Fish deaths

A mass fish death is when a large number of wild or farmed fish die suddenly and unexpectedly. They are more likely to happen in times of flood or drought.

What causes mass fish deaths

The most common cause of mass fish deaths is low levels of oxygen in the water. Although, fish deaths can happen for unknown reasons. 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 land and water temperatures decreasing the amount of oxygen in the water
• severe or sudden storms or floods
• low water levels in freshwater river systems over summer.

There are other events that remove oxygen from the water and lead to fish deaths, including:

• blackwater events
• algae and algal blooms
• acidic water entering lakes and rivers

Other causes of fish deaths include:

• changes in water quality
• sudden changes in water temperature
• fish diseases and infection
• pollution from pesticides and other chemicals
• water being released from dams.

It is usually a combination of factors that causes fish deaths rather than just one of them.

You should know

• Fish deaths can be caused by natural events and in many cases cannot be prevented.
• Mass fish deaths are often caused by a combination of factors.
• Sometimes the reason for fish deaths is unknown.
• Connecting rivers and their tributaries and providing water for the environment can prevent fish deaths.

Between December 2018 and January 2019 there were 3 significant mass fish deaths in the Lower Darling. Hundreds of thousands, possibly millions, of fish died due to a combination of drought, algal blooms and a sudden temperature drop.

Between October 2019 and May 2020, over 65 fish death events were reported by Basin state governments. These fish death events occurred due to dry conditions, bushfire runoff, or sudden changes in water quality following rain.

Fish need enough oxygen in the water

Most aquatic animals, including fish, get the oxygen they need from water through their gills. The amount of oxygen in the water that is available to aquatic animals is called 'dissolved oxygen'. This oxygen is in addition to the oxygen atoms that form a water molecule.

Oxygen from the air naturally dissolves when air comes into contact with water. This happens especially when:

• wind or waves make the water on the surface move, increasing how much air comes into contact with the water
• the flow of water in streams and rivers is faster, again increasing how much air comes into contact with the water
• aquatic plants photosynthesise and release oxygen into the water.

When the levels of dissolved oxygen are low, aquatic animals can suffer from stress and suffocation and die.

Whether fish can survive when oxygen levels are low depends on the species and different physical conditions, but few fish species will survive being in water with too little oxygen for very long. Larger fish species like Murray cod can die first because they need more oxygen given their size.

Fish can't always escape dangerous conditions

Fish will try to swim to another area to avoid harm if they can. But unlike land animals and birds, fish can't always escape a dangerous environment. Fish can't escape when:

• river levels are so low, that fish are trapped in disconnected pools
• there are barriers that stop the fish from moving
• the dangerous conditions arise very quickly.

What the MDBA is doing about fish deaths

Long-term actions

To help prevent fish deaths in the future, the MDBA and Basin state governments must work together to improve the health of the rivers in the Basin.

The Basin Plan is one of the most important tools for improving the health of the rivers in the Basin. The Basin Plan limits the amount of water that can be taken from the Basin each year, so that there is enough for our rivers, lakes and wetlands and the plants and animals that depend on them.

The MDBA and Basin state governments have worked in partnership with the wider community, to develop the Native Fish Recovery Strategy.

The Native Fish Recovery Strategy aims to improve the native fish populations in the Basin and will be implemented collaboratively with the Basin state governments, First Nations and the wider community.

Immediate actions

The MDBA and Basin state governments work together to identify the areas in the Basin that are most at risk of fish deaths. When weather and flow conditions create a high-risk of fish deaths, Basin state governments that are signed up to the Basin Agreement work to reduce these risks, by:

• adding water to improve water quality, where possible, in refuge areas to protect native fish
• using aerators to increase the amount of oxygen in the water (though this only has a small impact)
• moving fish to key waterholes to protect them during droughts (though fish don't always survive the journey)
• developing new technologies to increase turbulence in the water and improve water quality
• developing a water quality threat map to communicate to others where there are locations with potentially poor water quality.