Summer & Winter Kill

Safety & Procedures

Summer and winter kill occur in some Alberta lakes where large numbers of fish die from a lack of oxygen. Although the two phenomena are similar, the causes of low oxygen differ.

Summer Kill

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

Many water bodies summer kill regularly because they are shallow or have shallow bays where water temperatures rise and oxygen levels lower. The high temperatures also cause algae to bloom and then die out suddenly. 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 occur only in 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 to 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.

During the winter, oxygen normally enters the water of a frozen lake through inlet water streams, cracks in the ice, and slow diffusion through 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 water volume is low, winter kill will 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, it may only occur in certain shallow areas of the lake where fish are unable to escape to deeper and more oxygen-rich water.

Summer and winter kills are normal occurrences in some Alberta lakes. Winter kill especially occurs in many of our 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.

Posted: April 21, 2008