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When isotopes fail: importance of satellite telemetry and multi-site validation when estimating the foraging grounds of migratory species

O. I. Coffee*, D. T. Booth, J. A. Thia, C. J. Limpus

*Corresponding author:

ABSTRACT: A combination of δ13C and δ15N isotope ratios, capture-mark-recapture and satellite telemetry data were used to investigate the foraging locations supporting nesting loggerhead turtles on the Woongarra coastline, south Queensland, Australia. Known foraging grounds were available for a subset of these turtles and supplemented with samples taken from foraging turtles to determine whether foraging regions could be identified in untracked nesting turtles using the stable isotopic values of their sampled tissue. Despite the large distances between known foraging regions, no latitudinal gradient was observed in the isotopic values of these turtles. In addition, the isotopic values of foraging individuals from a single site encompassed the range of all loggerhead turtles sampled over the 2000 km north–south distribution along the east coast of Australia. This study demonstrates that assumptions common to δ13C and δ15N stable isotope inference in migratory species are not adhered to globally. Contrasting the successes of stable isotope analysis in the northern hemisphere, these results indicate that factors in the southwest Pacific such as differing prevailing oceanic currents, temperatures regimes and river run-offs may prevent the establishment of region-specific unique isoscapes needed for identifying the foraging region of turtles using their isotopic values. Therefore, we caution against the use of δ13C and δ15N stable isotopic values as suitable indicators of foraging regions for loggerhead turtles in Australia. These findings potentially highlight the need to re-evaluate when and where the use of isotopic analysis is appropriate for identifying foraging locations in marine turtle species.