Using Stable Isotope Analysis to Guide Management of Stranded Green Turtles (Chelonia mydas)

Abstract Gaining a better understanding of the foraging ecology of green turtles (Chelonia mydas) in different regions can improve the effectiveness of conservation and management strategies. New South Wales (NSW), in eastern Australia, has been identified as a region requiring improved information on foraging ecology and habitat use by green turtles. This study used stable isotope ratios of δ13C, δ15N, and δ34S from macrophytes collected along the NSW coastline from -35.722585°S, 150.230674°E through to -28.537292°S, 153.555205°E, between March- June 2021, and stored epidermis tissue from stranded green turtles found along the NSW coastline from 1998-2020. The aim of the study was to investigate diet composition and foraging behaviour of green turtles stranded along the NSW coastline. Mixing model analyses demonstrate that stranded turtles had been feeding in food-webs predominantly supported by algae (74.8%), then seagrass (19.2%), and trivial amounts of mangrove material (6%). Enriched δ15N values indicate the majority of sampled turtles feeding two to three trophic levels above primary producers within algal-based food webs. High variation was present in δ13C, δ15N, and δ34S values of macrophyte sources along the NSW coastline, limiting the potential to infer the foraging behaviour of stranded green turtles. The results show green turtles stranded along the NSW coastline feed primarily in algae-dominated food-webs, with evidence of omnivory. This novel finding demonstrates the importance of algae-dominated habitats for green turtle conservation in eastern Australia. These findings may reflect opportunistic foraging behaviour of stranded green turtles, with algae being the dominate macrophyte in NSW waters.