Has the invasive round goby caused new links in Baltic food webs?

The Ponto-Caspian round goby (Neogobius melanostomus, Pallas 1814) most probably was established in the Gulf of GdaAsk, Baltic Sea, in the late 1980's and has since become one of the dominant species in the region. In this study we assess the role of round gobies as prey for two important fish species in the Gulf of GdaAsk, cod (Gadus morhua) and perch (Perca fluviatilis). We compared their present diet with stomach analyses from the area prior the round goby establishment, as well as with diet analysis from Baltic regions where round gobies are absent. There were large differences in the diet between cods from the Gulf of GdaAsk 2003-2006 compared to cods in earlier studies (1977-1981) from the Southern Baltic Sea. There were also large differences in cod and perch diets from areas with and without round goby. Presently, round goby constitutes the most important prey for medium sized cods in Gulf of GdaAsk, and perch from the same area almost exclusively feed on gobiids. Stomach analysis, trophic level estimates, and stable isotope analyses all indicated that cod and perch in Gulf of GdaAsk after the round goby establishment belonged to a similar trophic level. Beside round goby, no mussel feeding fish contributed much to the diet of cod or at all to the diet of perch. Thus, it is likely that round gobies constitute a new energetic pathway from mussels to top predators. However, due to the short time elapsed after round goby establishment, we can only speculate on the species future impacts on Baltic food webs.