| Summary: | For several decades, the North Sea has been impacted by climate- and other human-driven changes such as chemical and noise pollution, marine traffic and intensive fishing activities. Indeed, the North Sea contributes to 4-5% of the world fish production with more than 5 million of tons of fish per year and is considered as one of the most productive fishing area in the world. This may be a real problem for the renewal of some fish species that play an important role in the food chain such as sandeels (Ammodytidae). They are the trophic link between plankton and marine mammals as grey seals (Halichoerus grypus) and harbour seals (Phoca vitulina); both piscivorous and sympatric species that inhabit much of the coasts and continental shelf waters of the North Sea. Classifying foraging resources and foraging distributions of marine mammals in relation to anthropogenic activities and environmental conditions is important to guide marine conservation. This study provided a rare opportunity to determine the poorly known 1) ecological niche partitioning and competition between sympatric grey seals and harbour seals and 2) temporal variations in their feeding ecology in the changing environment of the North Sea by using a multi-tracer approach - with stable isotope ratios of carbon, nitrogen and sulphur and mercury and selenium concentrations - to test one of the most pertinent ecological questions of our time: How do populations respond to environmental changes? As such, we have been able to 1) identify current, short-term and long-term variations in foraging resources and spatial distributions of sympatric grey seals and harbour seals in the North Sea, 2) describe influence of prey abundance variations on their foraging habits, 3) identify potential human infrastructures that may influence their foraging activities and 4) clarify the important use of a multi-tracer approach in ecological studies. In undertaking these analyses, we have identified substantial variations in foraging resources and spatial distributions of ...
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