Reclaiming our rivers from feral carp
Five years ago, after the millennium drought had broken, feral carp numbers exploded in Australia’s river systems. ECOS reported back then on new research underway investigating a biological control – the cyprinid herpesvirus 3 (CyHV-3 or the carp virus).
The National Carp Control Plan was established in late 2016 to coordinate the research, planning, and community consultation necessary for this program in Australia.
As part of the control plan, a team of Australian researchers is assessing the potential use of the carp virus to control carp numbers, while acknowledging the challenges this presents.
Carp and habitat damage – cause or symptom?
Australian rivers experience many environmental pressures, including pressure from carp. In fact, they thrive in rivers already degraded. That suggests carp may intensify environmental stress, not cause it.
These are resilient fish which means they can also survive – and spread – in the very ecosystems they succeed in degrading.
Matt Barwick, an ecologist appointed last year as coordinator of the $15-million National Carp Control Plan, says that as a result the ‘footprint’ carp leave on the environment extends over large areas.
“Their bottom-feeding behavior reduces water clarity, limiting sunlight penetrating down to macrophytes on the river bed, which then reduces the habitat and food source for invertebrates, native fish and water birds,” he explains.
“The cumulative effect of these impacts is to shift ecosystems from a predominantly clear-water state to a murky, nutrient-rich state referred to as eutrophication.”
Once an ecosystem shifts in this way, reversal can be difficult, meaning the river will remain muddy for some time, even as carp densities fluctuate in various locations within the system.
Choosing the right biocontrol agent for carp
Common carp are widely farmed in many countries in Asia, Europe and the Middle East where they are an important source of protein. The virus was first identified in Germany and Israel in 1998 and spread in the 2000s to more than 28 countries. The carp virus devastated farmed and wild carp in Israel, Indonesia, Japan and parts of Europe, suggesting possible value as a biocontrol agent here.
However, based on Australia’s experience with biocontrol viruses for rabbits, Dr Ken McColl, who is leading a team of CSIRO researchers as part of the control plan, cautions the virus is not a ‘silver bullet’ and broad-scale complementary control measures will be required for effective long-term reduction of carp impacts in Australia.
Based on lessons learnt from past use of viral biocontrol agents for invasive vertebrates, and on mathematical modeling, the carp virus will likely have the greatest impact in the first few years after release, he says.
After that, effectiveness may be diminished – but not lost – as virus and host adapt to each other.
The National Carp Control Plan team is carefully considering genetic strategies to work alongside the carp virus to skew the gender ratio of the remaining carp population after release of the virus for better control of numbers. New generations of more virulent, but still natural, strains of the carp virus may also be investigated.
Virus impact and management measures
If released, the carp virus is likely to kill more than 70 per cent of carp at release locations. That is, potentially, a lot of dead fish.
Because carp densities exceed 350kg per hectare in parts of the Murray-Darling Basin, one of the NCCP’s most important tasks is to safeguard water and habitat quality for people, stock and native species.
Dr McColl says rigorous testing has been undertaken to ensure impact and management measures are safe.
“Planning for post-release clean-up efforts begins with a virus release strategy based on sophisticated mathematical modeling to identify the best strategy for maximum impact from the virus,” Dr McColl says.
The model accounts for river flows, rainfall, carp biology, and barriers to fish movement such as dams and weirs. The importance of the model is that it will be used to investigate likely ecological consequences of virus release, including possible responses from other invasive species, how different quantities of dead carp affect water quality, and timeframes within which dead carp will need to be collected to maintain water quality.
Options for the productive use of carp following collection are also being explored.
Ongoing safety measures
A report to the European Commission in 2000 from the Scientific Committee on Animal Health and Animal Welfare stated: “There is no evidence, to date, for a fish virus causing disease in humans.”
Apart from humans, the CSIRO research has also investigated the potential risk of the carp virus to a wide range of native fish, lampreys, amphibians, reptiles, birds, mice and yabbies.
Following a rigorous study, Dr McColl says the work shows the carp virus does not multiply, nor cause disease, in any of the tested species other than common or koi carp.
He also adds that, in the 20 years since its recognition, the carp virus has never been reported in other species.
It is also unlikely that mutations in the current carp virus will result in a future expanded host range. For DNA viruses like the carp virus, mutations are rare, and jumps into new hosts are measured on time-scales of millions of years. This view is supported by a DNA virus of rabbits, the myxoma virus, which has been in Australia for over 60 years, but has never been found in a species other than rabbits.
On the path to reclaiming our rivers
Matt Barwick says that the carp virus, together with some other broad-scale complementary measure, will be imperative in countering the on-going carp problem in Australia. However, the control plan is fully committed to testing virus effectiveness, and safeguarding against health and environmental risks for humans and other animal species before virus release is contemplated.
The National Carp Control Plan is being developed in conjunction with the Fisheries Research Development Corporation, to be finalised by the end of 2018.