Fish kills may be common in Upper Peninsula following extreme winter conditions

After the heavy ice and snow cover experienced on Upper Peninsula lakes this winter and spring, it may be common for anglers and others enjoying the outdoors to discover a higher than average number of dead fish or other aquatic creatures, such as turtles, frogs, toads and crayfish, Department of Natural Resources fisheries managers said today. “Winterkill is the most common type of fish kill,” said Gary Whelan, DNR Fisheries Division research section manager. “Much of the U.P. saw very deep ice and snow, so winterkill may be particularly common this year in shallow lakes, streams and ponds. These kills are localized and typically do not affect the overall health of the fish populations or fishing quality.” Winterkill occurs during especially long, harsh winters – similar to the one experienced this year. Shallow lakes with excess aquatic vegetation and soft 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 or more 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, caused by decaying plants and other dead aquatic animals under the ice.” Dissolved oxygen is required by fish and all other forms of aquatic life. When daylight is greatly reduced by thick ice and deep 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. Once the oxygen is reduced, other aquatic animals die and start decomposing, increasing the rate at which oxygen is used for decomposition and further decreasing dissolved oxygen levels in the water, leading to increasing winterkill. For more information on fish kills in Michigan, visit 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 800-292-4706.