NORTHERN MICHIGAN – As Michigan lakes thaw this spring, you may notice dead fish or other aquatic animals. After a colder, snowier winter, fish kills may be more apparent than in recent years.
While alarming, the Department of Natural Resources (DNR) reassures that winterkill is a natural occurrence, affecting fish, turtles, frogs, toads, and crayfish.
“Winterkill is the most common type of fish kill,” said Aaron Switzer, DNR Fish Production Program manager. “It’s particularly common in shallow lakes, ponds, streams, and canals during seasonal changes. These kills are localized and don’t typically impact overall fish populations or fishing quality.”
Shallow lakes with abundant vegetation and soft bottoms are more prone to winterkill, especially when deep snow blocks sunlight, preventing oxygen production. Urban canals are also susceptible due to nutrient runoff and pollution from roads, lawns, and septic systems, particularly after storms.
Rapid temperature shifts in spring can further stress fish, making them more vulnerable. Fish struggle in winter due to minimal feeding and low energy reserves. When oxygen levels drop under ice and snow, bacteria use the remaining oxygen to break down dead plants and organic matter, accelerating depletion and leading to more fish deaths. This year’s extreme cold and potential rapid warming could worsen the impact.
Winterkill victims may not be visible until ice fully melts, as cold water preserves them temporarily. “If you see dead fish, they may appear fuzzy due to secondary fungal infection, but the actual cause of death is suffocation from lack of oxygen,” Switzer explained.
For more information, visit the DNR website. Report fish kills at Michigan.gov/EyesInTheField; these reports help the DNR manage aquatic resources. If you suspect a fish kill resulted from pollution or other non-natural causes, contact the nearest DNR office or Michigan’s Pollution Emergency Alert System at 800-292-4706.
