The deaths of thousands of fish in the Parramatta River triggered by low oxygen levels in the water is being investigated by the New South Wales environmental regulator.
The Environment Protection Authority said it was examining two fish kill events, one last week at Haslams Creek, which flows into Homebush Bay, and the other at Rydalmere on Sunday.
Thousands of dead fish have been observed along the riverbanks.
The events were likely due to natural causes, the EPA said, with preliminary investigations suggesting Sunday’s fish kill was due to storm runoff.
High intensity storms result in large amounts of organic matter – such as twigs and leaves – being washed into waterways.
An EPA spokesperson said this, combined with elevated water temperatures, had likely triggered a drop in oxygen.
Officials were collecting water samples and water monitoring was set to continue.
The spokesperson said it was distressing to see a significant number of fish had been killed and a variety of species were affected.
Flood waters north of Menindee Lakes, which experienced mass fish kills in 2019, have also prompted authorities to warn there could be further fish deaths in coming weeks.
Stuart Khan, the director of the graduate school of engineering at the University of New South Wales, said there was widespread concern about deoxygenated water in the Darling River and there was little that could be done to stop those floodwaters from arriving at Menindee Lakes.
He said the weather would play a role because heat, and warm water temperatures were more likely to contribute to fish deaths.
“DPI [the NSW Department of Primary Industries] is already preparing people,” he said.
“It’s overland flow of water across the floodplains, particularly floodplains that might not have flooded for some time.
“They build up all manner of biomass, sticks and leaves.”
Khan said in areas such as the Parramatta River there was more that could be done to manage stormwater.
He said if cities were designed to have more pervious surfaces – such as grasses, soils or permeable pavements – it could slow the pace of such events because water would infiltrate soils first before running off into waterways.
“In a city, it is particularly at least a consequence of our own urban design,” he said.
“The big problem is sudden influxes of water which comes from having lots of concrete.”
