Summer and Winter Kill

Important information for trappers: Registered Fur Management Areas (RFMAs) where open seasons exist for Wolverine, River Otter, Canada Lynx, and Fisher - Species Limits (Quotas) have been removed for the 2024/2025 trapping season, please see page 16 of the Trapping Regulations for more information.

Summer and winter kills are when a large number of fish die in a lake. This is caused by low levels of oxygen. Summer and winter kills are normal occurrences in Alberta lakes.

The amount of oxygen a waterbody can hold is related to the temperature of the water. Generally, the colder the temperature of the water, the more oxygen it can hold.

Lowered oxygen levels are caused by different factors, which vary between summer and winter.

Summer Kill

It is common after a spell of hot weather to see dead fish (usually Lake Whitefish, but can include other species) floating on lakes or washing up on beaches, referred to as a summer kill. This is due to stress to fish caused by being ‘squeezed’ between the water layer near the surface that’s very warm, and water further below that may have not have enough oxygen. These summer kills are rarely severe or cause a total kill, and fish that are already less resilient due to age or illness usually succumb first.

In the summer, oxygen levels in some waterbodies may be impacted a number of factors:

• Some lakes are shallow or have shallow bays where water temperatures rise and oxygen levels lower.
• High temperatures also cause algae to bloom and then suddenly die out. The decomposition of the algae further reduces the oxygen in the water.
• Lack of rainfall, which brings oxygenated water into the lake, can also contribute to summer kill.
• Summer kill may also occur in only portions of a lake (e.g. shallow bays). Some fish may survive by moving to deeper and colder areas where oxygen concentrations are higher.

In the event of a summer kill, recreationalists should be aware that there may be a foul odour in the area and it may contain high levels of bacteria due to decomposition. Recreationalists should:

• Avoid swimming and other forms of recreation (water skiing, paddle boarding) in areas with large numbers of dead and decomposing fish.
• Temporarily avoid fishing and consumption of fish from areas near recent fish kills. Do not consume dead fish from a fish kill, or use them for bait.
• Do not allow pets or other animals to have contact with or consume dead and decomposing fish from a fish kill.

Winter Kill

In the winter, oxygen levels can be lowered by a number of factors:

• Oxygen normally enters the water of a frozen lake through the inlet water streams, cracks in the ice and slow diffusion though the ice. A thick snow cover on a lake can reduce the amount of oxygen passing through the ice.
• Fish and other aquatic plants and animals use oxygen throughout the winter. Larger and deeper lakes contain a higher volume of water that helps maintain higher concentrations of oxygen. Shallow lakes, where the volume of water is lower, may winter kill more often.

If you observe a fish die-off

Please notify your local fisheries biologist.

• Fish should be disposed of in an approved landfill.

Updated: Apr 13, 2023