Large fish kill likely after record cold, heavy snow reduced oxygen in shallow lakes, Michigan DNR says
Dead fish and other aquatic animals may appear in Michigan lakes and streams in the coming weeks as winter ice melts — a natural phenomenon that the Michigan Department of Natural Resources assures is not cause for alarm, though this year's kill could be more noticeable than usual.
The state DNR said winterkill, the most common type of fish mortality event, is particularly common in shallow lakes, ponds, streams and canals during seasonal changes. The department said the deaths typically do not affect the overall health of fish populations or fishing quality.
"Winterkill is the most common type of fish mortality event," said Aaron Switzer, DNR fish production program manager. "It can be particularly common in shallow lakes, ponds, streams and canals during seasonal changes. It's a natural phenomenon, and these events are localized. They typically do not affect the overall health of fish populations or fishing quality."
Shallow lakes with large amounts of aquatic vegetation and soft bottoms are more prone to winterkill, particularly when thick ice and deep snow reduce sunlight for plants, the DNR said. Urban canals are also susceptible because of nutrient runoff and pollution from roads, lawns and septic systems.
The phenomenon could be especially visible this year, as Michigan experienced record or near-record cold temperatures and large snowfalls this winter, followed by potential rapid warming in the coming months, officials said.
Fish can become stressed in winter because of low energy reserves and minimal feeding, making them less able to handle low oxygen levels and temperature swings, according to the DNR. Rapid changes in water temperature due to unseasonably warm weather can lead to additional stress and mortality.
Fish and other aquatic life that die in late winter may not be noticed until well after ice leaves lakes because the animals may be temporarily preserved by cold water, the DNR said.
"If you see dead fish as a result of winterkill, the fish may appear fuzzy. This is because of secondary infection by fungus, but the fungus was not the cause of death," Switzer said. "The fish actually suffocated from a lack of dissolved oxygen from decaying plants and dead aquatic animals under the ice."
In addition to fish, the DNR said winterkill can affect turtles, frogs, toads and crayfish.
The DNR encourages residents to report fish kills. If someone suspects a fish kill is due to non-natural causes, they should call the nearest DNR office or Michigan's Pollution Emergency Alert System at 800-292-4706.