Rochester, MN (KROC AM News) -  With the cold winters and abundant snowfall, Minnesota lakes have seen the yearly process of the winterkill. This winterkill happened in lakes near Brainerd, Hinkcley and Twin Cities area, according to the Minnesota Department of Natural Resources.

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

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

Without the sunlight, the plants produce less oxygen, thus the plants die. When the plants die, that depletes the oxygen dissolved in the water, that kills the fish.

Trout species require high dissolved oxygen levels, while Bluegill and largemouth bass are also sensitive to low oxygen levels. Walleye, yellow perch, northern pike, carp and crappie can tolerate the low dissolved oxygen levels.

While winterkill rarely results in the death of all the fish in a lake, lakes with frequent winterkill events tend to be dominated by bullheads. Winterkill does have some benefits, though. In lakes with overabundant panfish occasional winterkill can increase growth rates for the fish that survive. Winterkill also greatly reduces carp abundance, which will lead 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 Minnesota Pollution Control Agency at 800-422-0798.

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