Countless dead fish have piled up on the surface of the Darling River in far west NSW as receding floodwaters become starved of oxygen.

Images of the Darling River covered with dead fish were circulating widely on Friday, revealing scenes reminiscent of the mass fish deaths that occurred in the Menindee Lakes region in 2019.

#UPDATE: video from #Menindee local Graeme McCrabb… @NSWDPIE_Water confirming millions native fish dead. Cause = Lack of oxygen as floodwaters recede, huge build up of biomass and fish whose populations boomed during the wet times. Heatwave exacerbating low oxygen levels 😢 pic.twitter.com/RQLR1JA38D

— Sara Tomevska (@STomevska) March 17, 2023

But while the 2019 mass fish kills were caused by an algal bloom during drought, this week’s deaths appear to have been a result of decomposing organic material brought into the river system by floodwater draining from Darling River’s floodplains.

One thing both mass fish kill events have in common is that they starved the water of oxygen that fish and other organisms need to stay alive.

There have also been reports of mass fish deaths in the Macquarie Valley this week.

Image: Large numbers of dead fish have been reported in the Macquarie Valley this week. Source: NSW Irrigators' Council

The increasing risk of mass fish deaths was highlighted in a Murray-Darling Basin water quality and dissolved oxygen report issued by the NSW Government on March 15.

The report revealed that measurements of dissolved oxygen in the Darling River upstream of Menindee were as low as 1.39 mg/L on March 14. The report notes that “native fish and other large aquatic organisms require at least 2 mg/L of dissolved oxygen to survive, but may begin to suffer if levels are below 4 to 5 mg/L for prolonged periods.”

High temperatures across western NSW in recent days have been exacerbating the situation, with warm water unable to hold as much oxygen as cooler water. Broken Hill reached 38.5ºC on Thursday, its highest March temperature since 2019.