Don’t panic about dead fish

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By TAYLOR HAELTERMAN
Capital News Service

LANSING — As lakes thaw this spring, you might spot dead fish, experts warn.

That’s startling, but winterkill this time of year is normal, said Gary Whelan, the fisheries research manager for the Department of Natural Resources.

It occurs when ice and snow cover a body of water and the fish run out of oxygen, Whelan said.

It happens most often in shallow lakes with a lot of plant growth. Snow piles up on the ice and stops sunlight from reaching underwater plants. Without sunlight, plants go from producing oxygen through photosynthesis to dying, decaying and using up oxygen, Whelan said.

If enough plants die under the ice, they use up all the oxygen in the water. So fish and other aquatic animals that need oxygen won’t survive, he said.

How many fish might die is hard to predict. 

And it could be trending to less often because lake ice decreases as the climate warms. That means winterkill conditions may occur less.

“We had kind of an odd winter,” Whelan said. “Usually, they’re worse if you get early ice cover and snow cover for long periods of time. 

“But this year we didn’t really get ice cover in much of our state until mid-January, or later. Then we got really heavy ice cover and a lot of snow,” he said.

The impacts of fish kills vary. Overcrowded lakes can benefit, but killing off a lake’s entire population is never a good thing, Whelan said.

In Minnesota, some residents try to prevent winterkill by aerating lakes as if they were big aquariums. Aeration increases oxygen in the water by opening up the ice and causing the water surface to interact with air.

The most common systems are underwater bubblers and pontoons that blow air into the water, said Brian Schultz, the assistant regional fisheries manager for the Minnesota Department of Natural Resources.

Minnesota has a statewide aeration permit program. Counties or groups like sporting clubs purchase the systems, pay for upkeep and pay a fee to the agency. 

The state has issued nearly 290 aeration permits for winterkill. Southern lakes, which tend to be shallower and more heavily vegetated, account for 255 of the permits, Schultz said. Fifty-five are in operation, about what’s expected this time of year.

Schultz has noticed a decrease in aeration in recent years.

Dead fish from a winterkill preserved by cold water may not be noticed until long after the ice thaws.

Michigan Department of Natural Resources

Dead fish from a winterkill preserved by cold water may not be noticed until long after the ice thaws.

That could be because people worry about the potential dangers of open water on the lakes and the high cost of running aerators.

Or it could be because of a turn toward boom-and-bust fishery. That’s when a lake with a large die-off has good fishing the following year because there are fewer predatory fish in the water, Schultz said.

“You don’t see that much winterkill anymore, and I’m thinking it has to do with climate change,” he said.

It’s harder to tell how climate change has affected winterkill in Michigan because its weather is moderated by the surrounding Great Lakes, but the DNR’s Whelan said he expects to see a reduction in kills as climate change continues and lakes get less ice.

Unlike Minnesota, Michigan doesn’t have an aeration program. The state prefers to deal with winterkill by managing bodies of water better and reducing nutrient loads, fertilizers and vegetation, Whelan said.

Aeration “is expensive and it doesn’t always work,” he said. “If you’re willing to put the investment in you can get some to work, but a lot of them will not work even with that aeration. It’s a double-edged sword.” 

Taylor Haelterman reports for Great Lakes Echo.