Minnesota DNR reports heightened fish mortality during prolonged heat

With hot summer weather creating stressful conditions for fish in lakes, rivers and streams across the state, the Minnesota Department of Natural Resources is sharing what people can do if they see a fish mortality event — and how they can help prevent them.

“We have been seeing chronically stressful conditions this summer for fish and are receiving continued reports of fish mortality events,” DNR Fish Health Supervisor Isaiah Tolo said. “People should be aware that they might see dead fish. We also want them to know what to do to report these events and what actions they can take to protect our fisheries.”

Current conditions and mortality events

Reports from the public, public utilities and fisheries staff have recorded surface water temperatures of 86 degrees Fahrenheit and higher in several Minnesota lakes and the Mississippi River. Cold water fish like lake cisco and other salmonids become stressed when water temperatures exceed 75 degrees and cool water species like northern pike can handle temperatures up to 84 degrees.

Fish mortality reports for 2026 so far include 212 reports spanning 102 waterbodies across the state, as far north as Lake of the Woods on the state’s northern border and as far south as Albert Lea on the southern border.

Among those reports, the Fish Health Laboratory received 65 reports for 29 waterbodies in a four-day period from July 16-20. Notably, fish are particularly susceptible to mortality in lake systems impaired by excessive nutrients, caused by urban and agricultural runoff. That’s because the conditions in these lake systems already pose a challenge for fish by accelerating decomposition and algae growth, and then are affected by low dissolved oxygen availability and warmer water temperatures.

The recent string of reports is the second of two "pulses" of elevated fish mortality events this summer. The first happened throughout June following rapid high temperatures after a colder winter and primarily affected lakes in the Twin Cities metro area. The current pulse is happening due to consistent high temperatures with little precipitation and is affecting a wider distribution of lakes across the state with varying levels of water quality.

A notable number of recent mortality events are affecting species that are sensitive to high temperatures. These include northern pike and muskellunge, cisco, and larger fish of all types because they have a smaller gill surface to body mass ratio compared with smaller fish. For example, several events report hundreds of dead northern pike with few other species affected. Dead and dying cisco have been reported on Lake Koronis in Stearns County. Large dead lake sturgeon have been washing ashore on Otter Tail Lake in Otter Tail County. The wide distribution of these events in the state indicates that fish populations across the state are experiencing similar conditions.

Water temperatures during Minnesota summers are increasing, and this summer is no exception. On average, according to climatology data, Minnesota lakes are about 3 to 4 degrees warmer and have lost an average of 10-14 days of ice cover in the last 50 years. The continued warming trends accelerate decomposition and algae growth. Research also indicates that summer fish kills are likely to increase with climate forecasts.

How to help

Minnesotans have a role to play in preserving and protecting the state’s fisheries. Anyone can report fish mortality events they observe on the Minnesota DNR website. This webpage also includes details about what can cause these events.

“Reports from members of the public are an important way we receive information about fish mortality,” said Tolo. “Those reports reach an interagency team of fish and water quality professionals.”

Homeowners and land managers

Those who own and manage waterfront property have an important role to play in protecting and restoring natural shorelines, which are important parts of good habitat for fish in their lakes. Promoting lake and stream-side tree cover and aquatic plants in lakes helps establish thermal refuges for fish. Information on natural shorelines and restoration is available on the DNR website. Riparian and non-riparian landowners alike can help reduce nutrient runoff into lakes and rivers, an important step in improving the overall resiliency of aquatic ecosystems. The Minnesota Pollution Control Agency has useful tips on preventing runoff, and restoration.

Fishing in hot weather

Anglers should be aware that fish are stressed due to warm water temperatures and low dissolved oxygen availability and are at a higher risk of experiencing angling mortality, which means fish are less likely to survive after being caught, handled and released.

Anglers should be cautious during heat waves, especially if they are targeting species sensitive to thermal stress such as pike, muskellunge and walleye. Avoid overhandling fish during these conditions.

More information about fish mortality events in general, as well as links to other resources and agencies involved in responding to these events, is on the Minnesota DNR website.