A number of causes, natural or unnatural, may lead to large numbers of fish dying. Most fish kills happen in the summer, but winter fish kills are not uncommon. Unnatural fish kills are usually the result of releasing heated waste or toxic chemicals. The sources may be industrial, agricultural, municipal, or residential. Pesticides used for farming and to control water weeds are another cause.
Some kills result from tampering with water levels or flow.
The Ministry of Environment and Energy and the Ministry of Natural Resources investigate fish kills to find their causes, suggest responses, set up preventive programs and collect evidence for possible court action. Speed is of vital importance in these investigations. Since fish decompose rapidly, especially in warm weather, evidence may be lost within hours. It is important, therefore, to report any fish kills immediately. In river systems, toxins and fish carcasses may travel downstream from where the kill occurred.