Fish deaths in the Lower Darling

The Lower Darling was subject to tragic fish death events in December 2018 and January 2019. The large fish death events covered a 40 kilometre stretch of the Darling River, downstream of Menindee Lakes. 

The exact number of fish deaths is unknown. Anecdotal estimates ranged from hundreds of thousands to at least 3 million. In considering this issue the independent panel, chaired by Professor Rob Vertessy, concluded that ‘over a million’ fish may have died in the series of fish death events.

Reasons behind fish deaths

The recent tragic fish deaths in the Lower Darling are a terrible reminder of the effects drought can have on our environment. We are keenly aware of the difficult times many people are facing due to the drought and when things are dry, it’s tough on the whole river system.

We cannot ignore the impacts of drought on the system.

Unfortunately, the main causes of this distressing event are:

• the lack of water flowing into the northern rivers
• the impact of many years of over-allocation of precious water resources throughout the entire Basin.

Lack of water during drought leads to water quality issues and can provide prime conditions for blue green algae to thrive. When previously high water temperatures drop quickly, as they did recently, algae dies and as it decomposes oxygen levels fall below critical levels, causing the fish to die.

Mitigating further fish death events

Basin governments are working together to find solutions that could mitigate further fish death events across the Basin.

Basin Governments are considering:

• the immediate risk of further fish kills and how we can mitigate that risk, including aeration and possible fish movement
• if there is water that can be released to improve water quality
• reviewing federal and state environmental watering priorities, to see if adaptation is needed
• long-term strategies to mitigate fish death events of this extent.

Operation of Menindee Lakes

There is a written agreement where New South Wales, Victoria, South Australia and the Australian Government directs the Murray–Darling Basin Authority (MDBA) to call water from the Menindee Lakes to meet consumptive and environmental needs.

In 1963, the New South Wales Government agreed with the Australian, Victorian and South Australian governments that water from the lakes could be shared to meet downstream water needs, when the volume of the lakes rises above 640 GL and until it drops below 480 GL.

When the Lakes rise above 640 GL, the MDBA can direct water to be released from the Lakes to meet downstream demand. The MDBA refers control to New South Wales when storage drops below 480 GL.

The MDBA calls on water to fulfil New South Wales, Victorian and South Australian entitlements when there is water in the Menindee lakes. When the MDBA operates the Lakes, it draws water from this storage first. This is because Dartmouth and Hume dams are more efficient as they don't have the high evaporation rates of the Menindee Lakes.

Menindee Lakes can hold up to 2,050 gigalitres and is estimated on average to lose 426 gigalitres a year to evaporation, and up to 700 gigalitres a year when the lakes are full. Dartmouth Dam, by comparison can hold up 3850 gigalitres with net evaporation of close to zero. 

The Menindee Lakes are currently under the sole control of New South Wales and have been carefully managed since December 2017. New South Wales is responsible for operating the lakes to meet local needs, including those at Broken Hill and downstream along the Lower Darling River, and also during flood events. For more information refer to the New South Wales Department of Primary Industries.

Current storage levels are available at River Murray data.