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Hot weather likely contributing to fish die-offs (published June 21, 2021)

Recent hot weather may be contributing to fish die-offs in lakes across the state, according to the Minnesota Department of Natural Resources.

“Spring and summer fish die-offs happen occasionally, but we are getting widespread reports of dead fish following the recent prolonged stretch of hot weather,” said Tom Burri, DNR limnology consultant.

The DNR asks the public to report fish die-offs. People should call the state duty officer at 651‐649‐5451 or 800‐422‐0798 if they encounter a large group of dead fish in a lake or a stream (the state duty officer is available 24 hours per day, seven days a week). Calling in a report provides a single point of contact for the incident. An early report also allows timely water sampling or other response actions, if needed. It’s especially helpful to know what fish types and sizes people see in a fish die-off.

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.

“People tend to be concerned when they find dead fish, and they can help by reporting what they see right away,” Burri said. “These reports help us determine whether an investigation is needed.”

If there is an immediate threat to life or property, call 911 first.

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, toxic chemicals discharged or spilled, 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.

More information on fish kills is available on the DNR website.