Winter weather can create conditions that cause fish and other aquatic creatures to die
To simplify the public’s ability to report fish kills, the Michigan Department of Natural Resources recently developed an online form for reporting fish kills in quantities larger than 25 fish. A fish kill of this size could have more factors involved that need further DNR investigation. The new Sick or Dead Aquatic Species form can be found in the DNR’s Eyes in the Field application at michigan.gov/eyesinthefield.
Information requested in the form includes waterbody and location (both descriptive and latitude-longitude coordinates), observation details, and any available photos showing the fish kill. Close-up photos showing any external disease signs such as bloody patches, unusual wounds or odd coloration are particularly helpful to DNR staff as they try to determine the cause of the issue and its seriousness. Entered reports and associated images automatically are forwarded to fish health staff for quicker evaluation and action.
“In the past, the public has notified DNR staff by phone or email about a fish kill,” Whelan said. “While this input is hugely valuable and desired, it was not the most efficient way for us to get reports about fish kills and often delayed response. We have designed a simpler way for the public to get involved as our eyes and ears using the DNR’s Eyes in the Field.”
The DNR reminds everyone that after the ice and snow cover melts on Michigan’s lakes this winter, it may be common to discover dead fish or other aquatic creatures. Severe winter weather can create conditions that cause fish and other creatures such as turtles, frogs, toads and crayfish to die.
“Winterkill is the most common type of fish kill,” said Gary Whelan, DNR fisheries research manager. “Given the harsh conditions of winter with thick ice and deep snow cover, fish kills may be particularly common in shallow lakes and streams and ponds. 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 mucky bottoms are particularly prone to winterkill. 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.
“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 in early spring,” 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.”
Dissolved oxygen is required by fish and all other forms of aquatic life. Once the daylight is greatly reduced by thick ice and deep 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 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.
