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Dead fish sit along the edge of the ice near the public access to Eagle Lake Monday. It's one of many shallower lakes in which the DNR expects to see sizable fish kills following the deep snowpack that cut oxygen to the lakes. Pat Christman / Mankato Free Press

More fish kills expected this spring in oxygen-starved lakes

MANKATO, Minn. — People who frequent shallower lakes this spring may notice more than the usual number of dead fish floating to shore once the ice melts.

And that's not necessarily a bad thing.

Brian Schultz, of the Department of Natural Resources office in New Ulm, said their agency measured some very low levels of oxygen in many lakes, "more so than usual."

Once a lake is covered with ice, the amount of dissolved oxygen in the lake depends on how much is produced by aquatic plants. Winter kill occurs when snow and ice limit the amount of sunlight reaching those plants.

"A lot of the lakes were good in early February, then we got all that snow," Schultz said.

The DNR uses several small, shallow lakes as walleye rearing ponds, pouring in tiny fish and then netting them later when they're fingerlings and stocking them in area lakes. He said they like to see a good winter kill in those rearing lakes.

He said if large walleyes and other predator fish are in the rearing ponds, more of those tiny walleyes get eaten. "We want those (bigger fish) to die, so when we put fry in there isn't any competition."

And while several area sportsmen's or angling clubs operate aerators on shallow lakes during the winter to keep oxygen levels up, many leave the aerators off some years to allow a winter kill.

"On some of these lakes, the clubs and DNR decided they're better managed as a boom or bust system. It's OK if they winter kill sometimes because it kills all the rough fish and they can have a good game fish population in just a couple of years," Schultz said.

He said that in the large southwest Minnesota DNR region there are 109 aeration systems and 70 of them were turned on this past winter. It is up to the clubs, not the DNR, to decide if they will be operated.

"We won't know the extent of the fish kill until after the ice goes out," Schultz said. "We had heavy snow and more ice. We got a lot of snow this winter. We haven't had a winter like this for quite a while."

He said that if people on lakes notice large fish kills this spring they can contact the DNR and they will visit the lake and analyze it.