Epsilon Archive for Student Projects

Abstract

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