Using stable-isotope analysis to assess recent diet and habitat use of stranded green turtles (Chelonia mydas)

Context: 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 for this species. 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.

Aims: The aim of this study was to use stable-isotope analyses to investigate diet composition and foraging ecology of green turtles found stranded along the NSW coastline from 2018 to 2021.

Methods: This study measured δ¹³C, δ¹⁵N, and δ³⁴S in macrophyte (mangrove, seagrass, and algae) samples collected along the NSW coastline, between March and June 2021, and epidermis tissue from green turtles that stranded in the same region between 2018 and 2021. In addition, stable-isotope values for animals known, or expected, to be consumed by green turtles were sourced from the literature. Macrophyte stable-isotope values were mapped to display spatial variations, then combined with literature-sourced animal matter and measured epidermis isotope values into stable-isotope mixing models. Mixing models were used to determine diet structure for each stranded green turtle, and their similarity to regional macrophyte isotope values to infer potential foraging locations.

Key results: High variation was present in δ¹³C, δ¹⁵N, and δ³⁴S values of macrophyte sources along the NSW coastline, limiting the potential to infer the recent foraging locations of stranded green turtles. High δ¹⁵N values in green turtle epidermis samples indicated that most sampled turtles were feeding two to three trophic levels above primary producers within algal-based food webs. Mixing models suggested that these food webs were predominantly supported by algae (mean +/- s. d. = 51.2 ± 6.1%) and animal material (31.4 ± 4.7%), with mangrove (13.4 ± 2.0%) and seagrass (4.0 ± 2.5%) representing much smaller proportions of overall diet composition.

Conclusions: The results showed that green turtles stranded along the NSW coastline feed primarily in algae-dominated food webs, with evidence of omnivory. These findings suggest that sampled individuals display diet compositions alternative to those of their Queensland counterparts, who feed primarily in seagrass-dominated habitats. Management and conservation programs in NSW should focus on limiting anthropogenic disturbances in algae-dominated habitats.

Implications: This novel finding demonstrated the importance of algae-dominated habitats for green turtle conservation and management in south-eastern Australia. The generated isoscapes are useful in examining isotopic variations in macrophytes along the NSW coastline.