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Stable isotope analysis of whale skin has been recurrently used to assess diet and movement patterns. Such studies rely on the untested assumption that the stable isotope ratios in the small skin biopsies analysed are representative of those throughout the skin. In balaenopterids, the ventral skin looks notably different from that of the dorsal region, which is smoother and darker. To investigate possible differences in isotopic ratios throughout the skin, we collected and analysed samples from dorsal and ventral positions in 28 fin whales ( Balaenoptera physalus ). No significant differences were found between these two skin positions, which might suggest that whale skin is likely a homogeneous tissue. Thus, the isotopic ratios determined at a specific point may be representative of the whole skin in whales. [ABSTRACT FROM AUTHOR]

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]