An expert with the Iowa Department of Natural Resources says the winter conditions are going to cause fish kills in some areas of the state. Fisheries biologist, Mike Hawkins, says winter started early and is hanging around longer, creating the potential for problems. “Ice formed on our lakes probably a little ahead of schedule this past fall, and we’ve had some pretty extreme temperatures that have really created quite a bit of ice on some of our northern and northwestern Iowa lakes. We’ve got up to 33 inches that we’ve measured in some locations,” Hawkins says.
When you add snowfall on top of the ice — much of the sunlight is blocked out. “When that happens we can see some oxygen levels start to sag and get to critical levels on some of the area lakes. Especially those that are little bit shallow,” Hawkins says. “On top of all of that, we had pretty dry conditions going into the fall, so many of the lakes were lower than what’s typical going into the fall.”
Fish use up the oxygen in the water under the ice, but there are other things going on that also deplete the supply. “Things like decay of organic matter or plants that are beginning to decay under the water can cause more of that biological oxygen demand,” Hawkins explains. “But when light is cut off from reaching those waters below the ice — photosynthesis can’t take place — so you don’t have any replenishment of that oxygen occurring.”
There’s a lot of conditions that determine how much oxygen is in the water, which Hawkins says makes it tough to determine exactly which lakes might experience a fish kill. “I would have actually predicted many of our shallower lakes in northwest Iowa would be in tougher condition than they are. But we’re seeing fairly good oxygen levels in some of those lakes and then we’ve had some unexpected low oxygen levels in other lakes that typically don’t show that pattern,” Hawkines says.
The DNR can predict what might happen in a particular lake based on past experience and from testing oxygen levels. But what happens under the ice won’t be known for sure until the ice melts away. “The extent of the winter kill isn’t really known until the ice goes off and we start to see some of those fish accumulate along the shoreline,” Hawkins says.
The DNR has taken steps to prevent winter fish kills from low oxygen by placing aeration systems at a few lakes with a history of winter kills. Hawkins says the winter kill doesn’t take out all the fish, but it can lead to an imbalance in the types of fish in a lake. He says they can address that by stocking new fish in some of the lakes to try and restore that balance.