As ice comes off southern Minnesota lakes, evidence of this year’s harsh winter is washing up along the shorelines on some shallow lakes. While dead and dying fish washing ashore in spring can be discouraging to see, it’s the result of a 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 Craig Soupir, Waterville area fisheries supervisor. “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 can actually benefit a lake, according to Soupir.
“In lakes with high numbers of common carp, for example, periodic winterkill can thin out their numbers and allow desirable fish species to fill the void,” Soupir said. “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; these lakes are known as “boom and bust” lakes and may have winterkill every four or five years. If stocked, fish in these shallow lakes typically have high survival rates, grow rapidly and can provide great angling.
While some larger and deeper lakes in southern Minnesota have aeration systems installed to reduce the probability of winterkill effects, aerated lakes can still be susceptible to winterkill. Research shows aerating very shallow lakes often does not work well for maintaining a fishery though harsh winters.
To report fish die-offs including winterkill, people should call the Minnesota Duty Officer at 651‐649‐5451 or 800‐422‐0798 (the officer line is available 24 hours per day, seven days a week). An early report allows timely water and fish sampling or other response actions, if needed. It’s especially helpful to know what sizes and types of fish people see in a fish die-off.
Follow-up stocking of fish may ensue after a winterkill event, if consistent with DNR lake-specific management plans. For more information, contact a local DNR area fisheries office (mndnr.gov/areas/fisheries).