Invasive common carp have flooded Australian rivers.Credit: David W Cerny/Reuters

An Australian plan to kill invasive carp by releasing a virus into waterways has come under fire from researchers who argue that the tactic will not eradicate enough fish.

Australia has fought for decades to control populations of the common carp (Cyprinus carpio), and in 2016 the government invested Aus$15 million (US$11.7 million) in a plan to investigate the virus approach. The proposal is to infect carp with a strain of herpes called CyHV-3, which has caused mass fish deaths in the United States and Japan.

Many researchers have previously warned of possible far-reaching consequences of the plan, including rivers clogged with decaying fish corpses and further disruption to native ecosystems. Now, six researchers argue in a letter published in Science1 on 22 February that the virus is unlikely to be effective.

They contend that more data — including some from field trials — are needed before a wide-scale release of the virus. “This is potentially a big ecological move that’﻿s being made on not enough data,” says Philip Stevenson, a virologist at the University of Queensland in Brisbane and a co-author of the letter. The other authors include Jonathan Marshall, a member of the science advisory board of the National Carp Control Plan (NCCP), which is managing and conducting research into the potential virus release.

The NCCP’s coordinator, Matt Barwick, says that he welcomes debate about the proposal, but he adds that the NCCP has already addressed the issues raised in the letter or has submitted research proposals to do so to its scientific advisory board. “It’s accurate that we need to confirm the likely efficacy, and that’s exactly what we’re doing,” he says.

Carp carnage

The common carp was first introduced to Australia from Europe in the nineteenth century, but the biggest disruptions to ecosystems happened after a particularly hardy strain escaped from a fish farm in the early 1960s. With a single fish capable of producing more than one million eggs in a breeding cycle, carp now make up an estimated 80–90% of the fish biomass in parts of the Murray–Darling Basin, the breadbasket region of southeast Australia. Carp feed on the river bottom, which stirs up sediment and reduces water clarity, hurting native aquatic plants and animals. The economic impact of the fish is estimated at Aus$500 million a year.

In 2016, the Australian government launched the NCCP to study and design a potential release of CyHV-3. Led by the Fisheries Research and Development Corporation, an Australian authority, the group tested the effects of the virus on animals including 13 native fish species, as well as chickens, mice, frogs and turtles, and concluded that it is safe for them. The virus is only known to affect the common carp and ornamental koi, in which the expected mortality rate is over 70%.

However, the Science letter says that the virus’s effectiveness depends on environmental factors, and that the disease develops only when water temperatures are between 16 ºC and 28 ºC. Populations of carp that survive the initial viral epidemic in hot or cold spots would be able to replenish the population quickly, say the authors.

Ken McColl, a research veterinary pathologist at Australia’﻿s Commonwealth Scientific and Industrial Research Organisation in Geelong, and one of the lead researchers on the carp herpesvirus project, says the NCCP is currently assessing environmental factors through outbreak simulations that take into account regional water conditions and climate. It is also considering a field trial to assess the technique, says Barwick. The NCCP intends to publish its completed risk assessment by the end of 2018. The ultimate decision on whether to go ahead with the plan will be made by Australia’s environment minister; the earliest possible date for the virus to be released would be late 2019, says Barwick.

In the Science letter, Stevenson and his co-authors raised the prospect that the virus could already be present in Australia. If so, releasing it again would have little effect. Barwick says that question will be addressed by a proposed programme to sequence the genomes of wild-caught carp. The programme will also examine whether other viruses already present in Australian carp could react with CyHV-3 and provide immunity to it.