van Eerden M.R., van Rijn S., Kilpi M., Lehikoinen A., Lilleleht V., Millers K. & Gaginskaya A. (2021) Expanding East: Great Cormorants Phalacrocorax carbo thriving in the eastern Baltic and Gulf of Finland. ARDEA 109 (3): 313-326

Great Cormorants Phalacrocorax carbo in the eastern Baltic are expanding in numbers. Eight colonies in Estonia, Finland and the Russian Federation were investigated in 2007. Brood size and condition of large nestlings were determined and indicate that food provisioning is not a limiting factor. Food items consisted of both pelagic and benthic prey, with marked differences among the colonies. Eelpout Zoarces viviparus and Roach Rutilus rutilus were the most important prey according to fish mass. For pelagic prey this was Sprat Sprattus sprattus. Additional data in later years support the conclusion that the species experienced a rapid expansion at the time of investigation. Two factors are likely to have contributed to improved conditions for Cormorants in this part of the eastern Baltic. First, the low abundance of predatory fish such as Cod Gadus morhua and Pikeperch Sander lucioperca as a result of overfishing by commercial fisheries as well as climate change has, most likely, caused small benthic and pelagic prey fish to thrive. Second, increased eutrophication has probably fuelled the fish production. Based on fish species taken and the number of Cormorants present, the conclusion seems justified that any interaction between Cormorants and fisheries is unlikely at the scale of the Finnish Gulf, although locally measurable effects of Cormorant consumption on commercial yield may exist. As a visible, relatively new apex predator to the system, the Great Cormorant may well be an indicator of the ecological condition of the system. Monitoring of Cormorants (numbers, distribution, food, breeding success) may therefore provide useful data which can be used to assess the recovery of the benthic and pelagic parts of the food web.