Reclaiming our rivers from the stronghold of feral fish

Nicknamed ‘river rabbits’ due to their ability to multiply and reach huge numbers, carp are one of the most invasive and damaging pests of our freshwater ecosystems. Just as biocontrol agents have been successfully used to control rabbits in Australia, we’re confident that a virus that has been killing carp overseas could do a similar job in our waterways.

Carp herpes virus

The virus, once known as koi herpesvirus, is now formally known as Cyprinid herpesvirus 3 (CyHV-3). Seven years of CSIRO research, supported by the Invasive Animals-Cooperative Research Centre (CRC), has shown that the use of CyHV-3 as a biocontrol agent could significantly reduce the number of common carp (Cyprinus carpio) in our rivers.

Naturally with any talk of a biocontrol agent, there is public debate and speculation. In particular, four key questions constantly arise.

1. Is it really necessary to control carp in Australia?

First introduced in Australia in 1859, carp became a major pest in the 1960s after the accidental release of a strain that had been adapted for fish farming. Within a few years they established themselves throughout the entire Murray-Darling Basin.

Carp now comprise up to 90% of the fish biomass in parts of the Basin. This is largely attributed to female carp producing up to a million eggs per year, and to the omnivorous fish’s tolerance for a wide range of habitats including degraded water. While we may not be able to ‘prove’ that carp directly caused the degradation of our rivers, their dominance must certainly contribute to the problem. It is unlikely that the Murray-Darling Basin could ever return to its previous glory while carp remain in such high numbers.

Our views in Australia are supported by research from the US. This showed that carp muddy their waters resulting in flow-on effects on plants, invertebrates, bird-life and native fish in shallow lakes. Researchers concluded that common carp damage the ecology of shallow lakes, particularly when carp density reaches levels similar to those in parts of the Murray-Darling Basin.

2. Will CyHV-3 be effective as a biocontrol agent?

CyHV-3 first appeared in Israel in 1998 and quickly spread throughout the world, killing-off common and koi carp. Ironically carp are farmed in many countries and are an important food source. So, while CyHV-3 has devastated carp farming, the overseas experience has demonstrated how it could be used successfully as a biocontrol agent here.

Testing of CyHV-3 in the high-security Fish Diseases Laboratory at CSIRO’s Australian Animal Health Laboratory (AAHL), in Geelong, Victoria, has proven that the same virus does in fact kill Australian carp, and it kills them fast.

The flip side is our rigorous testing to ensure that the virus won’t affect native Australian or important introduced species of fish. It has been shown to pose no danger to 13 native species such as Murray cod, various species of perch, eel and catfish, as well as a crustacean (yabbies) and a non-native fish species, the rainbow trout. Our work has shown that there are no clinical or pathological changes in these non-target animals, nor is there any evidence that the virus multiplies in these species.

Chickens, mice, frogs, turtles and water dragons have also been tested as representatives of a wider community of birds, mammals, amphibians and reptiles. Again the virus has shown no effect on them which also makes us confident that it won’t affect that other major group of mammals – humans.

Based on lessons learnt from past use of viral biocontrol agents for invasive vertebrates, we expect that CyHV-3 will have the greatest impact in the first couple of years after release. After that, its effectiveness may be diminished, but not lost, as virus and host adapt to each other.

Therefore, we need an integrated pest management program that utilizes other methods to complement our virus. These include new broad-scale technologies such as ‘daughterless’ technology to create male-only populations, as well as traditional regional methods such as trapping, the commercial collection of carp, and controlling access of carp to breeding grounds.

3. What happens to the dead carp?

courtesy Nigel Harriss
Juvenile carp aggregated below the Menindee Main Weir on the Darling River. Image: Nigel Harriss, NSW Office of Water.

If an image of rivers full of large dead carp floating on the surface is what springs to mind, rest assured that our research includes careful planning and modelling before release and follow-up strategies are recommended.

Carp breed in well-mapped specific sites along the Murray-Darling Basin. The virus is likely to be released in these sites where most carp are juveniles. Not only would this wipe out large populations of carp before they become mature, but bird life will probably clean up large numbers of the immature carp.

Study tours of Japan and Indonesia are part of the Invasive Animals-CRC program to study natural outbreaks of CyHV-3. Researchers will be reviewing the significance of dead mature fish, and strategies for dealing with them.

4. How can we be sure that widespread distribution of the virus is safe for people?

CyHV-3 has devastated carp farming around the world yet despite the large numbers of people working on these affected farms, there has been no evidence of any effect of the virus on them.

We have also exposed mice to CyHV-3, and found no evidence of disease. Mice were chosen as being a representative mammal, just like a human.

And finally, a report to the European Commission by the Scientific Committee on Animal Health and Animal Welfare stated that there is no evidence for ANY fish virus causing disease in humans.

The verdict

Given our very good understanding of both the biology of CyHV-3 and of carp in Australia, we are optimistic that this carp virus will make a significant impact on carp in this country. And for that, our river systems and native fish will be very grateful.

About the Author


Dr Ken McColl, Senior Research Veterinarian at CSIRO’s Australian Animal Health Laboratory, Geelong. Ken is a veterinary virologist and pathologist specialising in diseases of aquatic animals.


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