MN DNR Press Release:

Cold winters with abundant snowfall can lead to fish die-offs and the Minnesota Department of Natural Resources has already taken reports of this process, known as winterkill, occurring in lakes near Brainerd, Hinckley and the Twin Cities area.

“While seeing lots of dead fish can be disconcerting, we remind people that winterkill is normal and happens every year to some extent,” said Neil Vanderbosch, DNR fisheries program consultant.

Once a lake is capped with ice, the amount of dissolved oxygen present in a lake depends on how much oxygen is produced by aquatic plants. Winterkill occurs when snow and ice limit the amount of sunlight reaching aquatic plants.

Without adequate sunlight, the plants produce less oxygen. If the vegetation dies from lack of sunlight, the plants start to decompose, a process that further depletes oxygen dissolved in the water.

Trout species require high dissolved oxygen levels and may begin dying off when a lake’s dissolved oxygen falls below 5 parts per million (ppm). Bluegill and largemouth bass are also sensitive to low oxygen levels. Walleye, yellow perch, northern pike, carp and crappie can tolerate dissolved oxygen levels as low as 2 ppm.

Winterkill rarely results in the death of all fish in a lake, but lakes with frequent winterkill events tend to be dominated by bullheads.

Winterkill can have some benefits. In lakes with overabundant panfish, occasional winterkill can increase growth rates of the fish that survive. Winterkill can also greatly reduce carp abundance, which leads to increased water quality and more successful stocking efforts.

People who see numerous dead fish after the ice melts should report their observations to the State Duty Officer at 800-422-0798.