The research team collated data from 4,831 sites across a range of habitat types including rivers, lakes, wetlands and billabongs and used a model-based approach to estimate how many carp were in the waterways.

Arthur Rylah Institute for Environmental Research principal scientist Jarod Lyon said 96 per cent of the carp identified were found on the east coast, where they occupied 54 per cent of wetlands and 97 per cent of large rivers.

To measure the density impact threshold — the number of fish in the water per unit area — the study used "a rule of 100kg per hectare", Dr Lyon said.

“When they start getting above 100kg per hectare, you start having impacts on all sorts of biota, aquatic vegetation, native fishes, all the other birds and animals.

“In some wetlands, we found levels of up to 1,000kg per hectare — way above what we know causes impacts on our environment.

"Luckily, at the moment, it’s the exception rather than the rule.”

Dr Lyon said some of the sites were wetlands in central Victoria, floodplain wetlands in South Australia’s Lower Murray-Darling Basin and the Lachlan River catchment.

Common carp (Cyprinus carpio) is known for sucking up mud and having a negative impact on water quality.

Dr Lyon said the fish posed a real threat to Australia's biodiversity.

Controlling populations

The federal government first announced $15-million funding for a National Carp Control Plan and the research into the potential release of a controversial carp herpes virus (Cyprinid herpesvirus-3) as a biological control agent for the pest in May 2016.

The plan has been delayed multiple times and remains incomplete.

The Department of Agriculture, Water and Environment said it was working with the Fisheries Research and Development Corporation (FRDC) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to complete ongoing research that was expected to be completed in the second half of 2021.

The department said other than integrating the remaining scientific research, the plan was close to completion and would be considered by all Australian governments and released publicly once it was finalised.

But many commercial fishers, like Garry Warrick from Barmera, in South Australia's Riverland, are sceptical of the carp herpes virus and the impact dead carp could have on other native species.

"You are never going to get all the carp out, even with the virus," Mr Warrick said.

"It's only going to get 50%t in the long-term, at best."

He said he had been involved in some of the experiments for the National Carp Control Plan conducted in backwaters in the Riverland and was fearful of its results.

“They put six tonnes of dead carp into a backwater to see the effect it would have on backwaters and it actually killed all the live carp in the creek because they just ran out of oxygen and the water turned black," Mr Warrick said.

“So, it would probably kill all the native fish in there as well.”

Community role has 'Positive impact'

A female carp can produce 300,000 eggs in a single spawning event and has the potential to produce 1 million eggs over the breeding season.

While it is still unclear if the carp herpes virus release will go ahead, Dr Lyon says the research could also be used for site-specific control techniques including cages and manual removal.

"At a species level across their whole range it is not going to make a huge difference," he said.

"But it can have a really strong and positive impact on a site level where communities get involved in carp removal and reduction programs."

Mr Warrick is trying to do his bit by catching carp at Lake Bonney and in local creeks.

He said he had seen a lot of change in the water habitat over the past 35 years of fishing and believed one of the biggest problems was a lack of high rivers.

"Carp breed up because the rivers don't flow as much," he said.

"They silt up the river and they are overpopulated and knock the native species around a fair bit."

Fishing comp pulls 16,600 carp

Fellow Riverland local Kym Manning started a community initiative to reduce carp numbers nine years ago.

He invented SA's largest carp fishing competition, the SA Carp Frenzy,  at Lake Bonney.

"The idea was to get rid of some of the carp to give our native fish a bit more space," Mr Manning said.

He said the competition started with 350 competitors in 2012 and now attracted about 700 keen fishers every year.

But Mr Manning said it was shocking how quickly carp numbers could rise.

"After the flood in 2015, when they spawned in 2016, on the competition day we caught 16,600 carp in nine hours between 400 fishermen," he said.

"Normally that competition would only pull 1,000 carp, so you can see how fast they can explode."

Mr Manning said he had witnessed many changes over his lifetime.

"When I was young there was never any carp — my first recollection of carp was in the '70s," he said.

"I remember, when the yabby season would kick-off in the ['70s and '80s], your opera house net would be full of little carp in the backwaters.

"The carp have been a change for the worse."

Story By | Jesssica Scremmer