Important!

Contact Alberta Fish and Wildlife if you see large numbers of dead fish in any Alberta waterbody.

To find contact information for an Alberta Fish and Wildlife office nearest you, visit the Environment and Parks (AEP) website at:

Summer and winter kills – when large numbers of fish in a lake die - are caused by low levels of oxygen in the water. Summer and winter kills are normal occurrences in Alberta lakes.

The amount of oxygen a water body can hold is directly 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 the summer and winter.

Summer Kill

In the summer, oxygen levels in the water can be lowered by a number of factors:

  • Summer kills happen regularly in some water bodies because they 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 by microscopic organisms further reduces the oxygen in the water.
  • Lack of significant 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 staying in the deeper and colder portions where oxygen concentrations are high enough for survival.
  • The warm water also makes it possible for certain microorganisms to reproduce and subsequently cause diseases in fish. This can be a contributing factor for summer kill.

Winter Kill

Winter kill occurs in frozen lakes and ponds where the exchange of gases between the water below the ice and the air above is not enough to maintain oxygen levels that support fish.

How Winter Kill Can Occur

During the winter, 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. Despite this steady use, if lakes are deep enough, they may contain a sufficient volume of water to maintain oxygen above lethal levels. But in shallow lakes where the volume is low, winter kill will occur.

When and Where Winter Kill Can Occur

Depending on the length of the winter, the amount of snow cover, the amount of fresh water entering the lake and the number of fish and other life in the lake, winter kill may occur only every few years.

Like summer kill, winter kill may only occur in certain shallow areas of the lake where fish are unable to escape to deeper and more oxygen-rich water. Winter kills, especially, occur in many of Alberta’s regularly trout-stocked water bodies.

In some cases, the Fish and Wildlife Division uses artificial aeration during the winter to help prevent the die-offs.

To find the potential for winter kill at specific lakes, see:

Updated: Mar 23, 2016