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How 'Fish Kills' Reset The Ecology Of Lakes And Ponds

BLOOMINGTON, Minn. (WCCO) -- It's a stinky and unsightly sign of spring. Dead fish are floating up in lakes across the state.

The phenomenon of winter fish kill might be shocking at first, but the Minnesota Department of Natural Resources officials say it happens most years.

Ice and snow block oxygen and sunlight from the water. Then, plants die and use up oxygen when they decompose.

"As that oxygen goes down in the water, then we'll have fish start to die from suffocation," said Brian Nerbonne, a regional fisheries manager for the DNR.

Nerbonne says smaller, shallower lakes with less oxygen to begin with are more prone. Some lakes, like Penn Lake in Bloomington, have aeration systems. They pump oxygen into the water and also can break up any ice on the surface, which helps with the atmospheric exchange. But the aeration is only a partial solution to winter kill.

Fish Kills, Dead Fish
(credit: CBS)

"We try to manage [Penn Lake] as a fishing lake," Nerbonne said. "When we have a fish kill like this, we're gonna probably want to go back in and stock some fish."

The fish kill can also act as a reset for the ecology of a lake, allowing other animals to flourish.

"People shouldn't be too worried about it, you know, especially for some of these small ponds and basins where maybe we don't want to have fish in there necessarily," Nerbonne said. "That actually might be a beneficial thing that's happening."

Ice-out is still going on as you travel north in Minnesota, so that's where you'll likely see more fish kill right now.

The DNR asks people to report it when they see it.

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