Winter fish kills are a normal process every spring, especially in shallow lakes, but this year it could be much worse.
When snow and ice cover a lake it limits the sunlight that reaches aquatic plants, which then cut back on the amount of oxygen produced. If the vegetation dies from lack of sunlight, the plants start to decompose, which uses oxygen dissolved in the water. If the oxygen depletion is severe enough, fish die, the Minnesota Department of Natural Resources says.
“We know there’s significant dead fish,’’ Dave McCormick, assistant regional fisheries manager in St. Paul, told the Star Tribune. “I don’t think we’ve had this bad of a winter in 18 years.’’
A significant kill on at least one deeper lake, the North Center Lake near Lindstrom, suggests this could be one of the worst winterkills in decades, the Pioneer Press says. North Center Lake is more than 40 feet deep and has no recent history of die-offs, but last week, the Star Tribune says a patch of open water on the lake was filled with dead fish, including northerns, bass, walleyes and crappies, and the most alarming: carp and bullheads.
Certain fish need higher oxygen levels, so they are more prone to winterkills, but the DNR says fish like bullheads can tolerate lower levels of oxygen. Jack Lauer, regional fisheries manager in New Ulm, told the Star Tribune that dead bullheads are a sign of a significant winterkill.
“We’re just preparing for the worst, and if it’s bad, we’ll be doing more fish stocking than we normally would,’’ Lauer told the Star Tribune.
The DNR won't know the extent of this year's winterkill until the ice clears and they get reports of dead fish from lakeshore owners. Winterkills can be beneficial for fishing – a blank slate can produce a lot of game fish, the DNR told the Pioneer Press, but a strong kill can also make for pointless fishing the following year. The DNR will assess the lakes and determine if they should be restocked or let natural reproduction occur, the Star Tribune says.
Officials are hoping spring arrives soon, which will bring oxygenated snowmelt into the lakes, the Star Tribune says, and hopefully prevent more fish kills.