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The Minnesota Department of Natural Resources would like the public’s help in reporting fish die-offs in lakes and streams, which happen occasionally and usually result from natural causes.

“People tend to be concerned when they find dead fish, and they can help by reporting what they see right away,” said Tom Burri, limnology consultant with the Minnesota Department of Natural Resources who deals with water quality issues related to fisheries. “Reporting dead fish helps us determine whether an investigation is needed.”

People should call the state duty officer at 651‐649‐5451 or 800‐422‐0798 if they encounter several dead fish in a lake or a stream. Doing so provides a single point of contact for the incident. The state duty officer is available 24 hours per day, seven days a week. 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.

If ther is an immediate threat to life or property, call 911 first. For general information, people can also contact area DNR fisheries offices, but this is not the best way to report fish kills.

Bacteria a common culprit

In early spring, the retreat of lake and stream ice can sometimes leave behind dead fish, commonly referred to as winterkill. When snow and ice cover a lake, they limit the sunlight aquatic plants receive. The plants respond by reducing the amount of oxygen they produce. If vegetation dies from lack of sunlight, the plants start to decompose, a natural process that consumes oxygen dissolved in the water. Fish die if oxygen depletion becomes severe enough.

In mid-spring and summer, groupings of dead fish usually are the result of a common bacterial infection referred to as columnaris. Columnaris tends to affect fish as water temperatures warm and fish are stressed from the energy they spent on spawning. Columnaris infections can kill sunfish, crappies and bullheads, and occasionally, largemouth bass and northern pike.

Most fish diseases and infection issues found in nature tend to be concentrated in fish of a specific species and size range. In contrast, when an individual observes dead fish of vastly different sizes and from multiple species, human activity is a more likely a 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.