The retreat of ice from Minnesota's shorelines may soon leave a grim reminder of winter's effects around some lakes and ponds: dead fish.
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. When the plants die from lack of sunlight, they start to decompose, a process which uses oxygen in the water. If the decomposing plants use too much oxygen, the fish die.
Some species of fish are more vulnerable than others to winterkill. Trout are the most sensitive species. Bluegill and largemouth bass are also quite sensitive to lower oxygen levels. Walleye, northern pike, carp and crappie species have intermediate tolerance to lower oxygen levels, while bullheads and fathead minnows are the most tolerant of low oxygen. Winterkill seldom results in the death of all the fish in a lake. Chronic winterkill lakes (those which have frequent winterkill events) are usually dominated by bullhead species.
Winterkill can have some beneficial effects. Winterkill in lakes with overabundant panfish can result in increased growth rates of the panfish that survive. Winterkill can result in greatly reduced carp abundance, allowing for improved water quality and increased success of subsequent fish stocking efforts.
People who observe dead fish after the ice melts should report their observations to their local D.N.R. Fisheries office. Fisheries staff would be especially interested in knowing what kind of fish were observed, approximate numbers of each kind of fish observed, and an estimate of the sizes of each kind of fish observed.