Foraging site fidelity and stable isotope values of loggerhead turtles tracked in the Gulf of Mexico and northwest Caribbean

We used stable isotope ( delta C-13, delta N-15, delta S-34) analysis in combination with satellite telemetry to evaluate the foraging areas chosen by 88 loggerhead turtles Caretta caretta nesting in southwestern Florida. Nine turtles were tracked and skin-sampled in more than one nesting season to evaluate within-individual consistency in foraging sites and stable isotope values. Turtles migrated to 5 regions: Caribbean, Florida Keys, West Florida Shelf, northern Gulf of Mexico, and Yucatan Peninsula. The stable isotope ratios across these foraging grounds ranged from -21.16 to -7.69 parts per thousand for delta C-13, 3.27 to 13.99 parts per thousand for delta N-15, and 1.91 to 20.64 parts per thousand for delta S-34. We compared bulk skin tissue stable isotope values for all turtles by bioregion, year, body size, depth of putative foraging area, and linear distance from the closest shore; among these factors, only bioregion showed a significant effect on isotope values. There were subtle regional differences in mean delta C-13, delta N-15, and delta S-34, and an apparent north-south isotopic shift aligning strongly with ocean currents adjacent to the Florida Keys. The influence of coastal topography and shifting biogeographic boundaries such as the Loop Current may cause strong ocean water mixing that results in the observed similarities in stable isotope values among regions. These results indicate that stable isotopes alone may be an inadequate tool for identifying fine-scale (< 100 km) residency of sea turtles within this ocean region.