Inferring long-term foraging trends of individual juvenile loggerhead sea turtles using stable isotopes

Individual variation in diet composition has important evolutionary, ecological, and conservation implications. However, few studies of marine turtles have examined long-term foraging behavior of individuals. Using stable carbon (delta C-13) and nitrogen (delta N-15) isotope values from blood plasma, we examined long-term variation in resource use of 27 juvenile loggerhead turtles Caretta caretta captured multiple times in North Carolina between 2001 and 2010. Seven turtles displayed a marked increase in delta N-15 values consistent with a shift from oceanic to neritic habitat. These turtles, which we designated as 'recruits', had initial straightline carapace lengths (SCL) and delta N-15 values at first sampling that were significantly lower (p = 0.007 and p < 0.0001, respectively) than the remaining 20 'resident' turtles. For the resident group, the influence of SCL on delta C-13 (p = 0.02) and delta N-15 (p = 0.05) values was significant. Individual effects on delta C-13 values were also significant (p < 0.001), indicating that foraging patterns vary among individuals; however, individual effects were not significant for delta N-15 (p = 0.07). Stable isotope niche width calculations suggested that residents were consistent in resource selection over time, and individuals were specialized in resource use relative to the population. Also, the resident population was more specialized than generalized in resource use, and stable isotope mixing (MixSIAR) model results suggest their diet is largely comprised of horseshoe crabs Limulus polyphemus. These findings improve understanding of the foraging ecology of juvenile loggerhead turtles, and highlight the importance of examining the long-term diet composition of individuals.