The DNR would like your help in reporting fish die-offs in lakes and streams. Die-offs happen occasionally and usually result from natural causes. People should call the state duty officer at 651-649-5451 or 800-422-0798 if they encounter several dead fish in a lake or a stream. Doing so provides a single point of contact for the incident. The state duty officer is available 24 hours per day, seven days a week. If there is an immediate threat to life or property, call 911 first.
In mid-spring and summer, groupings of dead fish usually are the result of a common bacterial infection referred to as columnaris. Columnaris tends to affect fish as water temperatures warm and fish are stressed from the energy they spent on spawning.
More details: DNR fish die-off page