Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses

Abstract Sharks are a major component of the top predator guild in oceanic ecosystems, but the trophic relationships of many populations remain poorly understood. We examined chemical tracers of diet and habitat (δ 15 N and δ 13 C, respectively) and total mercury (Hg) concentrations in muscle tissue of seven pelagic sharks: blue shark ( Prionace glauca ), short-fin mako shark ( Isurus oxyrinchus ), oceanic whitetip shark ( Carcharhinus longimanus ), scalloped hammerhead shark ( Sphyrna lewini ), pelagic thresher shark ( Alopias pelagicus ), crocodile shark ( Pseudocarcharias kamoharai ) and silky shark ( Carcharhinus falciformis ), from the data poor south-western tropical Indian Ocean. Minimal interspecific variation in mean δ 15 N values and a large degree of isotopic niche overlap – driven by high intraspecific variation in δ 15 N values – was observed among pelagic sharks. Similarly, δ 13 C values of sharks overlapped considerably for all species with the exception of P. glauca , which had more 13 C-depleted values indicating possibly longer residence times in purely pelagic waters. Geographic variation in δ 13 C, δ 15 N and Hg were observed for P. glauca and I. oxyrinchus. Mean Hg levels were similar among species with the exception of P. kamoharai which had significantly higher Hg concentrations likely related to mesopelagic feeding. Hg concentrations increased with body size in I. oxyrinchus , P. glauca and C. longimanus . Values of δ 15 N and δ 13 C varied with size only in P. glauca , suggesting ontogenetic shifts in diets or habitats. Together, isotopic data indicate that – with few exceptions – variance within species in trophic interactions or foraging habitats is greater than differentiation among pelagic sharks in the south-western Indian Ocean. Therefore, it is possible that this group exhibits some level of trophic redundancy, but further studies of diets and fine-scale habitat use are needed to fully test this hypothesis.