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Diet and recruitment of juvenile green turtles at two foraging grounds in Fiji, South Pacific inferred from in-water capture and stable isotope analysis

Susanna Piovano*, Garrett E. Lemons, Ana Ciriyawa, Aisake Batibasaga, Jeffrey A. Seminoff

*Corresponding author:

ABSTRACT: Green turtles (Chelonia mydas) are listed as endangered on the IUCN Red List, yet in the South Pacific few conservation-relevant data are available for the species, especially relating to foraging and habitat use. Here, in-situ observations and stable isotope analysis (δ13C and δ15N) were used to evaluate green turtle diet and recruitment patterns at Yadua Island and Makogai Island, Fiji. Juvenile green turtles (N = 110) were hand-captured, measured and sampled. Stable isotope analysis was performed on skin samples and on putative prey items. ‘Resident’ turtles versus ‘recent recruits’ were classified based on their bulk skin tissue isotope values, which were compared with stable isotope values of local prey items and analyzed via cluster analysis. Green turtle diet composition was estimated using MixSIAR, a Bayesian mixing model. Recent recruits were characterized by ‘low δ13C/high δ15N’ values and ranged in curved carapace length (CCL) from 25.5 to 60.0 cm (mean ± SD = 48.5 ± 5.7 cm). Recruitment mostly occurred in summer. Green turtles identified as ‘residents’ had CCLs ranging from 43.5 to 89.0 cm (mean ± SD = 57.4 ± 9.0 cm) and were characterized by ‘high δ13C/low δ15N’ values; mixing model results indicate they fed primarily on invertebrates (40%), fishes (31%) and marine plants (29%). This study confirms the value of seagrass pastures as both an essential habitat and a primary food source for green turtles, and can serve as a baseline for evaluations of natural and anthropogenic-derived changes on local green turtles aggregations.