Stable isotopes reveal dietary differences and site fidelity in juvenile green turtles foraging around Sao Tome Island, West Central Africa

Green sea turtles are common in West Central Africa, but little is known about the occurrence of immatures in foraging grounds in the Gulf of Guinea islands, known for their volcanic origin and narrow coastal fringes. This study presents results of in-water surveys in foraging grounds off Sao Tome Island, in the Sao Tome and Principe archipelago, providing the first available data on the size distribution of immature green sea turtles of different life-stage groups on these islands. Two sites offering distinct types of food sources were studied, and isotopic signatures of immature turtles hand-captured at each foraging site were used to infer (1) how long they were established at the foraging sites and (2) their diet preferences. Recruitment in the region was estimated to occur at a minimum size of 34 cm curved carapace length (CCL), and resident immature turtles ranged from 53 to 87 cm CCL. Immatures sampled at each site showed clear differences in isotopic signatures, suggesting that they establish specific home ranges related to the available diet items and use them for extended periods of at least several months. Macroalgae were as important as or more important than seagrasses for the turtles' diets, and there was evidence that these individuals are not strictly herbivorous. Our study provides the first dataset for comparison with demographic data from other locations in West Africa, where current knowledge on green turtle foraging behavior is limited, and indicates that even oceanic islands that are geologically recent like Sao Tome may provide important recruitment and development habitats for juvenile green turtles.