The retreat of ice from Minnesota’s shorelines may soon leave a grim reminder of winter’s effects around some ponds and lakes. That is according to the Minnesota Department of Natural Resources (DNR).

In most cases the dead fish are the result of a normal process known as “winterkill.” When snow and ice cover a lake, they limit the sunlight reaching aquatic plants. The plants die from lack of sunlight, stop producing oxygen, and then decompose - a process that also 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 the probability and severity of winterkill.

Some species of fish are more vulnerable to winterkill than others. Trout are the most sensitive species, although bluegill and largemouth bass are a close second. Walleye, northern pike, carp and crappie species have intermediate tolerance to lower oxygen levels while bullheads and fathead minnows are the most tolerant. Lakes that have chronic winterkill are usually dominated by bullhead species.

Winterkill also can have some beneficial effects. In lakes with overabundant panfish, winterkill can result in increased growth rates of those that survive. It also can greatly reduce carp abundance, 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, the approximate numbers of each kind of each species, and a rough estimate of sizes.