Assessing resource use patterns of Mediterranean loggerhead sea turtles Caretta caretta (Linnaeus, 1758) through stable isotope analysis

Stable isotope analysis is a useful tool for studying the ecology of marine consumers, as carbon (δ13C) and nitrogen (δ15N) isotope ratios may reflect individuals’ patterns of diet and habitat use. Knowledge of foraging strategies has significant implications for the conservation of endangered loggerhead turtles Caretta caretta (Linnaeus, 1758). In this study, δ13C and δ15N isotope data were used to assess resource use patterns of the Mediterranean loggerhead turtles (Aeolian Archipelago, Southern Italy). δ13C and δ15N values from carapace scutes of 54 loggerheads of different curved carapace length (CCL) and health status were compared with those of eight potential prey items (benthic, pelagic and fishery discards). MixSIAR results suggested that pelagic prey (from goose barnacles to planktivorous fish) comprised most of loggerheads’ diet, with small variations (i.e. benthic prey or fishery discards) depending on size (δ13C and δ15N) and health (δ15N) of individuals. δ13C variations with turtles’ size might reflect changes in dietary habitats during life stages. However, the loggerhead turtles and their main source of prey (pelagic prey) had a higher variation in values of δ15N compared to δ13C. This suggested that smaller-sized turtles might preferentially feed on pelagic prey in oceanic habitats and then, as they reach a larger size, gradually enter neritic waters, including in their diet prey sources with higher δ13C and δ15N. Some turtles foraging on longline baits/debris also displayed a marked increase in δ15N. These δ15N variations might be explained by differences in diet (trophic differences) and somatic growth rates among individuals, or dietary dilution.