“This event is ongoing as a heatwave across western NSW continues to put further stress on a system that has experienced extreme conditions from wide-scale flooding,” the department said on Friday.
The deaths were most likely caused by low oxygen levels as floods recede,a situation made worse by fish needing more oxygen because of the warmer weather.
Bony herring were a boom-and-bust species,the department said,which thrive in floods but are more susceptible to stresses when water flows return to normal.
“NSW DPI understands that fish death events are distressing to the local community,particularly on the lower Darling-Baaka.”
Nature photographer Geoff Looney found huge clusters of dead fish near the main weir at Menindee on Thursday evening.
“The stink was terrible. I nearly had to put a mask on,” he said.
“I was worried about my own health. That water right in the top comes down to our pumping station for the town.