Stable isotope composition of multiple tissues and individual amino acids reveals dietary variation among life stages in green turtles (Chelonia mydas) at Ningaloo Reef

Diet is fundamental to an individual’s biology because energy acquired from food constrains growth and reproduction, which subsequently influences survival. It is, therefore, important to have a strong understanding of a population’s diet for species of conservation concern, such as the green turtle (Chelonia mydas). While the diet of adult green turtles is generally characterised as primarily herbivorous, growing evidence suggests variation in diet between and within populations is prevalent. We use complementary stable isotope analysis techniques to elucidate diet variation within a C. mydas population (ranging from small juveniles to adults) foraging at Ningaloo Reef in Western Australia. Analyses of multiple tissues and samples from ten individuals recaptured between 4 months and 4.5 years apart revealed that adults showed the highest levels of individual specialisation and consistency in diet over time. Analysis of red blood cell δ¹³C and δ¹⁵N values revealed macroalgae is likely the dominant food source for all size classes, and sub-adult and adults also ate animals (probably jellyfish). Compound-specific stable isotope analysis of amino acids indicated the main sources of essential amino acids for Ningaloo C. mydas were macroalgae or bacteria. Taken together, these results suggest C. mydas at Ningaloo conform to the general description of adult C. mydas diet as predominantly herbivorous, but diet varies with size and between adult individuals. Consideration of within-population diet variation will be important for predicting responses to stressors such as climate change, that directly affect foraging resources, as fitness consequences may vary for individuals with different diets.