If the black-mouthed butterfish is seriously established, there is a risk that it may compete with other benthic species such as flounder.

For the past 30 years, it has migrated up from Gdansk Bay where it is now very dominant.

- This is something we want to prevent at all costs here, Linda Calamnius, marine biologist, Gävleborg County Administrative Board. 

The catch of black-mouthed butterfish in Gävleborg is the northernmost find of the species in Sweden.

It is also very adaptable and has easy to find food and and habitats in both salt and fresh water.

- It would be a disaster if it spreads into our salmon waters like Testeboån, says Linda Calamnius.

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Watch when SVT Gävleborg's reporter tries to eat the invasive fish.

Photo: SVT