As ice comes off of southern Minnesota lakes, evidence of this year’s harsh winter is washing up along the shorelines. While dead and dying fish washing ashore can be discouraging to see, it’s the result of a natural process called “winterkill,” according to biologists with the Minnesota Department of Natural Resources.

“Shallow lakes like we have in southern Minnesota are most susceptible,” said Waterville area fisheries supervisor Craig Soupir. “Winterkill happens because the amount of dissolved oxygen available to fish becomes too low.”

Low oxygen environments are created when sunlight is no longer able to penetrate the ice, causing oxygen-producing plants to die and generate higher levels of carbon dioxide as they decompose.

Winterkill is a natural process that can actually benefit a lake, according to Soupir.

“In lakes with high numbers of carp, for example, periodic winterkill can thin out their numbers and allow desirable game species to fill the void,” said Soupir. “It can also improve water clarity and increase aquatic vegetation.

Soupir said populations of game fish can sometimes rebound quite dramatically in years following winterkill. Improved survival of young fish and increased growth rates for both young and adult fish can combine to create significantly improved angling opportunities.

It is not unusual for lakes in southern Minnesota to experience some winterkill on an annual basis. However, the severity of winterkill varies greatly depending on factors such as depth of snow and length of time it covers the ice, lake depths, water inflows and the rate at which oxygen drops over time. Most often, winterkill events on these shallow basins are partial and rarely do all fish in a lake die.

The fishery in some lakes is specifically managed around winterkill and are known as “boom and bust” lakes. These shallow lakes typically have high survival rates of stocked fish which grow rapidly and provide quick turnaround for anglers. These are the type of lakes that rely on frequent winterkills, which may happen every four or five years, to reset the lake and allow the fish population to achieve its boom times of quality-sized fish.

While some larger and deeper lakes in southern Minnesota have installed aeration systems to reduce the probability of winterkill effects, aerated lakes are still susceptible to winterkill. Additionally, research is finding aerating very shallow lakes often does not work well for maintaining a fishery.

Anyone observing dead or struggling fish should report their findings to the local DNR fisheries office, and share with them the species and approximate numbers and sizes of each kind of fish. That information will help DNR fisheries staff identify which lakes experienced winterkill and to what extent.

Follow-up stocking of fish may ensue, if consistent with DNR lake-specific management plans. For more information, contact a local DNR area fisheries office.