, which should motivate the implementation of existing guidelines to reduce sea turtle bycatch.