Recently, thousands of fish have died in Mille Lacs Lake near Brainerd, MN. It's normal for some fish to die every year, but this year's record-breaking heat, 9 consecutive days of >90° temps, and the 5/6th warmest July on record have increased stress on fish populations.

As lake temperatures rise, dissolved oxygen levels drop because warmer water holds less oxygen. "Surface temperatures have been approaching 80° in some cases," said Greg Berg, area fisheries supervisor for the MN DNR. "When we get those types of conditions, we tend to see some of the ciscos dying off out there. It's pretty common." 

Photo: Dead fish floating on the water’s surface at George Watch Lake in Centerville, MN. This photo was taken in July and reflects recent heatwaves and lower dissolved oxygen levels. Photo Credit: MPR News
Photo: Dead fish floating on the water’s surface at George Watch Lake in Centerville, MN. This photo was taken in July and reflects recent heatwaves and lower dissolved oxygen levels. Photo Credit: MPR News
A cisco is a type of freshwater fish, closely related to whitefish and trout. In MN, "cisco" usually refers to what are called lake herring. They typically live in deep, cold, well-oxygenated lakes and feed mainly on zooplankton and small aquatic organisms.

Fewer cisco can signal broader ecological stress in a lake. As climate change warms lakes and shortens winter ice cover, environmental conditions in some MN lakes are changing and may no longer support cisco survival.

This article draws on information from a recent MPR news article. Learn more about fish kills from the MNDNR or contact Kris Larson, Water Resource Specialist, at This email address is being protected from spambots. You need JavaScript enabled to view it..