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.
Highlights
Elucidating patterns of migratory connectivity for broadranging animals is central to defining spatio-temporal management priorities for these species
Leatherbacks that were satellite tracked in this study departed their Indonesian nesting beach en route to two broad regions (Figure 1): (i) the ‘western Pacific’ (n = 8), with primary destinations in the Sulawesi, Sulu, and South China Sea region and the North Pacific Transitional Zone (NPTZ), and (ii) the ‘eastern Pacific’ (n = 5), with the prevailing terminus centered in the California Current Large Marine Ecosystem
When linked with telemetry results, we found that all turtles tracked to the eastern Pacific were within the high-d15N group, whereas all but one turtle moving to western Pacific sites were in the low-d15N group (Figure 1)
Summary
Elucidating patterns of migratory connectivity for broadranging animals is central to defining spatio-temporal management priorities for these species. Satellite telemetry has been a primary tool to track long-distance movements of vertebrates from around the world [1,6,7], but the high cost of tags, satellite tracking data acquisition, and analysis (up to US $5K per animal) coupled with large body size requirements for tracked animals have precluded its application for many vulnerable species and age classes. These challenges have often resulted in extremely limited samples sizes, which thereby limit the value of telemetry as a tool for revealing migratory variability within focal populations. A new approach to rapidly assess the movements of large cross sections of imperiled populations could greatly enhance conservation of such animals
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