Carp were first introduced to Australia in the 1800s. Today, the ‘ecosystem engineers’ infest about one million square kilometres of the country’s waterways and growing.
“As a nation it seems we tried really hard to wreck our rivers,” National Carp Control Plan (NCCP) Coordinator Matt Barwick said of the deliberate introduction of the pest species.
“Carp are called ecosystem engineers because they change the ecosystem in ways that benefit them.”
The impacts of carp on waterways are well documented. As bottom feeders, they contribute to poor water quality by uprooting vegetation and stirring up sediment. This leads to increased turbidity, which is a problem for many native species that need to be able to see to feed.
Muddier water also means less sunlight can get through to aquatic vegetation – an important habitat for bugs and small fish. Reduced populations of these species mean reduced food sources for larger fish and birds. Not to mention carp also eat the eggs of native fish and frogs.
Numerous attempts have been made to reduce carp numbers in Australia, including fishing, screening them out of spawning habitats and placing traps on weirs and dams. Scientists have also tried luring male fish into traps by synthesising the pheromone female carp give out when getting ready to spawn.
But Barwick said it might take a biological solution to help stop the spread of one of the planet’s most invasive fish species, particularly as every state and territory, bar the Northern Territory, has a carp problem.
Fishing for a solution
It was a passion for native species that led Barwick to the NCCP, which was established by the Federal Government in 2016 to assess the viability of using carp virus – a type of herpesvirus specific to common carp – to kill off large numbers of the fish.
Carp virus was first observed in Germany in 1997 and has since turned up in 33 countries around the world. It has never been used as a biocontrol tool, which means it is important the NCCP answer some big questions: Can a biological control be used to reduce the impacts of carp in Australia? If so, how should this be done, what would the risks be, how can the risks be managed and what would it cost?
“The impact carp have on our ecosystems really knocks many of our native species about, which is why we need to try and fix the problem,” Barwick said.
“We’ve tried pretty much every tool in the pest control playbook. Some of them worked on a small scale, but none are effective over a scale of a million square kilometres. That’s why we need to explore biological control.”
The NCCP was meant to deliver its findings at the end of the year but was recently granted a 12-month extension to December 2019.
However, a lot of work has already been done, including the first continental-scale estimate of the total biomass, or tonnage, of carp in Australia. Research involving multiple locations and led by the Arthur Rylah Institute found carp biomass is about 1500 kilos per hectare in some areas, while in others it is as low as 20 kilos per hectare. Earlier studies have reported carp account for around 80% of the fish biomass in many rivers.
These numbers will be plugged into a computer modelling system that helps predict how the virus is likely to affect carp populations in different habitats.
Research has also been done to predict water temperature. As the carp virus is only active within a range of 16-28 degrees celsius, it is vital scientists know the water temperature at different locations to maximise the chance of it succeeding.
In a first for Australian scientists, CSIRO researchers discovered that if they had air temperature at any given location, they could predict the water temperature in that location – not just on the surface but throughout the water column.
“To me, it was a fascinating moment in science. Aussie researchers learned the data they needed didn’t exist, so they found a way to overcome the problem,” said Barwick.
“The application of this exciting new discovery is likely to extend well beyond carp research.”
Is carp herpes the answer?
There are many questions the NCCP will work to address over the next year, including what to do with all the dead fish killed by the virus and how the plan would impact water quality.
Although there is still a lot of research and stakeholder engagement to be done before the NCCP is ready to present its advice, Barwick said there is no evidence so far to suggest viral biocontrol is not viable for carp.
“As of today, all the evidence suggests it might be possible and I can see tremendous potential benefit if we were able to use it,” he said.
“However, I think I speak for everyone working on this program when I say we feel a great burden of responsibility to make sure we do the best research possible and use it to inform smart decisions … We need to ensure decision makers can be confident that, if approval is given to go forward, it is on the basis of good advice.”