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Dead fish after ice-out likely from winterkill

The melting ice from Minnesota's shorelines may reveal some dead fish, according to the Minnesota Department of Natural Resources.

In most cases it is normal winterkill. When snow and ice cover a lake, it limits the sunlight reaching aquatic plants. The plants die, stop producing oxygen, and decompose - a process that consumes oxygen. This oxygen deficit can kill other fish, although it seldom affects all fish.

Winterkill is worse in winters with abundant or early snowfall. Lower water levels in the fall and late ice-out dates increase winterkill.

Some species of fish are more vulnerable. Trout are the most sensitive, though bluegill and largemouth bass are a close second.

Walleye, northern pike, carp and crappie species have an intermediate tolerance, while bullheads and fathead minnows are the most tolerant. Lakes with chronic winterkill are usually dominated by bullhead species.

Winterkill also can be beneficial. In lakes with overabundant pan fish, winterkill can result in increased growth rates of those that survive. It also can greatly reduce carp, leading to improved water quality and more successful fish stocking efforts.

Those who see dead fish after the ice melts should report their observations to a local DNR Fisheries office. Staff are especially interested in knowing the type of fish killed, and the approximate numbers and sizes.