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

Abstract

Knowledge of the trophic ecology and interactions of marine top predators is fundamental for understanding community structure and dynamics as well as ecosystem function. We examined the feeding relationships of 4 heavily exploited elasmobranchs caught in coastal artisanal shark fisheries in south-western Madagascar in 2009 and 2010—Sphyrna lewini, Loxodon macrorhinus, Carcharhinus falciformis and Rhynchobatus djiddensis—using stable isotope (δ15N and δ13C) analysis. Relative trophic position (indicated by δ15N) and foraging location (indicated by δ13C) 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. djiddensis 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 δ15N 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.