Why did the Menindee fish deaths happen?

An independent panel determined there were three main causes for the fish deaths in the Lower Darling in 2018 and 2019.

• not enough water flowing in the river
• poor water quality
• a sudden change in temperature.

This page explains

• Where the fish deaths occurred and the number of fish that died
• What caused the fish deaths
• What is being done by the states and the MDBA to prevent further fish deaths

Menindee Lakes fish deaths

Between December 2018 and January 2019, three mass fish death events happened in the Lower Darling. Although this is often described as the Menindee Lakes fish kill, the events covered a 40km stretch of the Darling River, downstream of Menindee Lakes.

The exact number of fish deaths is unknown, but an independent panel chaired by Professor Rob Vertessy concluded that over a million fish may have died in the fish death events.

The main native fish that were affected were:

• bony herring
• Murray cod
• silver perch
• golden perch

What caused the fish deaths

The mass death events in the Lower Darling were caused by a combination of several factors, including:

• an extended period of very hot weather leading to layering of the water, with a layer of cooler water with very low oxygen levels near the bed of the river
• sudden temperature drop aided by other weather conditions caused this low oxygen water to mix depleting oxygen throughout the water below the threshold fish needed to breathe
• the lack of water flowing into the northern rivers due to low rainfall and high evaporation rates
• over-allocation of water resources in the Basin for many years.

Drought and record heat resulted in low river levels

The Murray–Darling Basin had been under stress for several years due to consistently low rainfall and record heat. 2018 was:

• the eighth driest year on record
• the hottest year on record for New South Wales and the Murray–Darling Basin
• the second hottest year on record Australia-wide.

High temperatures and high winds throughout the year led to record evaporation rates during winter and spring in New South Wales and Queensland. As a result there was very little water coming into the river system (low inflows).

For example, the New South Wales river systems normally receive 4,000 gigalitres of inflows each year. Between July 1st and December 31st of 2018, they only received 30 gigalitres of inflows. This is less than 1% of the water that normally enters the system.

Poor water quality in the Menindee Lakes and lower Darling River

When the Menindee lakes are full they are used by some native species as major 'nursery habitats' (areas which support young fish).

Heavy rainfall and flooding in 2012 and 2016 led to an increase in the numbers of fish living in the Barwon-Darling. By 2018, there were extremely high numbers of young and mature fish in the Menindee Lakes system and in the river.

However, from 2016 there were extended hot and dry weather conditions. The lack of water coming into the lakes (‘low inflows’) and reduced amounts of water being released from the lakes into the lower Darling River, resulted in poor water quality. By 2019 the remining water in the Menindee Lakes and lower Darling River had dropped significantly and was of poor quality and so could not sustain high numbers of fish.

A sudden temperature drop killed fish

The lack of water and the poor water quality meant the water had separated into layers, with very low oxygen levels in the water near the bed of the river. A sudden drop in temperature and increased wind from storms caused the water in the different layers to mix. When this happened, the fish were starved of oxygen and suffocated as the water became deoxygenated throughout.

You should know

• Mass fish deaths are due to a combination of factors.
• Looking after the health of the river is the best way to stop fish deaths occurring.

Questions you may have

“Could the MDBA have prevented the fish deaths?”

Fish died because of a combination of low water levels from drought, stress from prolonged heat and not enough oxygen in the water following a sudden drop in temperature. These were all natural conditions which could not have been prevented by the MDBA.

“Why wasn’t there enough water in the Menindee Lakes?”

There was less water in the Menindee Lakes because of very low rainfall and consistently hot weather. Not enough water came into the Lakes to improve water quality and to allow fish to move into the lower parts of the river system where water quality was better.

An Independent Panel found the MDBA shared water from Menindee Lakes conservatively in 2017 to 2018, focussing on maintaining storage in the upper lakes to assist communities. To do this, water releases were less than normal into the lower Darling, to make sure enough water was available for livestock and domestic needs (critical human needs).

“Is the MDBA mismanaging the water?”

The MDBA works with scientists, governments and communities to look after the health of the whole river system. Managing the Lakes is a shared responsibility between the Australian and New South Wales governments.

 “How is the Basin Plan helping the river?”

The Basin Plan sets the amount of water that can be taken out of the river. Reforms are being introduced to increase both the volume and management of flows from the northern Basin, down the Barwon-Darling, and into the Menindee Lakes. These changes will improve water availability to the Menindee Lakes in the long-term, but it will take time. In 2020 we are seven years into a 12 year plan.

“Who makes decisions about where water goes from Menindee Lakes?”

The rules for which government is responsible for water in the Menindee Lakes are complex and depend on the amount of water in the Lakes. This dates back to 1963, when 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 there is more than 640 GL in the Lakes the water is considered a shared resource and the MDBA can direct water to be released from the Lakes to meet downstream demand, as part of the Murray–Darling Basin Agreement.

When the water levels decline to 480 GL, the water is considered a resource for New South Wales only, meaning the New South Wales Government is responsible for how water is shared and released, according to its rules and frameworks.

The MDBA doesn’t operate the Lakes but calls for water from the Lakes to fulfil New South Wales, Victorian and South Australian entitlements when there is water available, within the threshold outlined above.

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 close to zero.

The New South Wales Government has to manage water in 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 NSW Department of Primary Industries website.

To find out more about fish deaths

• Read the Independent Panel’s reports on the fish deaths
• Learn about who is responsible for water operations