Summer and Winter Kill

Important! Contact Alberta Environment and Parks (AEP) if you see large numbers of dead fish in any Alberta waterbody. To find contact information for the office nearest you, visit the Environment and Parks (AEP) website at:

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

In the summer, oxygen levels can be lowered by 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.

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.

Updated: Jul 22, 2019