Carp are a freshwater fish native to Asia. Now the most abundant large fresh water fish in some areas — Australia considers them a pest.
Research to be presented at the International Water Conference in Brisbane next Tuesday has found a mass carp culling could have a major impact on native flora and fauna due to the amount of biomass in Australia's waterways.
Outdoor tanks set up in Prospect Reservoir, which supplies water to Sydney, were filled with different quantities of dead carp as part of a risk assessment by Water NSW.
The loads of virus-free fish varied from 250 to 6,000 kilograms per hectare and in every case but one the oxygen levels in the water dropped to zero.
"Above 1,000 kilograms per hectare the dissolved oxygen stayed at zero for up to five days," lead researcher Joe Pera said.
"That's a real concern because for such a prolonged period at low DO [dissolved oxygen] you'll pretty much wipe out the rest of the ecosystem."
In every tank there was a spike in nutrients, bacteria and algae with the levels increasing as the number of dead carp went up.
"We did a couple of experiments, one of them was actually an odour test, where you'd smell the water and pretty much in all the treatments it was undrinkable," the water quality scientist said.
A plan to release a strain of carp herpes into Australia's waterways to control the pest species, will be delivered at the end of the year after a range of research is completed.
The pest that affects the rest
Carp make up about 80 per cent of the biomass in the nation's river systems and cost the Australian economy an estimated $500 million a year.
Joe Pera supports the introduction of the biological control, but warns a quick clean-up is critical to maintain drinking water quality and protect native fish.
"They [native fish] wouldn't last long," Mr Pera said.
"They wouldn't be able to survive a scenario where there was no oxygen for a day that's for sure."
Mr Pera says his study applies more to still and shallow-water systems and that the figures would have to be diluted for deeper and faster-flowing areas.
Brisbane-based scientist Jonathan Marshall says the findings are a good indication of what might happen in the northern part of the Murray-Darling system, where rivers are more stagnant and contract to waterholes during droughts.
He says waterholes are where fish survive during dry times, but they're hard to reach which could make removing dead carp difficult.
"I think the worst-case scenario is that the intention of the National Carp Plan to improve native biodiversity could have the opposite effect and actually cause loss of native biodiversity," said Dr Marshall, who is also a member of the plan's science advisory group.
A difference in opinion
But another water expert, John Conallin, says the virus will give native fish a chance to reclaim territory and he rejects claims it will lead to an ecological disaster.
"I really think that it will be very short-lived if there is a water-quality problem," said Dr Conallin who works for the International Delft Institute for Water Education.
"And it will be only for one season for a lifetime of benefits."
Several other water-quality trials are also being carried out, including one by SA Water which will examine how the virus may affect South Australia's drinking water and the way it's treated.
The co-ordinator of the National Carp Control Plan, Matt Barwick, says the virus is the most well-researched pest animal biological control in the world.
"I'm glad that we're looking at worst-case scenarios because we need to understand under those circumstances what happens and can the risks be managed," he said.
"When you talk about controlling a pest species across a million square kilometres using a biocontrol agent, there is absolute complexity.
"This is a really challenging, complex issue. It doesn't mean we can't do it."
By Kerry Staight May 4, 2018