At least 10K dead fish wash ashore in Mille Lacs Lake die-off

MALMO — The stench emanating from Mille Lacs Lake was so strong earlier this month, Jim Hestad could smell it at his home 500 feet from shore.

“It smelled like death,” Hestad said. “I mean, dead and rotting fish, and fish guts that are rotting. ... It was putrefying the water, and it was kind of a mixture of fish tissue and guts and stuff, right at the point where the waves break onto the shore.”

From his dock in Bergstrom Bay, in Mille Lacs’ northeast corner, Hestad saw the corpses of cisco, bass, walleye, northern pike and muskies — some in the trophy range — as far as the eye could see. Raptors, vultures and seagulls flocked to the area to feast on the remains. He reported the fish kill on Aug. 12 to the Minnesota Department of Natural Resources, providing what he described as an “extremely conservative” estimate of 10,000 dead.

A few days earlier and miles down the shore, Daniel Dewald provided a similar estimate to the state agency after encountering the dead fish littering a beach south of Malmo. The Battle Lake man, his wife and his 1-year-old son were visiting family in the area and wanted to enjoy a wade in the water.

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Daniel Dewald

“The closest thing I could describe it [as], would be like if you've been to an ocean and smelled like a very strong sea smell,” Dewald said. “But it was, of course, you smelled bad fish. That's what it was. It was rank with fish.”

The DNR saw a significant spike in fish kill reports across the state this summer, especially from mid-July forward when temperatures heated up and dry weather pushed much of the state further into drought. In late July, the DNR knew of more than 200 mortality events . As of Wednesday, 14 of those reports were on Mille Lacs Lake.

Die-off events for cisco — also known as tulibee — in Mille Lacs occur almost every year, said Greg Berg, Mille Lacs area fisheries supervisor for the DNR. The fish is a cold water species, and the state’s second-largest inland lake represents the southern end of its range. With the hot summer this year, water temperatures in Mille Lacs consistently hit 77 to even 80 degrees in the latter half of July.

“The cisco that are dying are from a very abundant 2024 year class, the biggest on record, which is why it is so visible this year compared to others,” Berg said in an email. “ ... It is not uncommon to see other species mixed in as well later in the summer from heat stress and added hooking mortality.”

The maximum temperature for cisco is 70 degrees, said Isaiah Tolo, DNR fish health supervisor, and other cold water species like pike and walleye start to become stressed at 74 degrees.

“The most recent pulse of fish mortality is likely from the high winds causing some thermal mixing in the lake — increasing warm water mixing and blowing dead and dying fish to the northeast shore of the lake,” Tolo wrote in an email.

The wind on Aug. 8 blew from the west and southwest at 10-18 mph, likely pushing fish carcasses into the beaches and bays around Malmo.

Beth Holbrook, Minnesota DNR fisheries research scientist, explained that Mille Lacs is what’s known as a “polymictic lake.” The water continually mixes throughout the summer, because it is relatively shallow compared to its surface area. This means oxygen is continually distributed throughout the water column. So fish in Mille Lacs don’t usually struggle for lack of oxygen — but there is no refuge from the warm water, even in the deeper areas.

“Temperature alone determines mortality events, with hot, dry, still summers causing the highest temperatures and the most thermal stress,” Holbrook wrote. “Several other lakes with cisco mortality events this summer have also been shallow, polymictic lakes.”

While the sight and smell of tens of thousands of dead fish is enough to make life on the lake more than a little unpleasant for the humans who wish to enjoy it, Berg said this year’s die-off isn’t concerning to scientists on a population level for any of the fish species involved. This summer brought the second hottest heat wave on record for the state, with areas as far north as Voyageurs National Park reaching the 100-degree mark.

Fish kill reports in the state are slowing as overnight temperatures drop , helping to moderate water temperatures.

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Jim Hestad

“Although cumulative thermal stress might still be an issue for older, larger fish,” Holbrook said.

But temperatures are expected to continue to heat up overall in the coming years if climate change trends continue. The University of Minnesota Climate Adaptation Partnership estimates that in north-central Minnesota, the average annual temperature is projected to increase by 3.8-4.5 degrees by mid-century.

While projections point to warmer winters rather than hotter summers, precipitation trends point to continued drier summers. Much of the region is experiencing a “storage drought,” or depleted long-term water storage, that takes more than a few heavy rainfalls to replenish. And lower water levels are more susceptible to spiking temperatures when heat waves do arrive.

Dewald said he hopes when he returns to Mille Lacs for future family visits, he doesn’t find something like this again.

“Hopefully that doesn't become a more regular occurrence.”