(Michigan) As the ice and snow melt on Michigan's lakes, it's not uncommon to discover fish kills or die-offs, the Department of Natural Resources reminds lakefront property owners and recreational boaters and anglers. Typical Michigan winters with heavy snow and ice cover create conditions that cause die-offs of fish and other aquatic life such as softshell turtles, frogs, toads, and crayfish.
"Winterkill is the most common type of fish kill," said DNR Fisheries Division Chief Kelley Smith. "Particularly in shallow lakes and streams, it can have large effects on fish populations and fishing quality."
Winterkill occurs during especially long, harsh winters, such as occurred in Michigan this past winter. Shallow lakes with excess amounts of aquatic vegetation and mucky bottoms are particularly prone to this problem. Fish and other aquatic life actually 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 ends with large numbers of dead fish that bloat as the water warms in early spring," Smith 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 as winterkilled fish were actually suffocated from a lack of dissolved oxygen under the ice."
Trace amounts of dissolved oxygen are required by fish and all other forms of aquatic life. Once the daylight is greatly reduced by ice and snow cover, the aquatic plants stop producing oxygen and many die. The bacteria that decompose organic materials on the bottom of the lake require oxygen and work to use up the remaining oxygen in the water, once the plants stop producing it. This winter, many locations had perfect conditions for winterkill with heavy ice and snow cover, and a number of locations likely ran out of dissolved oxygen to support fish and other aquatic life.
"The DNR is still concerned about Viral Hemorrhagic Septicemia virus (VHSv) infections, particularly after a stressful winter such as the past one, and requests the assistance of anglers and citizens in reporting fish with symptoms of this disease," said Smith.
Information on VHS can be found at www.michigan.gov/dnrfishing.
If anglers or citizens see unusual fish or other aquatic life kills or see fish with clinical signs of VHSv, please e-mail information about the fish kill to DNR-FISH-Report-Fish-Kills@michigan.gov.
If you suspect a fish kill is caused by non-natural causes such as a chemical spill, please call your nearest DNR location or Michigan's Pollution Emergency Alert System at 800-292-4706.