Luke Pearce had arrived at Mannus Creek for a three-day mission to rescue the Murray-Darling Basin’s last population of Macquarie perch.
For 10 years Pearce had visited this spot on the edge of the Snowy Mountains that, just weeks earlier, was ravaged by fire. There had been rain and the creek was flowing fast.
But as Pearce and his colleagues stood on the bank – nets at the ready – the water turned “to a river of black porridge”.
“We got there at about midday with two teams. But we were too late,” he says.
Pearce is a fisheries manager in the New South Wales Department of Primary Industries. A week earlier, he had caught nine of the endangered perch and taken them to the tanks at Narrandera Fisheries Centre.
But Pearce says nine was not enough to be confident they could breed enough in captivity to replenish the river. About 100 specimens would be be ideal, but Pearce says the fish are in such low numbers that he was hoping for 20. Hence the rescue mission on 20 January.
“It was a front of black water coming down,” Pearce says. “The water was pretty bad to start with, but it went from green to inky black.
“It was a moment of complete despair and, really, a feeling of a missed opportunity. Maybe if we’d got there four or five hours earlier we may have been able to get one or two more.”
An electronic probe in the water monitoring the oxygen levels dropped to show zero within hours, Pearce says.
“Watching those oxygen levels drop like that I had grave fears we could have lost all the fish in that system. It was devastating having worked there for such a long time to then potentially lose all this.”
The river was too black to see any fish, but crayfish, shrimp and mayfly larvae were crawling out.
What happened at Mannus Creek is one example of what scientists have described as a “triple whammy” hitting rivers on Australia’s east coast and inland.
Drought and a long-term drying has delivered a cascade of mass fish kills since late 2018, with low river flows, low oxygen and algal blooms. Authorities and politicians warned repeatedly in 2019 that ongoing drying would see more mass fish kills.
Then Australia’s bushfire crisis struck across catchments. Now heavy rain has washed sludge and ash into rivers, robbing the remaining fish of oxygen.
At one time, Macquarie perch was one of the most abundant native fish in the Murray-Darling system – prized by anglers and also commercial fishers.
But a NSW government assessment of the fish in 2008 wrote the building of dams and weirs had compromised spawning areas and blocked the fish’s movement. Overfishing, pollution and predation by introduced species like redfin perch had also caused numbers to plummet.
Now, Pearce says there’s just one known population of the fish at Mannus Creek, near the town of Tumbarumba.
On 14 January, Pearce had been to the creek to see what was left after the fires. The vegetation, the animals and the usual audio backtrack of birds and bugs he was used to seeing was now a scene of carnage.
The area was littered with the burnt remains of wildlife – kangaroos, wombats, possums, birds and deer. “There’s wasn’t a sound – nothing at all,” he says. “There was nothing left – just black stalks.
“All that spoke to me about the speed and ferocity of the fires – these animals just couldn’t escape. We were really concerned about the potential impact if we had any rainfall.”
The rescue attempt for the Macquarie perch is part of the biggest fish rescue project the NSW government has ever attempted.
Since September, with water levels low across rivers, some 4,000 native fish have been taken from rivers and placed in tanks across the state – most in government facilities and some in private hatcheries.
NSW DPI’s Dr Trevor Daly, a senior fisheries manager working on threatened species, says rescues of threatened fish have included 1,630 olive perchlet, 740 southern pygmy perch, 292 Oxleyan pygmy perch, 107 southern purple spotted gudgeon, 98 eastern freshwater cod, 79 silver perch and 34 eel-tailed catfish.
Catchments targeted for rescues have included Gwydir, Border Rivers, Macquarie, Lachlan, and Upper Murray catchments in the Murray-Darling Basin, and in the Clarence and Richmond River catchments on the coast.
“We’ve been battling the worst drought on record and we know we can’t save every fish, but we are doing what we can to save as many as we can,” Daly says, adding the heavy rain will likely see more fish deaths.
The government says the rescue efforts are “critical” if populations of fish are going to recover when river conditions improve.
Some fish are being held in tanks, while others have been relocated to rivers that do have some water.
Prof Lee Baumgartner, a freshwater ecologist at Charles Sturt University, says the Murray-Darling Basin has gone through an extreme swing from wet to dry in just 18 months.
“The climate change science tells us that this could be the new normal and, if it is, then we have to manage that transition much better and share the water in a more equitable way.
“Fish need water in rivers. Birds can fly away if there’s drought, trees can drop leaves. But fish need water.”
How long it will take for conditions to improve at Mannus Creek is hard to say. That initial flush of “black porridge” could travel through in a matter of days, but the sediment could linger for months or even longer depending on river flows and rainfall.
Last week, Pearce returned to Mannus Creek and went a little further upstream. On the banks, the black sludge was about 15cm thick, but that visit has also given him some hope.
“I managed to catch another two – I released one,” he says. “I was astounded that there were still some fish surviving – I had real fears that nothing could survive. It’s given me some hope that they can still hang on.
“Hopefully they’re through the worst of it.”