It's not uncommon for the Department of Natural Resources to get reports of large numbers of dead fish floating in Minnesota lakes in spring and summer.
But fish die-offs have been more frequent and widespread than usual, as temperatures soared to record highs in June across much of Minnesota.
The DNR has received a couple hundred reports of fish die-offs in the last three weeks from lakes all over the state involving significant numbers of fish, said Tom Burri, a limnology consultant for the DNR.
“It’s pretty broad in the state right now in the last few weeks,” he said. “And it is definitely more than we’ve seen, other than the 1998 drought.”
When lakes heat up, fish get stressed, and they're more vulnerable to deadly infections — either viral or bacterial, Burri said.
"We've had a long stretch of hot weather, and it's stressful for fish,” he said. “It opens these fish up for these opportunistic infections to get in the system more than they (already) are. And it leads to the mortality of these fish."
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Climate change is likely a contributing factor as Minnesota lakes gradually get warmer, Burri said, increasing the stress on fish.
“Our most northern lakes in Minnesota also feel the heat, because those are some cold- or cool-water fish up there,” he said. “That rapid change in temperature stresses those fish out equally.”
Die-offs commonly affect multiple species, including carp, sunfish, crappies, perch, largemouth bass and northern pike, he said.
While infections have been the culprit behind the most recent wave of deaths, there can be other sources of fish kills, including the depletion of oxygen in a lake while it’s frozen, or a pollution spill.
Burri encourages people to report fish die-offs to the state duty officer by calling (800) 422-0798.
Providing information such as location, estimated number and size of fish and species is helpful, Burri said, as well as any obvious signs of a hazardous spill or other potential cause.