Ontogenetic changes in diet and habitat use in green sea turtle (Chelonia mydas) life history

Green turtles Chelonia mydas are endangered, long-lived marine reptiles that display an ontogenetic shift in diet and habitat use during development. During their early life stage, juvenile green turtles in the southwestern Pacific inhabit the pelagic zone where they feed omnivorously on neustonic material. At approximately 44 cm curved carapace length they recruit to inshore foraging habitats where they become primarily herbivorous. In this study we investigate the change in stable isotope (delta C-13 and delta N-15) composition of green turtle epidermal tissue throughout their life history to examine this ontogenetic shift in diet and habitat as it occurs in a southwestern Pacific green turtle population. Turtles that had recently recruited to foraging grounds in Moreton Bay, Australia had significantly higher delta N-15 isotopic signatures when compared with all other life history groups examined and significantly lower delta C-13 when compared with all age classes other than pelagic juveniles. Adult and large immature turtles had similar isotopic signatures and were both significantly enriched in C-13 when compared with hatchlings and small immature turtles. These results support previous observations that suggest pelagic juveniles are foraging in a different habitat and at a higher trophic level than turtles captured in the neritic environment. This is the first study to capture the entire life history of green turtles in terms of foraging ecology and supports the ontogenetic shift previously observed in traditional diet and behavioral studies of green turtles.