Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Two men installing aerator on frozen lake. DNR File

DNR fisheries staff install an aerator on frozen Prairie Lake to prevent fish from suffocating over winter.
© DNR File

February 2013

Avoiding winterkill

Keeping fish alive during the harshest season.

Kevin Harter

On a clear, cold mid-winter morning Brian Spangler, DNR fisheries technician and a crew of DNR workers and Chetek Lakes Protection Association volunteers trudge across frozen Prairie Lake on the 20th annual rescue mission to install aerators and create open water before oxygen levels plummet to the point that fish suffocate under the ice.

"Some lakes, well, you just know are going to winterkill," said Spangler as augers and chainsaws cut into six inches of ice and ATVs pulled three, 3-horsepower, 230-volt surface aspirating aerators into place.

"Smaller, shallow, fertile lakes create the perfect storm for winter fish kills," Spangler said.

Prairie Lake, which has been aerated since 1992, is one of them. It is part of the Chetek Lakes Chain, a 3,764-acre impoundment fed by a large, mostly agricultural watershed, which forms the headwaters of the Chetek River in northwest Wisconsin. The 1,619-acre lake is the largest in the chain, whose depths range from 12 to 22 feet.

"What we are doing here today will add oxygen to the water all winter and help prevent winterkill of fish and other organisms," Spangler said.

Winterkill is a natural process occurring when fish don't have enough oxygen. In ice capped winters, most of this oxygen comes from aquatic plants. However, when the ice and snow cover is thick, plants cannot get the sunlight needed, and instead of producing oxygen, die and decompose.

Fish kills follow. How many and how fast depends on the depth of the lake and the types of fish it contains.

Trout require the most oxygen, followed by bluegill, largemouth bass and walleye, according to DNR fisheries managers. Crappies, carp, northern pike and yellow perch can tolerate less oxygen. Bullheads and fathead minnows require the least amount of oxygen to survive.

Statewide, the Department of Natural Resources works with, and grants permits annually to cities, lake associations, park districts and similar groupsto install and maintain aerators on some lakes.

Increased pressure on large, recreational lakes has also had a domino effect, Spangler said. Because of it, more people have looked to "marginal" lakes for their cabin and fishing pleasure.

Aeration improves the quality of life on and around those lakes. Since it began in the northwest region in 1977, aeration has successfully been done on 19 lakes in Barron and Polk counties, totaling more than 3,000 acres.

"It's a big thing we do to increase angling opportunities and improve recreational opportunities," Spangler said. "Aeration has greatly reduced or eliminated winterkill and improved the sport fishing opportunities."

Photo of sign cautioning open water. Kevin Harter
One of several warning signs located at all Prairie Lake landings.
© Kevin Harter


Outdoor enthusiasts, including skiers, snowshoers and snowmobilers who frequent aerated lakes should take note and proceed with caution on aerated lakes. Such lakes are clearly marked – as required by state law – at public access points and the open water is cordoned off with fencing, rope and reflective tape.

Kevin Harter is the public affairs manager for the DNR's Northern Region.