Stable isotopes and epibiont communities reveal foraging habitats of nesting loggerhead turtles in the South West Indian Ocean

Abstract

Loggerhead sea turtles (Caretta caretta) occupy an array of marine habitats throughout their lives and understanding how this threatened species utilizes these habitats can help guide conservation efforts. This study used a combination of isotopic analysis (δ13C; δ15N) of turtle epidermis and epibiotic community composition to examine habitat-use patterns of nesting loggerheads from the South West Indian Ocean and related these to turtle body condition. First, we identified two foraging groups using relative changes in the turtle’s epidermis δ13C isotope values (of 170 individuals). Second, we show that epibiont communities (of 80 turtles) differed significantly between the two foraging groups, with oceanic/pelagic and neritic/benthic species occurring in higher abundances on turtles with depleted and enriched δ13C values, respectively. The complementary evidence from isotope and epibiont data thus indicates that nesting loggerheads of this population exhibit a bimodal foraging strategy. The relatively high variability in δ13C and δ15N values of the turtles’ tissue showed that they have a broad isotopic niche. While isotopic niches overlapped among turtles with different body conditions, very healthy individuals had a significantly narrower isotopic niche width than those in poorer condition. This suggests that turtles in very good condition are food specialists, and individuals with suboptimal body conditions utilise a wider range of food items and are more generalist foragers. These results show the value of using a combination of stable isotopes and epibiont communities as cost-effective tools that can be applied effectively to derive information on habitat-use patterns of migratory species.