A diet study of post-breeding Great cormorants (Phalacrocorax carbo sinensis) on Gotland

Sammanfattning: The Swedish population of great cormorant (Phalacrocorax carbo sinensis) has rapidly increased to over 40 000 breeding couples since the EU-protection 1980. Since the cormorant are high efficient predators living in large colonies, they are accused for reducing fish populations and competing with fishery. An intense conflict between cormorants, fishery and conservationist has arisen. Cormorants are top predators in the food web. It is therefore essential to attain knowledge regarding their ecological role and fish consumption, to be able to evaluate the effects on the ecosystem, but also to evaluate the competition between cormorants and fishery. This study aims to assess the prey species composition of cormorants on Gotland based on otoliths from regurgitated pellets. 397 pellets were collected and analysed from four different localities, divided into north and south. The pellets together contained 4363 otoliths which were identified to the lowest possible taxon. To correct for biases caused by eroded otoliths, and complete loss of otoliths, size correction factors (SCF) and numerical correction factors (NCF) were applied. Results shows that cod (Gadus morhua), European flounder (Platichthys flesus) and sticklebacks (Gasterosteidae) dominated the diet in the north area, while shorthorned sculpin (Myoxocephalus scorpius), cod and European flounder dominated in the south. The results contribute with valuable information regarding cormorant diet in the Baltic sea. This information increases the understanding how cormorants can affect fish populations and identify possible management measures. To fully evaluate the effects of cormorants on fish populations, ecosystem and fishery, additional studies are necessary. Keywords: Baltic sea, regurgitated pellets, otoliths, prey species composition, sea birds