Trophic ecology of common elasmobranchs exploited by artisanal shark fisheries off south‑western Madagascar

Knowledge of the trophic ecology and interactions of marine top predators is funda- mental for understanding community structure and dynamics as well as ecosystem function. We examined the feeding relationships of 4 heavily exploited elasmobranchs caught in coastal arti- sanal shark fisheries in south-western Madagascar in 2009 and 2010— Sphyrna lewini, Loxodon macrorhinus, Carcharhinus falciformis and Rhynchobatus djiddensis—using stable isotope (δ 15 N and δ 13 C) analysis. Relative trophic position (indicated by δ 15 N) and foraging location (indicated by δ 13 C) differed among species. Isotopic niche width was highly variable: more pelagic species, such as S. lewini and C. falciformis, had the broadest isotopic niches while the benthic R. djidden- sis had the narrowest. A high percentage of niche overlap occurred between R. djiddensis and 2 of the species, C. falciformis (93.2%) and L. macrorhinus (73.2%), and to a lesser extent S. lewini (13.3%). Relative trophic position of S. lewini significantly increased with size, suggesting a dietary shift with age. Sex differences in δ 15 N values were observed in L. macrorhinus, suggesting intraspecific niche partitioning. Variation in stable isotope values among these 4 highly exploited elasmobranch species indicates trophic structuring, likely driven by differences in diet and habitat use as well as by size and sex. This study provides the first baseline information on the trophic ecology of elasmobranchs caught in artisanal fisheries from south-western Madagascar.