Stretches of the Murray River could become an “ecological desert” because widespread flooding has reduced water quality, a scientist has warned, as New South Wales communities rally to save fish populations.
Dr Ian Wright, a professor at Western Sydney University’s School of Science, believes fish and other aquatic wildlife are dying in huge numbers along the river as dissolved oxygen levels plummet – and the ecological impact could be “catastrophic”.
“I’ve just got a feeling that this is like a silent killer affecting huge swathes of rivers right now,” he said.
“Large stretches of it could, to some degree, become an ecological desert. I don’t like using that term. But it is going to be catastrophic.”
Wright said there was “absolutely” the potential for fish kills similar to those seen at Menindee in 2019, when hundreds of thousands of fish died along a stretch of the Lower Darling River in NSW.
He said the fish were “probably just about all dead” across large sections of the Murray River. But unlike the 2019 event, which occurred during a period of drought, the deaths now were not as obvious because of the “enormous” volumes of water flowing through the system.
“The volumes still coming out are massive, so they’re probably getting washed away.”
The dissolved oxygen levels in the Murray River at Barham, in the western Riverina, on Monday had dropped to 0.5mg/L. According to the Murray Darling Basin Authority, the normal range is between 6–8 mg/L.
“It’s really alarming,” Wright said. “I have seen low oxygen but not this sort of a trend, and even a few weeks ago I would have said the fish were in serious, dire straits.”
Wright has sampled and tested the water quality of rivers for 35 years. He said 0.5mg/L is “like us as humans trying to hold our breath for a few hours”.
“These levels are so low,” he said. “The insect life living in rivers that the fish eat would be killed from this. So the fish, the invertebrates, we’re talking about a breakdown of the food chain.”
Wright’s comments come after reports of hundreds of dead fish in the Murray-Darling catchments in NSW after record flooding across large parts of the state.
Cameron Lay, the director of freshwater environment at the NSW Department of Industry, said it was difficult to predict if this event would be as bad as the 2019 fish kills.
“The current flooding and subsequent high inflows have been some of the highest on record,” he said.
“Fisheries officers are on the ground monitoring for fish deaths in areas of poor water quality and investigating fish deaths where access is possible.
“However, the number and spatial extent may increase as flows start to recede and temperatures increase.”
Lay said “pure numbers aren’t necessarily a measure of ecological impact”.
Wright said the responsibility of monitoring the water quality and fish kills was “falling on to the shoulders of citizen scientists” and volunteer groups.
In the NSW town of Barham and surrounding areas, community members and agencies are rallying to implement solutions to save fish, the Western Murray Land Improvement Group said.
The group’s executive officer, Roger Knight, said there has been “extensive fish kills” along parts of the Murray around Swan Hill and Tooleybuc.
“There’s a 1.15 metre cod dead at the caravan park here, and a lot of other really big yellow belly, and stressed fish,” he said.
Knight said community members have been using aerators to pump oxygen back into the river, with fundraising pages created to help.
“We’ve got people buying their own aerators at $1,800 a pop,” he said. “A couple of young guys down at Kyalite bought their own, just to put in the river.
“We’ve had a GoFundMe page get going in Swan Hill on a race day and raised $8,100 in a number of days. In Moulmein, they went to raise some money for aerators and in half an hour they raised $3,000 from the local community.”
Over the coming summer months, Wright said it was critical to monitor the temperature and presence of oxygen in the rivers across NSW – and to take care before going swimming.
“It’s actually quite hard finding information and we’re just lucky that Water NSW has that one or two sites,” he said.
“Unlike our beaches in Victoria and NSW – right up and down the coast, they test the water quality there regularly – good luck finding any data on swimming in rivers.”
A spokesperson for the NSW Department of Planning and Environment said real-time data for dissolved oxygen was available via the WaterNSW website.
“Multiple state and federal agencies are closely monitoring the unfolding hypoxic blackwater events across NSW,” the spokesperson said.
“People should always exercise extreme caution before swimming in flood waters or in rivers where fish deaths have occurred, as water quality problems are likely.”
Community members are encouraged to report any fish deaths or observations through the Fishers Watch phone line on 1800 043 536 or fill in a fish kill protocol and report form here.
