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The Mediterranean Sea is considered a biodiversity hotspot and hosts a broad diversity of shark species. Unfortunately, many shark populations have suffered a long story of overexploitation and their abundance has strongly declined over the last decades. A few studies have addressed the trophic role of sharks in the northwestern Mediterranean Sea, yet little is known about their isotopic niches. In this study, the isotopic niches of six shark species (Alopias vulpinus , Galeorhinus galeus, Hexanchus griseus , Isurus oxyrinchus , Prionace glauca and Squalus acanthias) from the northwestern Mediterranean Sea were determined and compared with those of sympatric predatory teleosts and marine mammals. Overall, δ13C and δ15N values were similar amongst all the shark species except for the three individuals of G. galeus , whose high δ15N values indicate recent immigration from a distinct area with a different isotopic baseline and hence were excluded from further analysis. The mixing models identified cephalopods as the dominant prey of the other five shark species and revealed that the medium-sized and large shark species studied here filled unique isotopic niches. Their trophic positions were similar to those of predatory teleosts and small dolphins but lower than those of larger dolphins, pilot whales and beaked whales. The high contribution of cephalopods to the diet of sharks in the northwestern Mediterranean Sea might be indicative of fish scarcity and deserves further research in the context of potential ontogenetic diet changes. • The isotopic niche of six medium-sized and large sharks was assessed. • Their trophic positions suggested they act as high-level mesopredators. • Cephalopods were the dominant prey group in the diet of five sharks. • The position of sharks in the δ13C-δ15N biplot revealed a unique isotopic niche compared to teleosts and cetaceans. [ABSTRACT FROM AUTHOR]

It is commonly assumed that the resolution of the isotopic niche of consumers can be improved with a larger number of chemical elements, but this is only true if steep environmental gradients exist at the appropriate spatial scale. Off Mauritania, the δ13C value is a useful proxy to understand the distribution of marine mammals along the inshore-offshore gradient, and the δ15N value to assess their trophic position. Here, the incorporation of δ18O values as an independent habitat tracer largely improved the resolution of the isotopic niche, because the δ18O gradients, mostly reflecting marked salinity gradients, spanned over broad spatial scales when compared to the home range of marine mammals. On the contrary, δ34S values did not improve much the resolution of the isotopic niche at the species level, although it was useful to identify individuals relying on food webs associated with anoxic sediments, such as seagrass meadows. This is because the δ34S gradient in the area develops over a small spatial scale, compared to the home range of the considered species. This study provides new insights into the use of a muti-element approach in isotopic ecology, and improves the understanding of habitat partitioning between the considered marine mammal species off North Western Africa. [Display omitted] • δ18O and δ34S complement δ13C as habitat tracers, but at different scales. • δ18O is useful to identify species using water mases of contrasting salinity. • δ34S is useful to identify individuals within a species relying on seagrass meadows. • Sighting and stranding reports validate inferences made from stable isotope ratios. • The gradient of the isotopic tracer should encompass the home ranges of the species. [ABSTRACT FROM AUTHOR]