Trophic ecology of a green turtle breeding population

While many migratory marine organisms converge at breeding areas, identifying foraging strategies away from these reproductive sites can be challenging. Adult female green turtles Chelonia mydas regularly migrate thousands of kilometers between nesting and foraging areas, making it difficult to identify foraging habitats that support nesting populations and to understand their feeding strategies. In this study, we use stable isotope analysis to investigate the trophic ecology and spatial distribution of foraging green turtles in the Greater Caribbean. Further, we explore the possibility that adult green turtles, originally considered to be herbivores, may, like their counterparts in the Pacific Ocean, display carnivorous feeding strategies. The wide range of carbon and nitrogen isotope values in bulk epidermis observed in the nesting population at Tortuguero, Costa Rica, could indicate that these turtles feed over several trophic levels. Isotopic niches — or the range of δ13C and δ15N values, which can be used as a proxy for ecological niche — varied among the 5 green turtle foraging aggregations sampled. Similarly, the isotopic composition of the primary producer Thalassia testudinum also varied substantially with geographic location. However, compound-specific stable isotope analysis of amino acids (AA-CSIA) indicated that individuals in the nesting population with different bulk δ15N values feed at the same trophic position. The combined results suggest that spatial differences in the isotopic composition of seagrass at the base of the food web, rather than differences in turtle foraging strategy, contribute to the isotopic variation in the nesting population. This study improves understanding of the foraging ecology of a highly dispersed and migratory species.