Warm weather brings dire forecasts for Minnesota fish

Fish kills aren’t uncommon in Minnesota, but a new study paints a terrible future for fish in northern lakes.

“We were mainly curious about how many of these mass deaths of fish might occur as a result of future climate change,” said Simon Tye, a PhD. University of Arkansas Department of Biological Sciences candidate and lead author of the study.

The study used a decade of documented fish killings in Minnesota and Wisconsin and more than one million water and air temperature profiles from 8,891 northern lakes.

It also used projections of future temperature rises due to climate change.

Assuming the current pace of climate change does not slow down, computer models predict a 600% increase in fish killings by 2100.

“It’s pretty drastic, it’s pretty amazing. And it’s worrying for the future of the fish in those lakes,” Tye said.

A scary future

“I was surprised at the scale of the impact,” said Nick Phelps, assistant to Professor Nick Phelps at the University of Minnesota, who helped compile the fish kill database. “It’s a scary future if that’s how it ends.”

“Right now, we estimate there are about 500 fish kills a year here in Minnesota,” Phelps explained. ‘The conservative estimates of this research suggest that by the end of the century there will be several thousand fish kills occurring each year.’

Pelican Rapids student Parker Sanchez launches his line as his father Fred pilots the boat at the Heart o’Lakes High School Fishing League event on June 13, on Big Cormorant Lake in western Minnesota.

David L. Wallis for MPR News | 2019

The change will be gradual over the decades with a significant increase recorded over the next 30 years.

While temperatures are expected to steadily rise, Phelps said more extreme weather events are likely to occur as well, with heat waves rapidly raising water temperatures and causing fish mortality.

The study also predicts that there will be less the fish kills during the winter. Winter fish kills often occur when lakes are covered in heavy snow that blocks sunlight, reducing the oxygen produced by aquatic plants. Tye said future climate predictions are for less snow and ice on Minnesota lakes, which should reduce winter fish mortality.

According to the researchers, the main causes of fish killings in the Upper Midwest are infectious diseases and low oxygen levels in the water. Water heating exacerbates both of these risks.

Warmer water contains less oxygen and warmer water stresses fish making them more susceptible to disease.

Recorded fish killings document large numbers of adult fish dying over a short period and typically washing ashore. But there are ripple effects on complex lake ecosystems that aren’t well understood, Tye said.

“There are a lot of damaging effects on young people, embryos and eggs, which we won’t be recording in our datasets, because no one is recording them,” he said.

A threat to hot and cold water species

Water heating is a well-documented threat to cold-water fish species such as salmon, trout or cisco.

But the researchers say there is also a significant threat to warm-water fish species.

“Most of the documented deaths involved perch and sunfish, the region’s main wild fish,” Tye said. “And those deaths were mainly attributed to a lower dissolved oxygen concentration.”

As fish kills occur more often, it will become more difficult to recover fish populations in a lake, Phelps said. If this prediction is confirmed for the rest of this century, Minnesota fishing will change significantly.

“There will be a shift from a system dominated by walleye, perch, to a system dominated by bass and bluegill when the water warms, and that’s because of the impact on production, food availability and everything in between.” , he said. We will see that the change occurs over time and is likely through mortality events, fish kills “.

Researchers believe these effects could be mitigated somewhat if temperatures didn’t rise as much as expected. And there may be management techniques to keep water bodies healthy as the water warms up.

“We know that healthy ecosystems are more resilient,” Phelps said. “And so the things that can keep our lakes healthy, like reducing shoreline development, preventing the introduction of invasive species, reducing chemical runoff, are all good things for the lake.”

But a changing climate could also mean managing a new normal where fish populations are different or much smaller. Tye suggests that such a substantial increase in fish mortality events will be difficult to manage.


Walleye is one of the species that could be affected by climate change.

Tom Robertson | MPR

“It will likely stretch the resources of the Department of Natural Resources, because if there are thousands of lakes that are rising in temperature, there are only so many that we can devote our time to rehabilitating,” he said. “So it just might become too messy for us to theoretically clean up.”

Based on the computer models performed for this project, Tye believes the projections in this study are conservative, not a worst-case scenario.

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