Fish kills and die offs

Dead fish laying on the beach of a Minnesota lakeAs many as 500 fish die-offs can happen each year in Minnesota, most of them from natural causes. If you see one, report it. Early reports allow timely water samples and other investigative responses to be taken if needed.

Most fish diseases and infection issues found in nature tend to be concentrated in fish of a specific species and size range. In contrast, when a die-off includes fish of vastly different sizes and multiple species, human activity is a more likely cause.

To report a fish kill

Call the state duty officer at 651‐649‐5451 or 800‐422‐0798 to report fish die-offs. The duty officer provides a single point of contact for the incident and is available 24 hours a day seven days a week. Try to describe the fish types and sizes affected by the kill.

Spring & 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. Columnaris infections can kill sunfish, crappies and bullheads, and occasionally, largemouth bass and northern pike.

Ice out

The retreat of lake and stream ice can sometimes leaves behind dead fish, commonly referred to as winterkill.

When snow and ice cover a lake, they limit the sunlight reaching aquatic plants. The plants cut back on the amount of oxygen they produce. If vegetation dies from lack of sunlight, the plants start to decompose, which uses oxygen dissolved in the water. If oxygen depletion becomes severe enough, fish die.

Winterkill potential is worse in winters with abundant or early snowfall. Lower autumn water levels increase the probability and severity of winterkill. Early ice-on and late ice-out dates also increase the winterkill potential. Wetlands and shallow, soft-bottom lakes are more winterkill-prone than deeper, hard-bottomed lakes.

Other causes

Human causes of fish kills can include water discharged at high temperatures; toxic chemicals discharged or spilled; pesticides and fertilizers; manure runoff; and low oxygen levels in a lake resulting from storm water that runs off urban or rural landscapes. Multiple causes often contribute to fish deaths.

General questions about fish kills? Call the closest area fisheries office.