Understanding Fish Die-Offs

Sometimes large numbers of dead fish, often called fish "die-offs" or "kills", are found floating on a lake’s surface or shoreline. Fish die-offs can happen at any time of year in all kinds of water bodies.

Die-offs from Natural Loss of Oxygen in the Water

Like people, fish need oxygen to survive. Weather, water quality and temperature, types of fish and plants, and rapid increases in algae growth can be linked to water losing oxygen, causing fish to suffocate. This type of die-off is most common in shallow lakes with high plant growth.

At night, aquatic plants use oxygen from the water.  A cool, breezy summer will usually prevent fish die-offs because wind and wave action recharges the lake’s oxygen levels.  Long periods of calm, hot weather can reduce the water’s oxygen level by reducing the recharging effect of waves.

During the winter, cold water holds more oxygen and, with reduced activity, fish require less oxygen to survive. But cold winters can prolong the period of thick ice cover on lakes, which prevents oxygen recharge from the atmosphere and plants. Winter die-offs may not be noticed until early spring once ice and snow have melted.

Who is responsible for dealing with the removal of dead fish?

• Individual landowners are responsible for the cleanup of fish from their shorelines.
• Municipalities are responsible for the public beaches that they own, just as MNR will remove fish from swimming beaches in our Provincial Parks.
• Landowners are encouraged to contact their local municipalities regarding disposal arrangements.

To dispose of dead fish or birds along your shoreline:

• Bury them or dispose of them in the garbage. 
• Take caution while handling the carcasses.  Wear rubber gloves or protective material, and dispose of the gloves or protective material in the garbage. 
• Be sure to wash your hands thoroughly with antibacterial soap and water after handling the carcasses. 
• And, also make sure to wash any tool that came into contact with the animal with a disinfectant, such as bleach.

What do I do if I see large numbers of dead fish?

• Report fish die-offs to the Ministry of Natural Resources at 1-866-929-0994.
• If you suspect the fish died as a result of a spill, call the Ministry of the Environment Spills Action Centre hotline at 1-800-268-6060.