Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile

The diet of Southern sea lion Otaria flavescens is poorly known along the coast of central Chile (32°-39°S), where a population of about 17,300 individuals occurs, in an ecosystem that sustains one of the world's most important fishing industries. The primary objective of this study was to reconstruct the diet and estimate Trophic Positions (TPs) of sea lions and their prey off central Chile using stable isotopes (δ13C and δ15N). Our results showed that the diet of sea lions is primarily composed of pelagic prey, with the jack mackerel Trachurus murphyi as the principal prey item in the diet of sea lion (1-99th percentile: 20-66%), while demersal prey accounted for only 0-2.8%. We also found regional differences on the relative contribution of prey to the diet of sea lions. Animals that were sampled close to major fishing areas showed an increase in the relative contribution of jack mackerel to the diet as opposed to animals sampled away from these areas that displayed a relatively more heterogeneous diet. Trophic Positions (TPs) of sea lions prey items ranged between 3.39 for jack mackerel and 4.48 for pink cusk-eel (Genypterus blacodes). The TP for sea lions was 4.57. Hence, our results showed a community composed by at least 5 trophic levels, with sea lion as the top predator. In summary, our study demonstrates that the Southern sea lion is displaying pelagic foraging off central Chile. We highlight the necessity of conducting further research on the trophic ecology and diving behavior of Southern sea lion to obtain a better understanding of their role as top predator.