About 500 fish kills occur in Minnesota every year but few are reported, according to a University of Minnesota study. A fish kill is defined as five or more dead fish in one location. Most fish kills in Minnesota result from natural causes such as disease and infection or low oxygen levels under ice in the winter.

However, human impacts can also lead to fish kills, including toxic spills; runoff of manure, pesticides or fertilizers; and high-temperature wastewater or stormwater discharges.

If a fish kill involves one or few species, then the cause is often disease. If it involves multiple species and sizes, then the cause could be toxic pollution or discharges.

Reporting fish kills

If you see multiple dead fish in a lake or river, call the State Duty Officer at 800-422-0798. Calls are answered 24 hours a day, 365 days a year. If there is an immediate threat to life or property, call 911 first.

Response and investigation

The MPCA, Department of Natural Resources (DNR), and Department of Agriculture (MDA) coordinate on fish kill responses. The DNR serves as the lead agency on fish kill investigations until a discharge/release has been identified as the cause. The MDA takes the lead for pesticide and fertilizer incidents while the MPCA investigates the environmental impacts from the release of hazardous materials, oil, or other pollutants such as manure. Local governments also help with fish kill investigations.

State and county specialists in water quality, watershed management, feedlots, fisheries, and laboratory analyses all work together to explore possible causes of fish kills.

If responding agencies believe a fish kill resulted largely from natural causes, then investigation and follow-up action will be limited. There is often little to be done other than let the dead fish decompose.

Some fish kills have obvious causes, such as a spill or identifiable discharge to a surface water. For example, a tanker truck tips over in a ditch, spilling a product toxic to fish that flows to a stream. Other fish kills have less-discernible causes that are often driven by rain events in a given watershed. An example would be an intense rain event that creates runoff to streams from many agricultural fields and city streets. Finding a clear explanation for a fish kill will depend largely on the lag time between the kill and its reporting, as well as the complexity of the cause(s).

Changing climate

As Minnesota’s climate continues to change, the state will have both prolonged wet periods and significant droughts or dry conditions. The low water levels in streams and rivers during droughts make them more vulnerable to “flash loading” of runoff-driven pollutants from agricultural fields and city storm sewers during intense rain events. Lower flows in the systems mean less water to dilute and buffer the impacts of toxic pollutants or high loads of organic matter. These effects of climate change may lead to more fish kills in the future.

Many actions that increase the state’s resiliency to climate change can also help protect fish and other aquatic life, including:

  • Water retention ponds and rain gardens to slow down and filter runoff
  • Rain barrels to reduce runoff, and pervious surfaces to let stormwater soak into the ground
  • Buffers of deep-rooted native plants along streambanks and lakeshore to help prevent erosion and reduce runoff
  • Trees planted along streams and lakes to help keep the water cool and absorb runoff

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