The Department of Primary Industries in New South Wales state said the fish deaths coincided with a heat wave that put stress on a system that has experienced extreme conditions from wide-scale flooding.
The deaths were likely caused by low oxygen levels as floods recede, a situation made worse by fish needing more oxygen because of the warmer weather, the department said.
Residents of the Outback town of Menindee complained of a terrible smell from the dead fish.
“We’ve just sort of started to clean up, and then this has happened, and that’s sort of you’re walking around in a dried-up mess and then you’re smelling this putrid smell. It’s a terrible smell and horrible to see all those dead fish,” said Jan Dening, a local.
Nature photographer Geoff Looney found huge clusters of dead fish near the main weir in Menindee on Thursday evening.
“The stink was terrible. I nearly had to put a mask on,” Looney said. “I was worried about my own health. That water right in the top comes down to our pumping station for the town. People north of Menindee say there’s cod and perch floating down the river everywhere.”
Mass kills have been reported on the Darling-Baaka River in recent weeks. Tens of thousands of fish were found at the same spot in late February, while there have been several reports of dead fish downstream toward Pooncarie, near the borders of South Australia and Victoria states.
Enormous fish kills occurred on the river at Menindee during severe drought conditions in late 2018 and early 2019, with locals estimating millions of deaths.