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23 March 2023 Expert commentary

Millions of dead fish were discovered this month in the Darling River at Menindee, in NSW’s far west. This is the latest mass fish loss to affect this stretch of river, after the 2019 event which resulted in up to one million fish dying during NSW’s devastating drought.

The native species Bony Herring (bony bream) comprise most of the fish found so far. However, locals have also observed Murray cod, golden perch, and carp among the dead.

CSIRO’s ecology, fish and hydrology experts – Dr David Post, Dr Klaus Joehnk, Dr Brenton Zampatti, Dr Francis Chiew, Dr Alison King, Dr Paul McInerney and Dr Carmel Pollino – are available for interviews on this topic.

Dr Carmel Pollino, Research Director – Water Security at CSIRO, said:

Why has this happened?

The fish deaths occurred because of low dissolved oxygen (DO) levels. Dissolved oxygen is a measure of the concentration of oxygen available for consumption by organisms within a body of water. It’s critical for their survival.

Low DO, and in turn fish deaths, are caused by a combination of prevailing weather conditions, the available water in the river up to and during the event, as well as water quality conditions.

Extensive recent flooding at Menindee and upstream has led to an immense boost in fish populations by providing suitable habitat for spawning and growth of young fish. After the floods receded, a high concentration of organic material and sediments remained in the water. Water quality deteriorated, causing lower dissolved oxygen levels and, as water levels receded, fish became more concentrated in the main river channel essentially competing for oxygen.

How does it differ from the 2019 fish kill?

Low dissolved oxygen levels are the most likely cause of fish deaths in both the 2019 and 2023 events, but the mechanisms causing the low DO are believed to be different.

Briefly, in 2019 low DO was due to severe drought and very low river flows. These patterns led to stagnant and stratified (or layered) water and development of an extensive blue-green algae bloom. The subsequent passing of a cold front weather system led to a sudden mixing of anoxic (deoxygenated water) bottom waters throughout the water column, and the DO levels declined as a result.

In 2023, water levels declined rapidly after flooding. Fish, organic material and nutrients became concentrated in a smaller body of water causing a decline in DO.

Will it happen again?

Yes, most likely.

In rivers that are regulated for consumptive water storage and use, we can expect more fish deaths during periods of water scarcity and after large floods. Such conditions can also occur in unregulated rivers, particularly during droughts and after floods. Extreme events, such as droughts and floods, are expected to increase in frequency due to climate change.

Predicting exactly where and when these events will occur is difficult, but understanding the impact of river regulation on river flow and hydraulics, and associated water quality and ecological processes can provide a guide to both predicting and managing future events. Monitoring and prediction are core to understanding why these events occur and how we can reduce their occurrence in the future.

Furthermore, some fish species like bony bream and carp reproduce en masse in suitable conditions, like floods, however natural mortality is usually high and brings the population back to a level that can be sustained by the environment. The explosion of young fish adds considerably to the overall oxygen demand.

How can we better prepare, protect, predict, respond?

There are several factors we should consider when considering water use and security:

Good water quality is critical to sustain life.

Accurate monitoring of water quality, river flows and fish populations is essential.

Allowing enough water to maintain connection in river channels (i.e. continuous flowing water) can allow fish to move away from low dissolved oxygen areas. The more free-flowing and connected a river is, the less likely the conditions leading to fish deaths are likely to occur, even under low flows.

Weirs and block banks obstruct fish movement. This can be partly mitigated by installing fishways or wholly by removing weirs.

Improving operation of structures (e.g. weirs) can allow for better fish movement, particularly when water quality deteriorates.

Ecologically sensitive weir pool and floodplain management can also help, by slowly ramping down after floods and minimising the ‘lake’ effects of weir pools during low flows.

A deployment of temperature and oxygen sensors at different depths can deliver the necessary data to help inform initiatives such as the recently launched AquaWatch mission to design and build a water quality monitoring system. Integrated monitoring and early warning systems allow ecosystem managers to choose appropriate mitigation actions. Hydrologists working with fish ecologists can then evaluate risks of potential fish deaths.


An AquaWatch water quality sensor at Lake Tuggeranong, ACT.
Recent flood waters from the Darling river fill Menindee lakes in the far west of New South Wales, Australia. Image/Shutterstock
Dead fish in the Darling River at Menindee. Credit Graeme McCrabb.

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