Extreme winters can be tough on freshwater fish, especially if the river they live in is subject to many freeze-thaw cycles. Ice can form and break up again, which can kill all the fish that don’t manage to escape to ice-free eddies or deeper water. Juveniles are particularly vulnerable.
Waterways that are involved in hydropower expansion can be particularly tough places for fish. The constant changes in flow conditions can cause frequent cracking in the ice cover. When the surface ice breaks up, cold air can work its way deeper into the water column and form a blanket of ice crystals on the bottom. That makes it hard for fish to survive, according to a study by ecologists from NTNU and the University of Umeå.
The researchers stressed the need for more studies and measurements so they can better predict what will happen to fish in rivers. They also concluded that existing models of flow conditions and fish communities also need to include winter conditions in their calculations. Today, most of these models are only based on ice-free conditions.