Deteriorating water quality conditions in the lower Darling-Baaka River at Menindee has locals bracing for another fish kill as dissolved oxygen levels fall below the critical threshold for fish health.
A number of fish death incidents have been reported in the Menindee area since August, including one just last week. Among the species spotted in these smaller scale events was the endangered Murray cod.
“We’re all bracing for the next fish kill,” said Australia Institute water researcher and fifth-generation Menindee farmer, Kate McBride. “We recognise that we’re moving into a time where this might be a reality.”
On Monday, dissolved oxygen levels in the Darling River at Menindee dropped below 1 milligram a litre – the lowest reading since 12 April, when dissolved oxygen levels dropped to 0.3 mg/L overnight.
Native fish and other large aquatic organisms require at least 2 mg/L to survive, and may begin to suffer if levels are below 4 to 5 mg/L for prolonged periods.
Upstream of Menindee at Nelia Gaari, above Lake Wetherell, the dissolved oxygen levels in the Darling River sat below 1mg/L for almost 24 hours on Thursday. As of 9am Friday it was 0.5 mg/L.
McBride said those numbers were “really alarming”.
“Seeing the low oxygen is certainly concerning,” she said, adding that although the numbers were inconsistent, even short periods could be harmful. “If we starve a human of oxygen for an hour, but they have great oxygen for 23 hours of the day, they’re still dead.”
Broken Hill resident Ross Leddra, the president of the Darling River Action Group, told the Guardian Australia he saw some large dead Murray cod floating near Menindee’s main weir last week.
“But there was also some struggling cod, completely stressed, visible as they came alongside the boat, gasping,” he said.
When dissolved oxygen levels drop, McBride said, the larger Murray cod are often the first fish to die.
“When you see those fish deaths, even in small numbers, you know that the fish are struggling in that area,” she said.
The NSW Department of Planning and Environment said the government was continuing to work closely with the Menindee community and partner agencies to mitigate the risk of major fish deaths.
“Of course it’s concerning when oxygen levels drop in the Darling River,” a department spokesperson said. “Hot conditions, low dissolved oxygen levels and algal blooms over the coming months are expected to put additional pressure on the Darling-Baaka River system which still has a significant biomass of fish competing for limited resources below Weir 32.”
That will “keep the risk of fish deaths very high”, the spokesperson said.
They said the NSW and federal governments were “carefully monitoring the situation and adjusting releases of good quality water from Lake Pamamaroo to prevent hypoxic conditions”. Up to 1,000ML a day was released each day for three days last week, which the department said “successfully improved dissolved oxygen throughout the water column and assisted fish health”.
The NSW government last month released its first formal response to the chief scientist’s report into the mass fish kills in March, in which up to 30 million fish died. The report said there was a “strong likelihood of more mass fish deaths” as conditions dry over summer.
“Water quality in Lake Wetherell is already deteriorating, which means the habitat available to fish with good quality water will rapidly diminish,” the report said. “The next mass fish death event may be inevitable and could be larger than the March 2023 event.”
Under worst-case scenarios, without significant inflows, the government estimated that Menindee Lakes could be at 480GL stored volume by March 2024, which would leave only 12 months of water remaining for critical needs.
McBride, who was closely involved in the water amendment bill (restoring our rivers) which passed the senate on Thursday, said this prediction showed why embargos on further water extraction were “desperately needed now”.
“There’s still quite a high biomass of fish from the mass fish kills that occurred in March,” she said. “You had 20 to 30 million fish die, and almost none of them were taken out of the river.
“That creates a really horrible water condition in that part of the river. So that’s really concerning as we see temperatures increase.
“It’s unfortunately like this almost perfect storm of seeing the potential of another fish kill.”
