After Michigan saw a more “normal” winter this season, people may notice more dead fish than in the past few years.
The Michigan Department of Natural Resources (DNR) said it is normal for winter conditions to cause fish to die naturally, as well as other creatures such as turtles, frogs, toads and crayfish.
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“Winterkill is the most common type of fish kill,” said DNR Fish Production Program manager Aaron Switzer. “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.”
The Michigan DNR said shallow lakes with a large amount of aquatic vegetation and soft bottoms are more prone to winterkill. Canals in urban areas are also susceptible because of the large amounts of nutrient runoff and pollution from roads, lawns and septic systems that flow into those areas.
What is winterkill?
Rapid changes in water temperature from warm or rapidly warming temperatures can cause fish to get stressed and sometimes die. The DNR said fish can become easily stressed in winter due to low energy reserves because feeding is minimal.
Fish and all forms of aquatic life need dissolved oxygen to survive, according to the DNR.
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 at which 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 the late winter may not be noticed until after ice leaves lakes, as cold water could 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.”
