The ontogeny of foraging behavior in juvenile green turtles (Chelonia mydas) from Palmyra Atoll: Results from stable isotope analysis

Background/Question/Methods

Juvenile green turtles (Chelonia mydas) shift to neritic foraging grounds after a developmental period in the pelagic environment and are believed to experience a corresponding shift from omnivory to herbivory. Recent evidence, however, suggests that direct dietary shifts may not occur in all populations and that some individuals maintain an omnivorous diet after recruitment to neritic habitats. Further support is needed to substantiate if these are widespread phenomena. The Palmyra Atoll National Wildlife Refuge (PANWR) serves as a foraging ground for a previously unstudied population of green turtles in the central Pacific Ocean. Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) provide information on diet and trophic position allowing for a more thorough understanding of the ontogeny of PANWR green turtle foraging behavior.

Scute samples were collected from juvenile turtles (n = 17) captured in four regions of the atoll between 2008 and 2010. Scute is keratinized, metabolically inactive tissue that retains the isotopic signature of the diet from the time of tissue synthesis. Successive layers were removed from each scute biopsy sample and analyzed for stable carbon and nitrogen. A linear mixed model was used to analyze the δ¹³C and δ¹⁵N signatures of each layer, providing insight on resource use over time.

Results/Conclusions

A trend of increasing δ¹³C values supports an ontogenetic shift from pelagic to neritic habitats in PANWR turtles. However, δ¹⁵N values of sequential scute layers suggest that PANWR turtles may not undergo a direct trophic shift from omnivory to herbivory. In addition, high levels of individual variation of δ¹³C
and δ¹⁵N signatures in sequential scute layers from individual turtles may imply that PANWR juveniles utilize multiple foraging strategies. Maintenance of an omnivorous diet even after recruitment to PANWR foraging grounds is one possible explanation for the enriched δ¹⁵N seen in some individuals. Another hypothesis is that increasing δ¹⁵N values may indicate a change in baseline environmental δ¹⁵N values or prey isotopic values between habitats. Additional studies of baseline values and prey items are needed before either potential explanation can be confirmed. High individual variability suggests that this may be a generalist population with individuals maintaining a variable and complex dietary repertoire including omnivorous prey items.