The Minnesota Department of Natural Resources encourages Minnesotans to contact the state if they see evidence of a fish die-off in a lake or stream. These fish die-offs happen occasionally and can result from a variety of causes.
“People can help by reporting fish die-offs right away,” DNR limnology consultant Tom Burri said. “These reports help us determine whether an investigation is needed.”
To report fish die-offs, people should call the Minnesota Duty Officer at 651-649‐5451 or 800‐422‐0798 (the officer line is available 24 hours per day, seven days a week). An early report allows timely water and fish sampling or other response actions, if needed. It’s especially helpful to know what sizes and types of fish people see in a fish die-off.
In early spring, the retreat of lake and stream ice can sometimes leave behind fish that died during ice cover, commonly referred to as winterkill. When snow and ice cover a lake, sunlight reaching aquatic plants is limited. The plants, in turn, reduce the amount of oxygen they produce. If vegetation dies from lack of sunlight or other cause, the plants start to decompose, which uses oxygen dissolved in the water. If oxygen depletion becomes severe enough, fish die.
In mid-spring and summer, fish die-offs are often the result of warming water and opportunistic infections that spread in fish populations that are already stressed after the spawning season. Species commonly observed in these die-offs include sunfish, crappies and bullheads, and, occasionally, largemouth bass and northern pike.
When die-offs of wild fish are the result of disease issues, the affected fish tend to be of a single species and size range. By contrast, when die-offs include multiple species and size ranges, human activity is more likely to be the cause.
Human causes of fish kills can include water discharged at high temperatures; discharges or spills of toxic chemicals, including pesticides and fertilizers; manure runoff; and low oxygen levels in a lake resulting from storm water that runs off urban or rural landscapes. Often, there are multiple causes contributing to fish deaths.