Finding the Lost Years: Using Stable Isotopes to Identify Habitat Use Patterns of Post-Hatchling Hawksbill Turtles (Eretmochelys imbricata) in the Eastern Pacific

Recent studies identifying eastern Pacific hawksbill turtles Eretmochelys imbricata that forage in both mangrove estuaries and nearshore reef habitats highlight the need for increased research to improve conservation strategies for this critically endangered species. With much still unknown about the ecology of hawksbills in the region, and as a part of an ongoing research project, we investigated the habitat use patterns of post-hatchling turtles during their first few years of life. We combined skeletochronology using humerus bones with stable isotope analysis of skin (n=57) and annual bone growth layers (n=24) of dead-stranded turtles from El Salvador, one of the last strongholds for this species in the eastern Pacific. Our early results indicate that hawksbills are likely using different habitats during different life stages. Stable isotope analysis from tissue samples collected from hatchlings (min 3cm curved carapace length, CCL) to adults (max 84.5cm CCL) suggests that multiple ontogenetic shifts in diet and/or habitat likely exist in this population. Results showed that the stable isotope values of both carbon (d¹³C) and nitrogen (d¹⁵N) were lowest in hatchlings and, to a lesser extent, adults (>70cm CCL) and were highest in post-hatchlings (<40cm CCL) and juvenile stages (~40-60cm CCL). This stable isotope pattern suggests that post-hatchlings and small juveniles in this region inhabit offshore habitats prior to recruiting to nearshore estuary and reef habitats. As a population that frequently interacts with gillnet and blast fisheries, these results help to inform us of hawksbill ontogenetic habitat use patterns during some of their most vulnerable points in development and can be used to create more efficient conservation policies for this region.