Summer Kill / Winter Kill
Last Update/Review: August 6, 2002
"Summer Kill" and "Winter Kill" are names of two phenomena that occur in some Alberta lakes where large numbers of fish die in the water body or a portion of the water body because of a lack of sufficient 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 the temperature of the water. Generally, the colder the temperature the more oxygen the water is capable of holding. Many water bodies summer kill regularly because they are shallow or have shallow bays where water temperatures rise and oxygen levels are depleted. Alga blooms also occur at this time, again as a result of high temperatures. The algae will live for a while but then suddenly die out. The resultant decomposition of the algae by microscopic organisms further reduces the oxygen in the water. Lack of significant rainfall, bringing 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). Other fish may survive by staying in the deeper and colder portions where oxygen concentrations are sufficient for survival.
The warm water also makes it possible for certain micro-organisms to reproduce and subsequently cause diseases in fish. This can be a contributing factor to summer kill.
Winter Kill
Winter kill occurs in lakes and ponds that are frozen over and where the exchange of gases between the water below the ice and the air above is not sufficient 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 consumption, if lakes are deep enough, they may contain a sufficient volume of water to maintain oxygen above lethal levels.
In shallow lakes where the water volume is not sufficient, 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 only occur 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 employs artificial aeration during the winter to help prevent the die-offs.
