In Michigan, as the ice and snow on lakes thaw this spring, it is not uncommon for people to come across dead fish and other aquatic animals. While it may be alarming, the Michigan Department of Natural Resources explains that this is a normal occurrence as harsh winter conditions can cause fish, turtles, frogs, toads, and crayfish to die.

Shallow lakes with excessive aquatic vegetation and soft bottoms are particularly vulnerable to this problem, especially when deep snowpack reduces sunlight for the plants. Urban canals are also at risk due to the high levels of nutrient runoff and pollution from roads, lawns, and septic systems that flow into these areas, particularly during large storm events.

“Winterkill is the most common type of fish kill,” said Gary Whelan, DNR Fisheries Division research manager. “As the season changes, it can be particularly common in shallow lakes, ponds, streams and canals. These kills are localized and typically do not affect the overall health of the fish populations or fishing quality.”

While fish and other aquatic life typically die in late winter, they may not be noticed until a month after the ice leaves the lakes as the cold water temporarily preserves their bodies. Fish can also be affected by sudden changes in water temperature caused by unseasonably warm weather, leading to stress and, at times, mortality.

“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,” Whelan said. “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 from decaying plants and other dead aquatic animals under the ice.”

During winter, fish can become easily stressed due to low energy reserves since feeding is minimal. As a result, they are less equipped to handle low oxygen and temperature fluctuations. Dissolved oxygen is essential for fish and other aquatic life. However, when ice and snow cover greatly reduce daylight, aquatic plants stop producing oxygen, and many die. The bacteria that decompose organic materials on the bottom of the lake use up the remaining oxygen in the water. As other aquatic animals die and start decomposing, the rate of oxygen usage for decomposition also increases, further reducing dissolved oxygen levels in the water, leading to increasing winterkill.