Ontogenetic dietary changes of green turtles (Chelonia mydas) in the temperate southwestern Atlantic

The present study combines esophageal lavage (n = 74), stomach content (n = 52) and stable isotope analysis (n = 126) to understand the ontogenetic dietary shift of green turtles (Chelonia mydas) inhabiting the temperate waters off Uruguay. Based on esophageal and stomach analysis, green turtles in the region start consuming macroalgae soon after recruiting to neritic habitats; however, gelatinous macrozooplankton is still a major component of the diet of neritic juvenile green turtles measuring of less than 45 cm in curved carapace length (CCL). Conversely, turtles larger than 45 cm CCL were predominantly herbivores, with a gradual increase in the occurrence of macroalgae with size. Stable isotope analysis confirmed the dietary pattern revealed by esophageal lavage and stomach contents analysis, and also revealed that most of the green turtles smaller than 50 cm CCL found in Uruguayan waters had moved from Brazil only a few months ago. This conclusion is based on the large differences in the delta N-15 values of potential prey from southern Brazil and Uruguay and on a strong signal from Brazilian macrophytes in the skin of most green turtles from Uruguay. Turtles larger than 50 cm CCL, conversely, made a more prolonged use of Uruguayan foraging grounds. Furthermore, according to the stable isotope ratios in their skin, some turtles remained year round in Uruguayan coastal waters. The overall evidence indicates that green turtles inhabiting the coastal waters off Uruguay exhibit a rapid, but not abrupt, dietary shift after recruiting to neritic habitats and are best described as omnivores than as pure herbivores, with a relevant role of gelatinous macrozooplankton in their diets. Furthermore, most of the turtles spend only short periods in the area and their primary foraging grounds are in Brazil.