Isotope analysis reveals foraging area dichotomy for atlantic leatherback turtles

Abstract

Background: The leatherback turtle (Dermochelys coriacea) has undergone a dramatic decline over the last 25 years, and this is believed to be primarily the result of mortality associated with fisheries bycatch followed by egg and nesting female harvest. Atlantic leatherback turtles undertake long migrations across ocean basins from subtropical and tropical nesting beaches to productive frontal areas. Migration between two nesting seasons can last 2 or 3 years, a time period termed the remigration interval (RI). Recent satellite transmitter data revealed that Atlantic leatherbacks follow two major dispersion patterns after nesting season, through the North Gulf Stream area or more eastward across the North Equatorial Current. However, information on the whole RI is lacking, precluding the accurate identification of feeding areas where conservation measures may need to be applied.

Methodology/principal findings: Using stable isotopes as dietary tracers we determined the characteristics of feeding grounds of leatherback females nesting in French Guiana. During migration, 3-year RI females differed from 2-year RI females in their isotope values, implying differences in their choice of feeding habitats (offshore vs. more coastal) and foraging latitude (North Atlantic vs. West African coasts, respectively). Egg-yolk and blood isotope values are correlated in nesting females, indicating that egg analysis is a useful tool for assessing isotope values in these turtles, including adults when not available.

Conclusions/significance: Our results complement previous data on turtle movements during the first year following the nesting season, integrating the diet consumed during the year before nesting. We suggest that the French Guiana leatherback population segregates into two distinct isotopic groupings, and highlight the urgent need to determine the feeding habitats of the turtle in the Atlantic in order to protect this species from incidental take by commercial fisheries. Our results also emphasize the use of eggs, a less-invasive sampling material than blood, to assess isotopic data and feeding habits for adult female leatherbacks.

Figure 1. Trends in isotopic values for δ¹³C and δ¹⁵N in plasma, RBC, and egg-yolk of nesting leatherback turtles.

Each colour represents data for the successive clutches laid by one female; each sampled clutch is represented by a point.

Figure 2. Effect of remigration interval (RI) on δ¹³C and δ¹⁵N leatherback tissus (plasma, RBC and egg-yolk).

Values are mean±SD. The black symbols represent the values of Wallace et al. for Atlantic and Pacific populations of leatherback turtles, provided for comparison. The map shows the two major patterns of migration of Atlantic turtles, from nesting beaches to foraging areas, following Ferraroli et al. and Hays et al. .

Figure 3. Location of the study site, and leatherback turtle blood sampling.

A Map of Awala Yalimapo nesting beach in French Guiana. B Blood sampling of a large female leatherback turtle (Dermochelis coriacea) during a nesting event.