In addition, dead mussels have also been found.
The authorities in the region believe that a combination of high temperature and low oxygen conditions is behind the occurrence.
There is however no single explanation for the deaths, said the Uusimaa environmental authorities and the Finnish Game and Fisheries Research Institute.
Cold-blooded fish needs more oxygen in warm weather than in any other time. The Oxygen level decreases in warm water. Besides, algae bloom also reduces the oxygen content.
“When the concentration of oxygen in water decreases, the combined effect of temperature and hypoxaemia can be lethal,” said the authorities.
The dead fishes have been discovered in the Taasia River in eastern Uusimaa, Somero in southwest Finland, and Kisko and Eura rivers in Satakunta region.
