NEWS RELEASE
MICHIGAN DNR
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As ice and snow cover melt on Michigan lakes this spring, you may see dead fish or other aquatic animals. Given the return to a more “normal” Michigan winter this year — with more cold days and near-average snowfall across the state — you may notice more dead fish than you have in the past few years. While such sights can be startling, the Department of Natural Resources reminds everyone that it is normal for winter conditions to cause some mortality of fish and other creatures such as turtles, frogs, toads and crayfish.
“Winterkill is the most common type of fish kill," 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 kills are localized. They typically do not affect the overall health of fish populations or fishing quality.”
Shallow lakes with a large amount of aquatic vegetation and soft bottoms are more prone to winterkill, particularly when a deep snowpack reduces sunlight for the plants. Canals in urban areas also are quite susceptible, due to the large amounts of nutrient runoff and pollution from roads, lawns and septic systems that flow into these areas, especially after large storm events.
Fish also may be affected by rapid changes in water temperature due to unseasonably warm or rapidly warming temperatures, leading to stress and, sometimes, mortality. Fish can become easily stressed in winter due to low energy reserves because feeding is at a minimum in winter. They are then less able to handle low oxygen and temperature swings. That could be the case this year with the record or near-record cold temperatures and large snowfalls Michigan experienced, and potential rapid warming in the coming months.
Fish and all forms of aquatic life need dissolved oxygen to survive. When ice and snow cover reduce the daylight that reaches the water depths, aquatic plants stop producing oxygen, and many die. Bacteria use the remaining oxygen in the water to decompose the dead plants and other organic materials on the lake bottom. With available oxygen reduced, more aquatic animals die and start to break down, speeding up the rate that oxygen is used for decomposition. This further decreases dissolved oxygen levels in the water, creating a cycle of increased winterkill.
Fish and other aquatic life that die in late winter may not be noticed until well after the ice leaves lakes, as the cold water may temporarily preserve them.
“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,” said Switzer. “The fish actually suffocated from a lack of dissolved oxygen from decaying plants and dead aquatic animals under the ice.”
Visit the DNR website for more information on fish kills in Michigan. You can report fish kills at Michigan.gov/EyesInTheField; these reports are valuable to the DNR’s management of aquatic resources in Michigan. If you suspect a fish kill is due to non-natural causes, call the nearest DNR office or Michigan's Pollution Emergency Alert System at 800-292-4706.
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