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Highly migratory marine species pose a challenge for the identification of management units due to the absence of clear oceanographic barriers. The population structure of North Atlantic fin whales has been investigated since the start of whaling operations but is still the subject of an ongoing scientific debate. Here we measured stable isotopes of carbon, nitrogen and oxygen in skin samples collected from 151 individuals from western Iceland, Galicia (NW Spain), the Azores archipelago and the Strait of Gibraltar (SoG). We found spatiotemporal differences in stable isotope ratios suggesting that fin whales sampled in these four areas may share a common feeding ground within the Northeast Atlantic at different times during the year. Our results also suggest that SoG whales use this common feeding ground in summer but exploit Mediterranean resources during the winter months, further supporting the existence of a limited but current exchange of individuals between these two basins. • δ 13C, δ 15N and δ 18O values were measured in fin whale skin from four North Atlantic areas. • Spatiotemporal differences were found in the stable isotope profiles. • All whales may share a common Atlantic feeding ground at different times during the year. • Gibraltar whales use this common feeding ground in summer but the Mediterranean in winter. [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]