With each passing day of ice cover, there is a growing likelihood that several Iowa lakes and ponds will experience some level of natural winter fish mortality.
“We lose fish every year during the winter, but this has been a hard winter and we are expecting to see more lakes with some winter mortality, and a few lakes, like Minnewashta, are experiencing oxygen levels lower than typical,” said Jim Wahl, supervisor for fisheries in northwest Iowa.
Minnewashta, Upper Gar and the lower end of East Okoboji Lake have low oxygen levels and ice anglers reported seeing dead fish through their underwater cameras in Minnewashta. Other lakes with low oxygen levels are Eldred Sherwood in Hancock County, Clark Lake, in Cerro Gordo County, and Sabula and Spring lakes on the Mississippi River.
“Our lakes froze early and have been under ice in some areas for three and a half months. We are still finding 20 to 33 inches of ice so it will be around for some time. The last time we saw a winter like this was in 2008,” he said.
When lakes freeze early and receive a blanket of snow, it effectively shuts off the sunlight from reaching aquatic plants, which stops photosynthesis and the flow of oxygen into the water. The longer the ice and snow cover the lake, the less oxygen is in the water and available to the fish. Wahl said low oxygen levels have been found in a variety of lakes, from the north to the south.
An additional factor in Lake Minnewashta could be the decomposition of a late season algae bloom that robbed additional oxygen from the water compounding the early ice, snow cover factor.
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. Aeration systems keep a section of the lake from freezing allowing some oxygen into the water but does not guarantee a lake won’t winterkill during extreme winters.
Wahl said even during the most severe winter, a complete kill is seldom observed and fish populations rebound quickly.
“We expect some fish loss but we just won’t know the extent of it until we can get our survey boats on the water and sample lakes that had low winter oxygen readings,” Wahl said.