Carp-culling project will be a one-hit wonder, experts warn

A federal government plan to slash carp fish numbers in Australia’s waterways by infecting the pest-species with a herpes virus will be a one-hit wonder, University of Sydney experts are warning.

The warning published last week in Australian Zoologist comes as the Fisheries Research and Development Corporation investigates whether to go ahead with a planned release of the virus in an effort to help rebuild native fish numbers in Australia's waterways.

“The release of this herpes virus in our waterways will undoubtedly cause a single epidemic of herpesvirus disease resulting in massive deaths among carp,” said the study’s lead author Associate Professor Joy Becker of the University’s School of Life Environmental Sciences.

“However, there’s little evidence to suggest that we will see repeated outbreaks of a magnitude to counter the reproductive potential of the surviving carp.”

This conclusion is based on a review of evidence from around the world examining the impact of the koi herpesvirus (CyHV3) on common carp in natural and farmed environments.

Associate Professor Becker and her author-colleagues, Professor Michael Ward and Dr Paul Hick from the Sydney School of Veterinary Science, say that the likelihood of carp population being controlled by releasing the virus is significantly reduced due to herd immunity and the carp’s “remarkable fecundity”.

They warn this means the government’s $15m culling program, which was announced in parliament by former deputy prime minister Barnaby Joyce, could be a one-hit wonder.

The research team said their great concern is how quickly the CyHV3 virus reaches balance in host populations, which occurred within two years in a study in Japan.

Dr Becker, who is a member of the government’s National Carp Control Program Scientific Advisory Group, has communicated her advice to the NCCP and to colleagues at scientific meetings.

Fast facts

• In some areas of Australia’s largest river catchment – the Murray-Darling Basin – carp are reported to dominate fish communities, comprising 80 to 90 percent of the biomass.
• Common carp are an introduced species in Australia.
• Carp show a range of ecological characteristics that provide a competitive advantage over most Australian native fish species. A hallmark of carp is prolific breeding; the species is highly fecund, with 80,000 eggs for fish of 1.25 kg and up to 1.5 million for fish of 6 kg.
• Common carp is the third most farmed fish species in the world.
• In 2016, the Australian Government Department of Agriculture and Water Resources announced the National Carp Control Plan based on the use of Cyprinid herpesvirus 3 (CyHV3) to reduce carp densities.
• CyHV3 is host specific and natural infections have only been detected in common carp and varieties of the species such as koi carp.
• Infection with CyHV3 causes koi herpesvirus disease (KHVD), a high mortality disease in common carp affecting all age classes of both wild and farmed fish.
• KHVD is exotic to Australia, as no outbreaks have been recorded.
• KHVD is characterised by irregular patches on the skin and severe gill necrosis and inflammation. CyHV3 infections occur in water temperatures between 16°C and 26°C with optimal transmission and development of viremia between 22°C and 24°C.
• Surviving carp develop anti-CyHV3 antibodies and can have enhanced resistance to the disease but can also become persistently infected carriers and shed CyHV-3.
• From experimental challenge, 25 to 65 percent of carp develop anti-CyHV3 antibodies, which persist for at least 65 weeks.