European carp numbers are rising as floodwaters expand across the Darling River system, which could spell the end for many native fish.
While flooding was initially seen as a huge positive for native species after recent mass deaths on the Darling, University of New South Wales ecologist Derrick Cruz said it was a double-edged sword.
"Native fish will definitely benefit to some extent, but we also expect to see an increasing number of carp," he said.
"If only one per cent of carp eggs survive to adulthood, we're still facing an influx of potentially millions of carp across the Murray-Darling over the coming years."
As the floodwater recedes, it will be the carp, not the native fish, that will come out on top.
Native fish die in blackwater events, which occur when the floodwater stagnates and deoxygenates.
But carp can survive because they are uniquely adapted to breathe oxygen above the surface of the water.
Major efforts have been made to reduce the carp population by electrofishing and trapping, exclusion from breeding areas and stranding spawning carp in floodplain habitats by lowering water levels.
But this flood event may undo much of that work.
"It's estimated there are an average of 200 million," Mr Cruz said.
Restocking may be in vain
The NSW government has spent $10 million on restocking programs to bring back natives after fish kill events in drought years.
More than six million fish have since been released, including Murray cod, tiger perch and rainbow trout.
A few weeks ago, 130 Murray cod, each about a metre long, were rescued from the rivers during the drought and returned to their native waterways near Tamworth.
NSW Agriculture Minister Adam Marshall said fish had been released from Menindee to Tamworth and all the way down to Eden.
"The native fish drought response is the largest operation of its kind ever undertaken by the NSW government, providing native fish species a lifeline from the effects of diminishing flows along rivers as a result of the crippling drought," he said.
But all that work may go to waste as the carp numbers explode.
No silver bullet
It was hoped a virus could be developed to wipe out the carp population, but Mr Cruz said its release had been delayed.
"If it is released we expect to see high levels of mortality in carp," he said.
"But until the government is certain we won't see unforeseen flow-on effects, then from my understanding that's on hold."
He said the virus would have to be used with a suite of other measures to address longstanding problems.
"Native fish have suffered for quite some time now and that's the result of many decades of water extraction, anywhere a causeway might inhibit migration, pollution, and the removal of snags," Mr Cruz said.
Despite the role of floods in spreading carp, UNSW professor Richard Kingsford, the director of the Centre for Ecosystem Science, said the focus continued to be on building up native species and conditions.
"Floods are essential for the rivers," Dr Kingsford said.
