Persistent winter has DNR watching for fish kills

Tuesday, March 5, 2019
It will take some time for a thaw near Isthmus Park on East Lake Okoboji. DNR officials think heavy snow cover will lead to a fish kill this spring. (Photo by Russ Mitchell)

Approximately 2 feet of snow sits atop just as much ice in many of northern Iowa's lakes and ponds, according to the Iowa Department of Natural Resources. DNR officials suspect the snow cover may keep sunlight from reaching aquatic plants. Fish need oxygen from the aquatic plants to survive.

The DNR uses aerators on 10 of Iowa's natural lakes, including local bodies of water like Silver Lake, Center Lake, Ingham Lake and Five Mile Lake.

"Aeration systems do not inject oxygen into the water," a statement from the DNR said. "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."

Iowa DNR Fisheries Biologist Mike Hawkins explained aerators were installed in northwest Iowa during the 1980s with the thought they would be beneficial to the ecosystem. He said more recent science has shown some of the local lakes' shape and depth often help prevent winter fish kills.

DNR officials had to contend with variable and sometimes unsafe ice conditions between December and January, and a number of the state's aerators were unable to be safely started this season. Hawkins said, at this point, the aerators might do more harm than good.

"The oxygen isn't uniform from top to bottom in a lake, when oxygen levels start to fall," he said, noting the lowest levels are typically on the lake bottom. "By turning the aeration system on late in the season, you can actually mix that. We may still have a zone of higher-oxygen water right below the ice and, by turning the system on, we kind of deplete that."

He went on to say, the DNR normally wouldn't be watching for a fish kill this late in the season, but a forecast for freezing temperatures into March may leave conditions unchanged for longer than most years.

Hawkins said fish kills are natural and almost always take only a portion of the aquatic population. Pockets of fish sometimes can't survive when they are unable to reach an area of higher oxygen content, such as moving water or an inflow. Hawkins said other gases can reach toxic levels under the ice and contribute to a fish kill.

"Although sometimes it looks pretty bad, the system can recover fairly quickly," Hawkins said.

He said species like pan fish often bounce back to catchable sizes in two to three years. That period of new maturity is often the best fishing, according to Hawkins.

"It opens up room," he said. "A lot of times, a lake is only producing what it can produce. There's only a certain amount of productivity that's available for growth of fish, so basically all the voids are filled and things are at their maximum rate of growth. When you have a winter kill or any kind of renovation we see it when we do renovations on purpose ecological space is opened up and growth rates go high."

Since the DNR's initial Feb. 26 statement, oxygen levels seem to be improving. Hawkins said local DNR officials saw some ideal levels on Silver Lake and Center Lake at least twice what he would consider to be low. He said factors like inflow from the surrounding watershed, a late cap over and areas of ice reopening periodically over the previous months may have bumped the stats this year. Still, he said things remain very weather dependent, and the DNR's aerators can't prevent a fish kill in and of themselves.

"I would hazard a guess that we may not see as frequent of winter kill on these systems but, if mother nature's going to throw us a really hard winter with low water and very little water input to give fish refuge, our aeration systems probably aren't going to be the thing that saves the day," Hawkins said. "They're not the silver bullet."

Case in point, Hawkins said Ingham Lake in Emmet County showed low oxygen levels. He said that's not unheard of for the lake.

"Ingham Lake has been pretty prone to winter kill, and the last two winter kills we've had there were with the aeration system running," Hawkins said.

And while fish in the Iowa Great Lakes may have a rough winter under the snow and ice, Hawkins doubts the same can be said of the invasive curly leaf pondweed. He said the prop-tangling weed was found below the ice a few weeks before the snow cover. The DNR plans to sample the northern portion of East Lake Okoboji to see if the hearty nuisance has been starved at all by the snow covered conditions.. Hawkins wasn't terribly optimistic.

"It doesn't need a lot of light to maintain life," he said. "If anything's going to survive under that ice on very, very dim sunlight, it would be curlyleaf pondweed."

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