Alien fish in Swedish waters

A round goby (Neogobius melanostomus) was caught in late July off the Swedish coast near Karlskrona. This is the first find of its kind in Sweden. The species, which originates from the Black Sea and probably spread to the Baltic via ballast water, has been found in the Gulf of Gdansk since 1990, in the southern Baltic. Today it is one of the most common coastal fishes there, so it was expected that it would show up in Swedish waters sooner or later, according to researcher Gustaf Almqvist of Stockholm University.

Goeran Pettersson is the man behind the sensational catch, which he made as he was bottom angling for perch in Saltsoesund outside Karlskrona. Goeran, who is from Sibbehult in Scania, has experienced many fish catches in the waters surrounding Karlskrona, but he had never seen this species before. He was even more surprised when, later that day, he caught three more of them.

'I'm interested in fishing, and after having compared them with pictures on the Internet and from an account in the magazine Svensk Fiske (Swedish Fishing), I immediately understood that what I had pulled in were round gobies,' says Goeran Pettersson.

Goeran alertly froze one of the fish and reported the find to the Swedish Board of Fisheries Coastal Laboratory, which then conveyed the find to Gustaf Almqvist at Stockholm University, who, together with Sven Kullander of the Swedish Museum of Natural History, was able to confirm that it was indeed a round goby. It was a 96 mm long male that was estimated to be two, or at most three, years old.

'It was only to be expected that it would become established in Swedish waters sooner or later. Roe and fry can be spread via ballast water. It's logical that the first specimen was found in Karlskrona considering the intensive boat traffic between Karlskrona and Polish waters,' says Gustaf Almqvist, who defended his doctoral dissertation on the round goby at Stockholm University earlier this year.

In his dissertation he showed how the round goby has become one of the most colourful features of the southern Baltic in less than three decades. The fish primarily feeds on mussels. It can be 30 cm long and is a popular food fish in its original home region. The species has rapidly adapted to Baltic conditions and locally can dominate coastal fish populations. This has also meant that they compete with domestic bottom-dwelling fishes, and studies have shown that it can edge out the European flounder from certain areas. It has also become an important component in the diet of key predatory fishes such as cod and perch, and during some part of the year can constitute the most important prey for these fishes. Since the other prey of these predators do not feed on sea mussels to any great extent, the round goby is a new link between mussels and predatory fishes.

It is too early to determine whether these are merely a few stray specimens or whether the species has truly become established in Blekinge.

'Since the round goby winters in deeper waters, where they are harder to catch, targeted survey fishing probably can't get underway until spring. The fish may very well already exist in other areas along the Swedish coast, especially in harbour areas with especially intensive trade relations with the southern Baltic,' says Gustaf Almqvist.