Round goby

June 6th 2019 was the date when a fisherman north of Oulu, Finland, brought a strange fish to the Metsähallitus SEAmBOTH team. The fish was identified as a round goby, Neogobius melanostomus. It was the northernmost finding of this alien species in Finland, probably even in the world. The species has started to spread from the Caspian and the Black Sea regions in the 1990s and has now reached the Baltic Sea, many of the rivers in central Europe and for example the Great Lakes in North America.

A dead round goby from two diffent angles on a boat.
A round goby found by a fisherman in Oulu. Picture by Suvi Saarnio.

In Finland, the species spread fast around the Archipelago Sea and the Gulf of Finland. The previous northernmost finding was in Raahe, about an hour south of Oulu. Metsähallitus marine team did an official round goby count there, the way it’s done by the alien species specialists around the Baltic Sea and found 12 round gobies on a half hour observation period. On a one-hour dive, about 20 individuals were found, between a few cm long juveniles to almost 20 cm long adults.

A round goby hiding on the bottom of the sea. Light and striped color.
Round goby can be almost any color – from light sandy color to striped to almost black. Picture by Pekka Tuuri.  

In Sweden, the species has not spread north from the very southern Sweden. Yet. So the round goby has yet to be found in the Swedish SEAmBOTH area. It might just be a matter of time, so quickly has this aggressive invasive species spread around the world.

What makes this species so fast to spread then? It’s a very strong competitor, it tolerates a wide variety of temperatures and salinities, it doesn’t really care whether the water is a little low on oxygen or if there are pollutants like oil in the water or not. It eats almost anything and loves other fish species eggs. At least in Finland it competes in food with at least the flat fish. During their breeding season round gobies can be really aggressive towards all other fish. The parasites don’t bother them as much in their new adopted surroundings because they didn’t bring their native parasites with them, and the local parasites and diseases haven’t really caught up with them yet.

All in all, it’s a success story. Except for the local fish, which are sometimes outcompeted by the bold and capable round goby.

Two fishes staring at each other on the bottom of the sea.
Native inhabitant black goby (left) looks eye to eye to alien species round goby (right). Picture by Pekka Tuuri.

The round goby is here to stay, that’s for sure, now we just have to learn to live with it. One option would be to start eating it like the local people do in its native countries around the Caspian and the Black Sea. It’s a delicious fish when canned or smoked and it’s fished in its new homeland Poland and exported canned.

If you see a goby-like robust fish with a black ring with a white round ring around it on its dorsal fin, you’ve found a round goby, and you should inform the national alien species specialists, who want to monitor the spread of this species.

A round goby in a fish tank.
You can recognize a round goby from the black spot on the dorsal fin. Picture by Janica Borg.  

Written by Essi Keskinen

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