The Darling-Baaka River at Menindee is on the brink of another environmental catastrophe, with dead fish already appearing along 30km of the river compromised by the last fish kill in March, according to experts.
The office of the chief scientist is due to report by 31 August on the causes of the March disaster, which killed millions of bony herring (also known as bony bream) and thousands of other native fish and carp.
But a survey by the NSW Department of Fisheries conducted in June, which has not been released, has revealed the poor state of the river between Menindee and Pooncarie.
The Guardian understands the fisheries survey found the river already had very low levels of oxygen due to the breakdown of fish carcasses, which is expected to worsen when temperatures increase.
The survey found only two mature Murray cod in a 140km stretch of river between Menindee and Pooncarie, suggesting the population has been decimated.
Menindee resident Graeme McCrabb said dead fish, including cod, were being found every day around the weir pool at Menindee township, raising concerns about the quality of the town’s drinking water, and about what might occur in coming months.
Wayne Smith, who owns a property 60km downstream from Menindee, said there were already high levels of algae in the river due to the nutrient load from the fish.
“The water quality just looks bad; it’s like grey dishwater,” he said.
Several major fish kills have occurred in the stretch of river near the township.
In January 2019 hundreds of thousands of fish died because of lack of flow and high temperatures leading to stratification of water and a lack of oxygen in the few pools providing refuge to fish.
In January the region experienced record flooding. In March, millions of bony herring, which boomed during the floods, died in a fish kill that dwarfed the events of 2019.
Menindee local Ross Files, who was born in the town in 1939, said fish kills were not regular events.
“We never had a fish kill in the river until they started growing cotton in the catchment up north. It never happened,” he said.
The March fish kill is still being investigated by the chief scientist, but a possible cause could be that the millions of new bony herring that spawned during the floods became trapped at the upper weir and consumed all the oxygen in the water.
The office of the NSW chief scientist has also been asked to investigate longer-term issues around management of the river system and future emergency responses.
The NSW minister for water, Rose Jackson, said the government was trying to be upfront about the risks and was actively involved in ongoing discussions about the risks of more fish kills.
“Mass fish deaths are a terrible outcome for the natural environment and local communities around the Menindee Lakes,” she said.
“I am advised that there are still large numbers of bony herring and carp in the reach of Darling River downstream of Lake Pamamaroo. When water temperatures start to warm up again as we move toward spring and summer, there remains a risk of further fish deaths in the Darling River at Menindee, as fish in an already stressed condition may succumb to any decrease in dissolved oxygen and increased competition for depleting resources.
“This is particularly the case for bony herring, which boomed during the recent floods, and many are now in poor condition and more susceptible to environmental stresses.”
She said the government was continuing to release water from Lake Pamamaroo to support fish populations.
Dr Darren Saunders, the deputy chief scientist, said his report was on track to be finished in about three weeks.
“We have spent time listening to the community and we are very conscious of their concerns,” he said.
“We are now in the middle of doing our own sampling of the river and lakes and we are looking at developing improved emergency response measures to be put in place as soon as possible.
“But we are going to have a data driven response wherever possible.”
