"The invasion of foreign species is a result of human activity, usually the movement of cargo ships. If we want the cheap goods they bring, we inevitably have to put up with foreign species," said Henn Ojaveer, professor of marine ecosystems at the Pärnu College of the University of Tartu and one of the authors of the study.

Ojaveer and his colleagues looked at the effect invasive species have on natural benefits, including human food, marine services, tourism and economy.

The biggest impact follows a tiny water flea (Gammarus tigrinus) first found outside of its habitat in Estonian waters – Pärnu and Muuga bays. "It is very numerous and the populations of several other species of zooplankton have fallen considerably since its introduction. We also know that the Baltic herring and sticklebacks love to feast on it, and it's long tail tends to clog up fishing nets for an effect on fishing," Ojaveer said.

Another foreign species causing major problems is a segmented worm a few centimeters in length called Marenzelleria neglecta that can be found everywhere in the Baltic. The fish do not really like it, while its concentration can really explode in places. Digging up to 40 cm into the bottom sediment, it changes the physical and chemical makeup of the sediment and the water directly above it," Henn Ojaveer shared.

Round goby

The round goby (Neogobius melanostomus) is the most studied non-indigenous species in the Baltic Sea, affecting fishermen, human food, other fish and marine birds. Seals also feed on the round goby. The fish themselves mainly eat edible mussels that many other species of fish and seabirds eat.

"For example, the goby have eaten all the mussels that bird populations used to eat in the shallows of Lithuania, and the birds have disappeared," Ojaveer said. Because the round goby's diet is similar to those of the windowpane and spotted flounder, there are signs of their populations shrinking in areas inhabited by the goby.

People were slow to accept the round goby as an edible fish even though nets pulled up more goby than Baltic herring in some areas. By now, the fish increasingly finds its way onto our plates. "Its first buyer price is higher, and Latvians have devised special goby fishing gear," the professor remarked.

Importance of prevention work

Henn Ojaveer said that the biggest problem is not knowing which non-indigenous species we have to face next, not just in the Baltic Sea but globally. That is why it is important to engage in prevention to avoid species transfer. "There are no examples of success in trying to get rid of a foreign species once it establishes a foothold. Even in cases where the non-indigenous species has been repelled in the early stages of invasion, it has always returned eventually."

Problems with non-indigenous species in the Baltic Sea do not compare to those in the Mediterranean that sees a new foreign species come through the Suess Canal almost weekly. Several foreign species in the Mediterranean are poisonous and can sting and, therefore, pose a threat to human health. This causes economic damage to tourism and the fishing industry.

Still, foreign species should also be seen as an opportunity. This is evidenced in the fact that the round goby has been accepted as an edible fish over time. "The goby is also food for several important species of predatory fish, like haddock and pike perch, who have grown larger on the new diet," Ojaveer added.

The researchers analyzed around 100 articles published in the last few decades that deal with the effects of non-indigenous species in the Baltic Sea. The paper is part of a series of articles that map different aspects of the life of foreign species.

The study was published in the journal Science of The Total Environment.

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