Round goby contributes to Baltic flounder decline

Flounder numbers in the Baltic Sea have fallen as oxygen and salinity levels change, while the invasive round goby competes with the fish for food, ERR reports.

Hiiumaa fisherman Imre Kivi said his Thursday morning catch fit into two boxes. He said catches had been better four or five years ago and that there had been less flounder each year since.

Flounder researcher Tenno Drevs said the fish population depends mainly on how much fresh water enters the Baltic Sea through the Danish straits. In the past, strong inflows from the North Sea brought both salt and oxygen, he said. While salinity continues to fluctuate, the seabed area without oxygen is growing.

At the start of this year, the Baltic Sea’s water level was very low, raising hopes that storms would bring in fresh water. That did not happen, and oxygen levels are now at their lowest ever, Drevs said. As a result, deepwater flounder no longer reach local waters.

Kivi also blamed the round goby, a non-native species that has established itself in the Baltic Sea and shares the flounder’s food supply. He said the goby is so active that it can prevent flounder from staying still in the sand.

Kivi said some fish advertised as coming from Hiiumaa are actually brought in from further south. He said local flounder are generally rounder, while fish from Poland are more elongated. The flounder season lasts until the sea freezes.