DNR officials say warm weather and recent flooding have combined to kill fish in Big Sandy Lake in Aitkin County, about 60 miles west of Duluth.
Major rain last month washed a lot of decayed organic material into the lake. When it reached warm water, microbes in the material consumed dissolved oxygen that fish need to survive.
Mike Bruesewitz, Aitkin area fisheries manager for the DNR, says warm water species like bass and panfish should be OK.
"I think the cool water species, walleye pike, perch, maybe the pike more than anything, they're a little bit more of a cool water species," Bruesewitz says. "They're maybe going to have a harder time of it. We might see an increase in stress in these fish such that they don't grow as well this particular year."
But Bruesewitz says fish populations are resilient and usually rebound without much intervention. High temperatures have also been blamed for killing thousands of fish in several southern Minnesota lakes.
Bruesewitz says dead pike, perch and crappies have been found in Big Sandy Lake.
"If you have heavy rain, it doesn't even have to be a huge rain like we had up here, releasing the organic materials into these streams at higher temperatures, you can get the oxygen depletion," he says.