The impact of industrial trawlers on the coastal ecosystem

Coastal fishermen and managers have long argued that the large harvest of herring from industrial trawlers in the Baltic Sea can force species such as grey seals and cormorants to move closer to the coast in search of food other than herring. This has never been investigated with scientific methodology before – but will now be carried out within the framework of the project The impact of industrial trawlers on the coastal ecosystem. If seals and cormorants eat more of the coastal predatory fish, it can have effects on the entire coastal ecosystem – for example, the amount of filamentous algae can increase and thus amplify the symptoms of eutrophication.

Baltic herring is the grey seal's favourite food

Since the grey seal has herring as its favorite food in the Baltic Sea, Agnes will look at whether a possible spread into the archipelago is due to the reduced availability of large herring – a decrease that is probably largely driven by the large-scale trawling in the open sea.

Harder predation pressure can trigger a "trophic cascade"

Since seals and cormorants can eat a lot of perch and pike when they search for food further into the archipelago, the plan is to look at how this can affect the rest of the coastal ecosystem. Previous research indicates that a higher predation pressure on pike and perch can trigger a so-called trophic cascade, where a higher predation pressure leads to fewer predatory fish, which in turn leads to an increase in one of their prey – the stickleback. The increased amount of sticklebacks in turn increases the predation pressure on small grazers such as amphipods, which play an important role in grazing on the filamentous algae that otherwise deteriorate habitat and water quality. Increased predation by seals and cormorants could in theory have major effects on the coastal ecosystem, and that is something that will be examined in this research project.

Data from several different environmental monitoring programmes will be used

All work within the project will be based on data already collected from several different environmental monitoring programs. In addition, Agnes will use seal and cormorant observations that the public has reported in Artportalen.