DNR Says Fish Kills Common in Spring

By Gary Whelan, Michigan DNR

The Department of Natural Resources reminds everyone that after the ice melts on Michigan's lakes, it is not uncommon to discover dead fish or other aquatic creatures. Typical Michigan winters with heavy snow and ice cover create conditions that cause fish and other creatures such as soft-shell turtles, frogs, toads and crayfish to die.

"Winterkill is the most common type of fish kill," said DNR Fisheries Division Production Manager Gary Whelan. "It is particularly common in shallow lakes and streams. These kills are often localized and typically do not impact fish populations or fishing quality."

Winterkill occurs during especially long, harsh winters. Shallow lakes with excess aquatic vegetation and mucky bottoms are particularly prone to this problem. Fish and other aquatic life typically die in late winter, but may not be noticed until a month after the ice leaves the lake because the dead fish and other aquatic life are temporarily preserved by the cold water.

"Winterkill begins with distressed fish gasping for air at holes in the ice and often ends with large numbers of dead fish that bloat as the water warms in early spring," Whelan explained. "Dead fish and other aquatic life may appear fuzzy because of secondary infection by fungus, but the fungus was not the cause of death. The fish actually suffocated from a lack of dissolved oxygen under the ice."

Dissolved oxygen is required by fish and all other forms of aquatic life. Once the daylight is greatly reduced by ice and snow cover, aquatic plants stop producing oxygen and many die. The bacteria that decompose organic materials on the bottom of the lake use the remaining oxygen in the water.

For more information on fish kills in Michigan, visit www.michigan.gov/fishing. If you suspect a fish kill is caused by non-natural causes, please call your nearest DNR office or Michigan's Pollution Emergency Alert System at 1-800-292-4706.