Stable Isotope Tracking of Endangered Sea Turtles: Validation with Satellite Telemetry and δ15N Analysis of Amino Acids

Abstract

Effective conservation strategies for highly migratory species must incorporate information about long-distance movements and locations of high-use foraging areas. However, the inherent challenges of directly monitoring these factors call for creative research approaches and innovative application of existing tools. Highly migratory marine species, such as marine turtles, regularly travel hundreds or thousands of kilometers between breeding and feeding areas, but identification of migratory routes and habitat use patterns remains elusive. Here we use satellite telemetry in combination with compound-specific isotope analysis of amino acids to confirm that insights from bulk tissue stable isotope analysis can reveal divergent migratory strategies and within-population segregation of foraging groups of critically endangered leatherback sea turtles (Dermochelys coriacea) across the Pacific Ocean. Among the 78 turtles studied, we found a distinct dichotomy in δ15N values of bulk skin, with distinct “low δ15N” and “high δ15N” groups. δ15N analysis of amino acids confirmed that this disparity resulted from isotopic differences at the base of the food chain and not from differences in trophic position between the two groups. Satellite tracking of 13 individuals indicated that their bulk skin δ15N value was linked to the particular foraging region of each turtle. These findings confirm that prevailing marine isoscapes of foraging areas can be reflected in the isotopic compositions of marine turtle body tissues sampled at nesting beaches. We use a Bayesian mixture model to show that between 82 and 100% of the 78 skin-sampled turtles could be assigned with confidence to either the eastern Pacific or western Pacific, with 33 to 66% of all turtles foraging in the eastern Pacific. Our forensic approach validates the use of stable isotopes to depict leatherback turtle movements over broad spatial ranges and is timely for establishing wise conservation efforts in light of this species’ imminent risk of extinction in the Pacific.

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

“…5A). Recent studies have integrated telemetry data to validate marine geographic patterns in d 13 C or d 15 N values of highly migratory animals such as seabirds (Jaeger et al 2010) and sea turtles (Seminoff et al 2012) over broad spatial scales (e.g., within ocean basins). Here, we present the combined d 13 C and d 15 N spatial characterization for a highly migratory animal at a regional scale in the NWA.…”

“…In sea turtles, epidermis, keratin, and red blood cells reflect a longer-term foraging history (Reich et al 2008). Therefore, such tissues collected for turtles at breeding areas reflect their dietary history at foraging grounds prior to migration to the breeding area (Wallace et al 2006, Caut et al 2008, Vander Zanden et al 2010, Zbinden et al 2011, Pajuelo et al 2012, Seminoff et al 2012.…”

“…80% probability of group membership (Rocque et al 2006, Seminoff et al 2012) (340 out of 375 turtles). Finally, a chi-square test was performed to test for inter-annual variation in the proportion of turtles using different foraging grounds for nesting beaches that were sampled in two consecutive years, whenever sample size allowed (i.e., CNS, MEL, and JUN beaches; Table 1).…”

Spatial and isotopic niche partitioning during winter in chinstrap and Adélie penguins from the South Shetland Islands

“…In addition, advances in the use of stable isotope analysis (SIA), which is based on the principle that animals ''are what and where they eat'' Bearhop 2008, Bond andJones 2009), now provide the ability to estimate foraging niches from tissues grown during the nonbreeding period when individuals are away from land (e.g., Polito et al 2011a). In combination, direct tracking and SIA provide a robust toolset to quantitatively track the movements and foraging niches of wide-ranging marine organisms, including seabirds, throughout much of their annual cycle (González-Solís et al 2007, Block et al 2011, Polito et al 2011a, Seminoff et al 2012. Interestingly, recent studies using the approaches above have noted clear patterns of spatial and temporal foraging niche segregation within and between sympatric seabird species during the winter months when they are away from breeding colonies (Thiebot et al 2011, Ratcliffe et al 2014.…”

Modeling and mapping isotopic patterns in the Northwest Atlantic derived from loggerhead sea turtles

“…Satellite-tracked individuals often constitute training data for the development of models to geographically assign individuals of unknown origin (e.g., Jaeger et al 2010, Ceriani et al 2012, Seminoff et al 2012. However, to apply telemetry assignment models with confidence, it is critical to assess their performance by conducting external validation.…”