Food web interactions in a Baltic Sea coastal area

In the brackish water of the Baltic Sea, marine and freshwater species co-occur in unique combinations. In Himmerfjarden, an embayment of the northern Baltic Sea, the zooplanktivorous trophic level is dominated by a typical marine species, herring (Clupea harengus). This species constitutes the basic prey for the dominant pelagic piscivorous species, the freshwater pikeperch (Stizostedion lucioperca). In this paper we have quantified the predation pressure by pikeperch on herring. Most of the herring production in the area is made up of young-of-the-year fish (YOY), which also dominate the diet of pikeperch. However, due to an intensive fishery the pikeperch population is considerably reduced and the estimated consumption of YOY herring by pikeperch is only about 25% of the herring production. Managing the population to increase pikeperch could be of interest from three aspects: (1) Pikeperch is economically a very valuable species and an increased population will significantly benefit the local fishery. (2) With an increased pikeperch population, predator-prey relationships would be more similar to those expected if humans had not influenced the ecosystem by means of the fishery. (3) An increased pikeperch population could, through cascading trophic interactions, contribute to decreased phytoplankton biomasses and improved water quality in the eutrophied Himmerfjarden.