In creeks, rivers and flood waters across the Murray-Darling Basin, an uncountable and unfathomable number of invasive carp are turning waters into bubbling masses of flapping and flailing fish.
“It’s quite a sight,” said Dr Matt Herring, an environment consultant. “I walked through one of the schools of carp a few days ago and it’s the first time I’ve trodden on fish with every step.”
European carp – an invasive species introduced more than 100 years ago – are spawning and growing in huge numbers, putting smaller native fish under more pressure as the invaders known as “river rabbits” stir up water, compete for food and damage aquatic vegetation.
The chief executive of the New South Wales Irrigators’ Council, Claire Miller, said this week the number of carp was “jaw-dropping”.
“It looks like the water is bubbling.”
Experts say while the carp boom is not good news for some native fish, there will be some winners – including native water birds.
In parts of the basin around the Murrumbidgee flood plain in NSW, Herring had seen an array of water birds feasting on the carp, which was also food for the endangered Australasian bittern.
“Part of me imagines if these were native fish – to see this would be wonderful. But the experience is ruined because I know these carp are recognised for doing so much damage,” Herring said.
John Koehn, an adjunct professor at Charles Sturt University’s Gulbali Institute, is a native fish expert who has also studied carp for 40 years.
He said the massive spawning event was not unusual for wet times and it would be months before it was known how many of the carp survived to adulthood.
“There’s definitely a downside,” Koehn said. “But the pelicans are probably pretty full and predatory fish like Murray cod are having a good feed.
“Golden perch populations are going berserk and a lot of wetland species are doing really well.”
Sign up for Guardian Australia’s free morning and afternoon email newsletters for your daily news roundup
Carp began booming in the 1960s after one genetic strain escaped from a fish farm in Victoria. They are filter feeders and forage in riverbeds, damaging aquatic plants. Their feeding style causes turbidity in the water that makes it hard for some native fish to survive.
Carp have spread through a river basin already heavily modified and degraded with diversions, blockages, weirs and extraction for farming.
Mark Lintermans, an associate professor at the University of Canberra, led research last year on the rising risk of extinction for Australian freshwater fish.
More than 20 species were given a higher than 50% chance of becoming extinct over the next decades.
He said the flooding was giving carp access to more places and they would compete with native fish for food and spread parasites.
“It’s not good news, but carp are as much a symptom of degradation as a cause,” he said. “This just highlights how much there is to do to save our threatened fish.”
Prof Richard Kingsford, the director of the Centre for Ecosystem Science at the University of NSW and an expert on water birds, said colonies of pelicans and cormorants would also be feeding on the carp.
“In a boom time in a natural system with no exotics, the native species [of fish] would have been doing well and would have exploded,” he said.
“But ideally you want to drop the carp numbers down and give the native fish a chance to rebound.”
Once waters start to recede, many of the carp will die as they get stuck in isolated pockets of water.
Water birds don’t feed on dead fish, but Kingsford said other birds such as kites, eagles and crows would. But it was likely there would be too many dead fish to go around, he said.
Dr Adam Kerezsy, a fish biologist and consultant based at Lake Cargelligo near the Lachlan River in central west NSW, has been sampling water and detecting high numbers of carp since October.
He expects mass mortality among the carp in the coming weeks as water recedes. He said the carp were very visible as they are happy to come to the surface, but the flooding was also helping some native fish populations.
“All the native fish have bred as well. From an ecological perspective, there are winners and losers and without something magical, we are going to have to learn to live with [the carp].”
Jack Gough, the acting conservation director for the Invasive Species Council, said natural disasters that were being intensified by climate change tended to favour invasive species.
“This is because they reproduce quickly, lack natural predators and are able to take over ecological niches while the native wildlife are under stress.
“At the moment our agencies are understaffed and underfunded and that is undermining their ability to respond quickly to new incursions and population booms and to conduct vital research into new control techniques.”
In October 2022, a six-year national project to investigate the potential to introduce a herpes virus to control the carp delivered a final report to governments.
More research was needed, the report said, in particular to test the vulnerability of native species to the virus.
A Department of Agriculture, Fisheries and Forestry statement said the project’s findings were being considered and any potential virus release would need more research and agreement across jurisdictions, which was ongoing but could “take several years to complete”.
There remained uncertainties on the “efficiency and effectiveness of the virus” in safely removing carp that in some basin areas were 90% of the fish in the water.