What you need to know about winterkill:
The Wolter Report (Max Wolter, DNR fisheries biologist, Hayward): At this point in the winter, both anglers and fish biologists start to think about the risks of winterkill of fish. Winterkill is most commonly a result of low oxygen in lakes that ice has covered for several months. Ice cover prevents the exchange of oxygen between air and liquid water, which is the primary means that lakes receive oxygen in the summer. As for the lakes that are more at risk, research tells us the more productive lakes, those with more nutrients, have an increased risk of winterkill. More plant and algae growth in the summer can mean more decomposition in winter – and the decomposition process consumes precious winter oxygen. Snow cover can play a factor as well, with less snow cover allowing plants to make oxygen later into the winter. Depth, however, is by far the most important determinant of whether a lake is at risk for winterkill. Generally speaking, winterkill is considerably more likely in lakes less than 10 feet deep. Lakes more than 16 feet deep are rarely prone to winterkill, though there are exceptions to that rule.