Invasive round goby, Neogobius melanostomus - suitable prey for native cod, Gadus morhua?

Invasive species is a worldwide issue that is considered to be the one of the main reasons to losses of biodiversity. The round goby, Neogobius melanostomus, is an invasive species in the Baltic Sea that originates from the Black- and Caspian Sea. It was probably introduced via ballast water and since the discovery of the first speci-mens in 1990 round goby has spread and is now established along a majority of the Baltic coasts. To investigate the success of an invasion it is important to understand the competitive effects from the intruder but also the predator-prey interaction be-tween the introduced species and native predators. Round goby competes with native fish species for resources but it has also been shown to be an important prey to native piscivorous predators such as cod, Gadus morhua, perch, Perca fluviatilis and great cormorant, Phalacrocorax carbo. The present study was carried out to investigate cod predation on round goby in the Karlskrona archipelago over seasons. The aim of the study was to see if round goby is a common prey for cod in this area and if round goby make up a part of the diet all year round or just certain parts of the year. Cod condition over the seasons were also investigated to get an idea of the quality of round goby as prey. Round goby looks very similar to the native black goby, Gobius ni-ger, and to be able to separate the two species in stomach content an anatomical study was carried out. Stomach content from 116 cod caught in the Karlskrona archipelago was then analysed. Round goby was more common during the spring periods compared to the winter season. Cod condition was also higher during the spring periods com-pared to the winter season. Differences in round goby presence might be the result of a seasonal shift in cod diet or lack of spatial overlap between the two species. For cod condition the differences might be due to round goby consumption but further anal-yses have to be carried out to exclude other explanations for seasonal change in cod condition