Fish mortality events can be generally classified as either kills and fish die offs and refer to deaths of multiple fish in one area. A fish mortality event as small as 25 fish must be reported by state employees, but events can occur on a much larger scale, resulting in the loss thousands of fish of one or more species.
Human activity typically causes fish kills, which are caused by an acute pollution event or industrial process.
Fish die offs are more common and related to environmental factors that negatively affect the health of fish.
Both types of events may occur for a variety of reasons, alone or in combination, some of which can include but are not limited to:
- Diseases.
- Fish stranded in isolated water and spawning related stress.
- High temperature/low water level.
- Low dissolved oxygen in the water.
- Chronic excessive nutrients, sewage or wastewater release.
- Chemical spill.
- Pollutant runoff.
State response
Four state agencies will assess a fish kill report, consult each other and respond appropriately based on the situation and circumstances.
The state agencies contacted will be the:
- Minnesota Department of Agriculture.
- Minnesota Department of Health.
- Minnesota Department of Natural Resources.
- Minnesota Pollution Control Agency.
Agencies will consult with each other to determine the appropriate response following guidelines created to ensure an appropriate repsonse.
Reported fish kills are posted weekly in the Environmental Quality Board Monitor.
The DNR is interested in documenting fish disease and mortality events.
See a kill: Call the state emergency number at 800-422-0798 or 651-649-5451. Report the waterbody, date, fish species and number of dead or dying fish to the state duty officer.
Questions about fish health: Call the closest area fisheries office or email the DNR fish health laboratory.
DNR investigating dead brown trout
Causes
The most common factor causing fish kills and die-offs is the lack of dissolved oxygen which may occur for several reasons. The level of oxygen dissolved in surface waters is more limited than that in the atmosphere – and more variable.
Levels of dissolved oxygen can change with the time of day, temperature, the presence of large algal blooms, lake treatments and other variables.
Fish have several ways to adapt to changes in the level of oxygen in water, but rapid drops in oxygen or prolonged low levels can kill some fish.
Fish in low dissolved oxygen situations may be observed at the surface of the water gasping for air, and prolonged stress may lead to large die-offs.
Mortality events caused by low oxygen levels often are considered either summer kills or winter kills. Both events are more likely to occur when water quality is degraded.
Summer kill refers to a die-off in a water body during peak summer temperatures for various reasons, especially low dissovled oxygen.
The level of dissovled oxygen in water generally drops as temperatures warm, and warm water promotes the growth of algae and other microorganisms that consume oxygen.
Summer kills often are accompanied by dense algal blooms.
The retreat of lake and stream ice can sometimes leaves behind dead fish, commonly referred to as winter kill.
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.
Winter kill potential is worse in winters with abundant or early snowfall. Lower autumn water levels increase the probability and severity of winter kill. Early ice-on and late ice-out dates also increase the winter kill potential.
Wetlands and shallow, soft-bottom lakes are more prone to winter kill than deeper, hard-bottomed lakes.
Fish are susceptible to a variety of diseases such as cancer, health disorders caused by contaminants and low water quality.
Disease caused by pathogens such as bacteria, viruses and fungi also affect fish. Infectious diseases caused by pathogens occur most often when other environmental stressors are impacting the health of fish such as low water quality.
Fish that are affected by infectious disease may appear lethargic and have difficulty swimming or keeping themselves upright.
Some infections may also cause obvious wounds on fish, such as bloody lesions, tumors/growths, excess mucus production, discoloration, loss of scales and bulging or sunken in eyes.
Viral pathogens of fish are not transmissible to humans; however, some common bacterial and parasitic pathogens of fish are also infectious to humans.
When a fish die-off is attributed to an outbreak of pathogenic bacteria in fish, it is best to avoid touching dead or dying fish or wading/swimming in the water. Never consume dying or dead fish during fish kills or fish die offs.
Not all cases of fish disease warrant investigation by the DNR but it is still worth reporting these events so that their occurrence can be recorded.
Learn more about fish diseases from the DNR fish health laboratory.
Fish are susceptible to a variety of diseases such as cancer, health disorders caused by contaminants and low water quality. Gas bubble disease in fish is caused by supersaturation of dissolved gases (primarily oxygen, nitrogen and carbon dioxide) in water. Gas supersaturation can be caused by physical, chemical or biological processes. Hydroelectric structures like spillways or turbines can physically entrain gases in water leading to supersaturated outflows where fish live. Environmental processes such rapid changes in barometric pressure during major weather events can lead to supersaturation of gases in water. Eutrophic lakes are also susceptible to large blooms of algae that can lead to the occurrence of gas bubble disease in fish. Fish living in waters with gas supersaturation can develop gas bubble disease, a condition where gas bubbles (emboli) form in blood and other fluid vessels or muscle, connective and organ tissues. Formation of these emboli causes internal injuries to fish and can lead to organ and nerve damage or cardiovascular failure Fish suffering from severe gas bubble disease may have abnormal behavior such as swimming or floating near the surface. Gas bubbles may be observed on the skin of the face (blisters) or be observed in the eyes, fins, or gills. Fish may appear bloody from ruptured blood vessels. Fish exposed to chronic gas supersaturation may also have lesions from fungal or bacterial infections as gas bubble damage to fins and skin can develop secondary infections. Depending on the severity of the damage, fish may completely or partially recover or die. 
Gizzard shad (Dorosma cepedianum) die offs on the Mississippi River in Southern Minnesota are a common event during the winter and spring that are usually due to the natural biology of this species. Gizzard shad reproduce quickly, with enormous recruitment events occurring almost every year in this region of the state. Gizzard Shad in Minnesota are also at the northern-most reach of their native range and are sensitive to the cold of Minnesota Winters. This species has been determined to have a higher susceptibility to cold stress (relative to other MN native fishes) and to have difficulty acclimating to temperature changes in the early spring. When mortality events of shad occur, they are often noticeable because of the propensity of shad to gather in dense shoals in river backwaters, especially those with warm water outflows from industrial operations that utilize water cooling. Smaller gizzard shad die off events can be caused by temporary shutdowns of warm water outflows, but larger events that span long stretches of the MS river are also common, especially during harsh winters or flashy spring temperature fluctuations. 
Monitoring
Hundreds of fish mortality events may occur throughout Minnesota each year. Reports are archived so that the frequency and distribution of events can be monitored.
The majority of fish kill and die-off reports occur in the Twin Cities metropolitan area.
Reports outside of the metro area tend to occur in areas with a high density of lakes and regions where the health of lakes is impaired by degraded water quality, poor shoreline habitat and other factors.
Learn more about water quality at: