Mass fish deaths are more likely to occur in summer, during January and February.
Deaths are usually the result of sudden events or changes in the local environment.
Fish will naturally avoid adverse environmental conditions, however escape is not always possible.
Low levels of dissolved oxygen can cause stress (and even death) to fish and other aquatic animals which rely on oxygen in the water to breathe.
Larger fish species, such as Murray cod, tend to become stressed and/or die first due to their greater oxygen requirements.
Even when low oxygen is the cause, the reason for the low oxygen levels can’t always be found, and there may be other causes of stress.
When there is a mass fish death in summer, it is often because of natural events such as:
- high air and water temperatures decrease the amount of oxygen in the water
- severe or sudden storms or floods
- sudden destratification of water bodies
- low water levels during droughts
There are other events that remove oxygen from the water and lead to fish deaths, including:
Other causes of fish deaths include:
- rapid changes in water quality and temperature
- fish diseases and infection
- pollution from pesticides and other chemicals
- cold or low oxygen water released from dams
- not enough resources to maintain high fish numbers
It is usually a combination of factors that causes fish deaths rather than just one of them.
Last updated: 5 May 2025