| Summary: | peer reviewed The average temperature of the North Sea has risen twice as fast as the oceans of the world with already detectable consequences to the ecosystem. The North Sea is a critical habitat to different marine mammal species including the grey seal, Halichoerus grypus, the harbour seal Phoca vitulina, and the harbour porpoise, Phocoena phocoena. Long-term approach of their feeding ecology is crucially needed to understand their response in term of global changes and management of their population. A multi-tracer approach (THg and Se; 13C, 15N, 34S values) was applied on harbour seals and grey seals (blood; German and Scottish coasts, n=174 and n=80, respectively) during different time periods (from 1900 to 2017). Additionally, the same tracers were analysed in harbour seals (hair; from 1930 to 2017) from the Natural History Museum of Stockholm (n =10), free-ranging and stranded harbour seals from the North Sea (German and Belgian coasts, n=40 and n=6, respectively). Data acquired previously on harbour porpoises from the southern part of the North Sea were integrated (muscle; 1994-2018, n=188). Our multi-tracer approach revealed: (1) an increase of trophic niche size and foraging distribution (from inshore to offshore), an increase of foraging resource items (more flatfish and less sandeels) for Scottish grey seals over time; (2) long-term changes having affected the North Sea are reflected in hair collected from harbour seals. We found a linear concomitant increase of THg and 13C values, reflecting a higher exposure combined to a more intense use of wind farm locations for foraging activities. Wind farms areas are particular ecosystems with an abundant marine life and different isotopic values. To conclude, our results showed a high degree of plasticity in foraging resources for harbour seals and in foraging spatial distribution for grey seals. More research is needed to infer on impacts of wind farms, also in relation to disturbance. 14. Life below water
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