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Thousands of fish have died in Mille Lacs Lake in recent weeks, leaving some property owners with the unpleasant task of cleaning up a stinky mess along their shores.

It's common for some fish to die during every summer in Minnesota, as lake temperatures heat up and dissolved oxygen levels drop.

But this year's warmer-than-normal weather has increased stress on cold-loving fish like the tullibee, also known as cisco.

“Surface temperatures have been approaching 80 degrees in some cases,” said Greg Berg, area fisheries supervisor for the Minnesota Department of Natural Resources. “When we get those types of conditions, we tend to see some of the ciscos dying off out there. It's pretty common.”

Berg said there is an abundant population of young cisco in Mille Lacs this year. Like trout, they prefer cold water and a high level of dissolved oxygen.

Mille Lacs is a shallow, wind-swept lake, so its water mixes frequently, instead of forming layers like many deeper Minnesota lakes do during warm weather. That means the lake’s water temperatures and oxygen levels stay about the same from the top to the bottom, Berg said.

With climate change causing lakes to warm and fewer days of winter ice cover, conditions in some Minnesota lakes are not conducive to ciscos’ survival, Berg said.

“Mille Lacs is on the very southern range of cisco habitat, so they’re just kind of barely hanging on temperature-wise,” he said.

Warmer lake temperatures also can increase hooking mortality, when game fish caught and released by anglers die due to stress. 

Berg said the DNR encourages anglers to use proper catch-and-release tactics, including handling the fish gently with wet hands and returning it to the water quickly.

Fish kills typically subside as the amount of daylight decreases and temperatures cool, so “hopefully, most of the damage has already occurred,” Berg said. 

The DNR will be sampling the lake’s fish population in September to assess how many cisco survived the summer die-off, he said.

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Kirsti Marohn

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This year's hotter-than-normal weather has increased stress on coldwater-loving fish like the tullibee, also known as cisco.

Dead fish float to the surface of a lake.

Thousands of fish die in Mille Lacs Lake as temperatures climb

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