Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic using stable isotopes

Differential foraging area use can affect population demographics of highly migratory fauna because of differential environmental changes and anthropogenic threats among those areas. Thus, identification of foraging areas is vital for the development of effective management strategies for endangered migratory species. In this study, we assigned 375 loggerhead turtles (Caretta caretta) nesting at six locations along the east coast of the United States to their foraging areas in the Northwest Atlantic (NWA) using carbon and nitrogen stable isotope values (delta C-13 and delta N-15). We first evaluated the epidermis delta C-13 and delta N-15 values from 60 adult loggerheads with known foraging grounds. Twenty-two females from 6 nesting beaches and 23 males from one breeding area were tracked with satellite transmitters to identify their foraging locations following breeding, and 15 adult turtles were sampled at one foraging ground. Significant trends were observed between both delta C-13 and delta N-15 values of satellite-tracked loggerheads and the latitude of the foraging grounds to which the turtles migrated, reflecting a geographic pattern in the stable isotope values. Both delta C-13 and delta N-15 values characterized three geographic areas-with distinct abiotic and biotic features-used by adult loggerheads in the NWA. Discriminant analysis assigned all 375 female loggerheads to one of the three foraging areas; 91% were assigned with probabilities of >= 80%. The proportion of nesting turtles using each foraging ground varied geographically; most turtles nesting in northern beaches (72-80%) tend to forage at higher latitudes while most turtles nesting in southern beaches (46-81%) tend to forage at lower latitudes. Stable isotopes can reveal the foraging location of loggerhead turtles in the NWA, which will allow robust analyses of foraging ground effects on demography and improve the design of management strategies for the conservation of loggerhead populations. The conclusions and methods developed in this study are also relevant for other populations of sea turtles and for other highly migratory species.