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‘The smell is next level’: millions of dead fish spanning kilometres of Darling-Baaka river begin to rot

A “wall of dead fish” spanning tens of kilometres is moving along a stretch of the Darling-Baaka river near the town of Menindee, with temperatures forecast to reach 41C in the area on Saturday.

“The smell is just next level,” resident Graeme McCrabb said. “Imagine the smell if you put a dead fish in your sink and let it rot for a few days – but we have millions of them.”

Reports emerged on Friday of the mass fish kill near the western New South Wales town, with the numbers of dead fish likely in the millions.

Related: ‘Unfathomable’: millions of dead fish blanket river near Menindee in latest mass kill

Experts said the rotting fish and hot weather would probably further deplete the water of oxygen in the coming days.

“There’s an enormous amount of fish. That wall of dead fish is moving its way downstream,” said McCrabb, who thought any attempts to try to clean up the rotting bodies would be “futile” due to the sheer scale of the task.

He said water birds in the region – in particular cormorants and kites – were taking advantage of the chance for a feed.

“They’re all smashing it,” he said.

The NSW Department of Primary Industries said Friday the fish kills were related to low oxygen levels as the extreme flooding in the region in January had receded.

As nutrients and organic matter ran into the river, this pulled oxygen from the water, with the current hot conditions making the oxygen depletion – known as hypoxia – even worse.

The environment minister, Tanya Plibersek, said she was “devastated” to see the pictures of dead fish, describing the fish kill as “tragic”.

“We need to better understand the causes of these kills to better prevent them,” she said.

She said delivering the Murray-Darling Basin plan “in full” would help rebuild native fish populations.

The same stretch of water was hit by a series of mass fish deaths in the summer of 2018-19, prompting an independent review.

Prof Fran Sheldon, of Griffith University’s Australian Rivers Institute and a member of that review panel, said as the fish start to rot, this would further deplete oxygen in the water.

She said the boom in numbers of Bony Bream was a natural event and many would have died anyway because of a lack of food.

During the floods, all fish would have taken the opportunity to breed but bony bream were particularly susceptible to low oxygen events, and were less able than species such as Murray cod to find ways to escape, she said.

Releasing environmental water held downstream could help stem the spread of poor quality water, Sheldon said, while upstream regulators should consider restricting the amount of water used for irrigation to allow the system to recover.

Prof Lee Baumgartner, a fish ecologist and also a member of the review panel, said the fish kill was “like Groundhog Day”.

He said the review panel had recommended that improvements be made to operational strategies so that state and federal agencies could react more quickly to potential fish kills.

“That just hasn’t happened yet,” he said.

“During our review [of the 2018-19 events] we found that the climate in western NSW has significantly shifted. We see the transition from drought to flood to drought is happening more quickly than ever.”

Related: Second mass fish death in Menindee in two weeks prompts calls for water quality monitoring

The Bureau of Meteorology said there is now a 50% chance of an El Niño developing before the end of this year, which increases the risk of hotter and drier conditions for country’s south-east.

“That means we could be heading back to 2018 and 2019 again. That’s a concern,” added Baumgartner.

NSW Greens MP Cate Faehrmann said the fish kills were “an existential threat” to the river’s health and the community’s water security.

“This is categorically a catastrophe, regardless of whether this is a consequence of receding floods or water mismanagement, the NSW and federal governments should be acting now to clean up the millions of rotting fish which are spanning kilometres of the river,” she said.

A spokesperson for the Commonwealth Environmental Water Holder, Dr Simon Banks, said: “It’s devastating to see large-scale native fish deaths. Commonwealth agencies have been supporting NSW in their response to this serious fish kill, including by providing water for the environment to do what we can to improve water quality.”

The spokesperson said the timing, size and location of releases from Menindee lakes into the lower reaches of the Darling river had been “adjusted in response to conditions and local advice”.

“Unfortunately, under these conditions there are limited intervention options, however, agencies are continuing to monitor the situation and respond where possible.”