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Spirit Lake - As more snow and bitter temperatures dominate the forecast across Iowa, the mild start of winter is a distant memory. More than two feet of snow sits on many lakes and ponds across northern Iowa with 20-24 inches of ice below all that snow. Heavy snow on top of ice blocks sunlight from reaching aquatic plants and stops them from producing oxygen. This combination can create low oxygen conditions that may lead to fish kills.
Winter fish kills are common on Iowa shallow lakes during long winters with lots of snow cover. Larger water bodies with greater depth hold more volume allowing them to tolerate long periods of ice and snow.
Some public lakes and private ponds have winter aeration systems that maintain open holes. Aeration systems do not inject oxygen into the water, they simply create open areas where gases can diffuse at the water/air interface and create refuge. These systems rely on the size of the open water area to maximize effectiveness. The DNR has aeration systems at ten natural lakes.
Dangerous and variable ice conditions early this winter prevented state officials from turning some systems on in December and January. A few systems were started and then turned off after large portions of the lake opened back up.
Mike Hawkins, Fisheries Biologist with the Iowa Department of Natural Resources in Spirit Lake, says, “Ice conditions through the end of January were treacherous. Open holes and weak areas prevented us from finding a safe time to start the aerators.“
“The aerator at Clear Lake was started in December and then was shut down because it had caused a large part of the lake to open back up during windy and mild weather,” says Scott Grummer, Fisheries Biologist at Clear Lake.
Hawkins and Grummer decided not to restart several of their aeration systems after ice finally became solid on most lakes in late January and early February. “Having open water so late in the season really shortens the time the lakes will stay ice covered, drastically reducing the chance of a winterkill,” says Grummer. Hawkins adds, “We have a difficult decision to make in a very short window. We are always weighing safety against the risk of the coming season. High water, late ice-up, and very little snow cover earlier this year all added up to a low risk of winterkill.”
DNR crews are responsible for putting up safety signs on the ice around the hole once the system is running. Hawkins says crews couldn’t put signs up during the early part of the season because the ice was unsafe in parts of the lake.
Hawkins notes that turning the systems on later in the season usually isn’t possible or advised. “We reach a point where ice thickness is too great to create a large hole. Turning the system on when oxygen levels get low can cause water to mix under the ice and create even worse conditions.”
Grummer has four aeration systems running this winter. He says the value of those systems is greatly diminished right now. “The open holes created by the aerator at Rice Lake have shrunk substantially because of the extreme cold. Even with the aerator running, it is having very little impact on oxygen in the lake.” On Friday, Feb. 22, the eleven aeration holes were down to just a few feet in size on this 1,000 acre lake.
Dissolved oxygen levels are monitored on natural lakes throughout northwest Iowa and so far levels are still safe in most locations. Natural lakes winterkill from time to time and is part of the natural cycle. “Fish species in these lakes have evolved under these conditions and their populations usually rebound very quickly. Some of the best fishing in these shallow natural lakes can be two to three years after a winterkill,” says Hawkins. “Even with aeration, lakes still winterkill. Harsh Iowa winters can trump all our best efforts.”