(Provided by MDNR)

The Michigan DNR reminds everyone that once the ice and snow cover melts on Michigan's lakes, it may be common to discover dead fish or other aquatic creatures. Winter conditions often can cause fish and other creatures such as turtles, frogs, toads and crayfish to die.

"Winterkill is the most common type of fish kill," said DNR Fisheries Division Hatchery Manager and fish health expert Martha Wolgamood. "As the season changes it can be 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."

Shallow lakes with excess aquatic vegetation and soft bottoms are prone to this problem. Canals in urban areas also are quite susceptible due to the large inputs of nutrient run-off and pollution from roads and lawns and septic systems that flow into these areas, particularly from large storm events.

Fish and other aquatic life typically 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 temporarily are preserved by the cold water. Fish also may be affected by rapid changes in water temperature due to unseasonably warm temperatures leading to stress and sometimes mortality. This is likely the case with the record or near record temperatures coupled with the large rain events Michigan experienced in February 2017.

Fish can become easily stressed in winter due to low energy reserves because feeding is at a minimum in winter. They then are less able to handle low oxygen and temperatures swings.

Dissolved oxygen is required by fish and all other forms of aquatic life. Once daylight is greatly reduced by ice and 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 and other aquatic animals die and start decomposing, the rate that oxygen is used for decomposition is additionally increased and dissolved oxygen levels in the water decrease even more, leading to increasing winterkill.

For more information on fish kills in Michigan, visit the DNR's website. If you suspect a fish kill is caused by non-natural causes, call the nearest DNR office or Michigan's Pollution Emergency Alert System at 1-800-292-4706.