New fish romping in the southern Baltic

In less than three decades the round goby has become one of the most colourful features of the southern Baltic. The fish, which comes from the Black Sea, has rapidly adapted to Baltic conditions and can locally dominate coastal fish populations. This has led to competition with indigenous fish species, such as the flounder, but it has also become a significant contribution to the diet of important predatory fishes, such as cod and perch. This is shown in a dissertation in systems ecology at Stockholm University in Sweden.

The human dissemination of alien species is regarded as one of the greatest threats to maintaining biodiversity. The round goby was probably brought to the Baltic from the Black Sea via the ballast water of huge freighters, which is one of the most common ways for alien water-dwelling organisms to spread in global terms. Gustaf Almqvist at the Department of Systems Ecology, Stockholm University, has been studying the progress of these fish in the Gulf of Gdansk for the past five years.

He reports that the fish are so common there that when you wade into shallow water you can see them jumping and splashing around your feet.

'Since the species is an entirely new ingredient in the fauna of the Baltic, this is a golden opportunity to study, on the one hand, how it is adapting and, on the other hand, how it is affecting typical Baltic species,' says Gustaf Almqvist.

His research shows that the species evinces differences in sexual maturity, growth, and longevity under different conditions, which probably facilitates its adaptation to strictly separate environments.

Together with Swedish and Polish colleagues, Gustaf Almqvist has shown how the new species has the potential to compete with indigenous fishes, like the flounder. Even though the fish seldom grows to be more than 20 cm long, it is relatively large in comparison with related indigenous fishes (sand goby, common goby, black goby). It is aggressive and territorial (the males guard the young) and therefore forces out other bottom-dwelling fishes from major coastal areas.

The two most common predatory fish in the southern Baltic, the cod and the perch, have started to exploit the new species as prey, and in coastal areas the round goby is at the top of their menu.

Mussels, above all blue mussels, are important food for round gobies, and since other fish species in the southern Baltic do not eat blue mussels to any great extent, the round goby constitutes a new link between mussels and predatory fish.

'More energy, but also more poisons, that can aggregate in mussels can therefore be transported from mussels higher up the food chain,' says Gustaf Almqvist.

Everything indicates that the future spread of the species within the Baltic will be dependent on the climate, since it needs long warm summers to establish stocks.

'It's difficult to predict the future spread of the species in the Baltic, but considering changes in climate and the fact that there is no shortage of blue mussels in the Baltic, I find it hard to believe that it will not spread further,' says Gustaf Almqvist.

Gustaf Almqvist wrote his dissertation in the interdisciplinary research project AquAliens (funded by the Swedish Environmental Protection Agency) and has been employed by the Coastal Laboratory of the Swedish Board of Fisheries in Oeregrund and by the Department of Systems Ecology, Stockholm University.