Sustainable management of coastal ecosystems, the role of harbour seals in the marine food web.

Sustainable management of our coastal seas is of highest priority for biodiversity conservation and one of the Global Sustainability Goals.

While it is clear that overfishing and eutrophication are the main drivers for the current catastrophic state of the sea, including collapsed size and age-structure of herring and cod stocks along Swedish coasts, the role of top predators and in particular seals remains debated, and authorities struggle to identify efficient conservation measures to promote the rebuilding of fish stocks.

It is currently assumed that marine mammal predation by default has a negative impact, removing fish that humans could have caught, and seal hunting quotas are gradually increasing. However, recent research in marine food-webs show top predators can have a positive effect. Seals target a multitude of non-commercial fish species with different ecosystem functions.

We propose a three step action plan to quantify the magnitude and variation in effects of the impact of marine mammals in Swedish marine ecosystems. 1) Characterize the trophic niche of the harbour seal, and draw ecosystem flow charts. 2) Quantify long-term changes in prey composition of seal diet, and assess any associations with fishery statistics on abundance. 3) Provide recommendations to authorities on sustainable management of marine ecosystems with regard to top predators and define a road map to fill knowledge gaps.