Everyone agrees that introduced carp pose a serious threat to native fauna and flora in numerous countries and regions. Australia is almost certainly the best-known affected area, possibly because, as a result of the authorities’ view on the matter, koi do not appear on the list of live freshwater fish species whose import is permitted. Thus, Australian hobbyists are being denied significant access to this rewarding aquatics sector, while the trade, too, is being denied opportunities enjoyed by their colleagues in other parts of the world.
All this is the result of koi belonging to the same species as the common carp (Cyprinus carpio), whose invasive qualities are beyond doubt. In fact, the situation is so desperate that, as I reported some time back, the Australian authorities have decided to release the herpesvirus 3 pathogen (CyHV-3)—the agent that causes the deadly carp and koi herpesvirus disease—into Australian waters in an attempt to eradicate established carp populations.
Studies carried out over a seven-year period at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Animal Health Laboratory have shown that the virus is pretty effective, taking just over a week to kill its victims. Therefore, the situation seems quite clear: Release the virus and eradicate the carp. What could be simpler?
Well, I’m sure that readers of this column know that few things are as simple and clear-cut as this.
Aside from the daunting challenges presented by millions of rotting carp corpses, with the plethora of logistical and environmental complications that they would give rise to, to say nothing of the high costs involved in the program—estimated to be around $18 million—there are other factors that cast doubt on the reasoning behind the proposed virus release.
One of these relates to the actual efficacy of the virus itself, because not all carp will die, leaving virus-resistant survivors. Indeed, and worryingly—one would assume—for the proponents of the release, genetic polymorphism has been detected in carp. This means that carp vary in their genetic makeup and, thus, their response and sensibility to external factors, which could include pathogens. In a paper titled Genetic Evolution and Diversity of Common Carp, Cyprinus Carpio, published in the Central European Journal of Biology in September 2009, the authors, Dimitry Chistiakov and Natalia Voronova, state that "Knowledge of genetic variation and population structure of existing strains of both farmed and wild common carp Cyprinus carpio L. is absolutely necessary for any efficient fish management and/or conservation program."
To what extent this has, or has not, been taken into account by the Australian authorities is not clear, but the implications are serious, as it casts doubt on the efficacy of the proposed virus release. All you need is for a certain percentage of the carp population to survive and be resistant to the virus for the seeds of a potentially disastrous scenario to be sown.
It is also known that infected carp will seek warm water and that temperatures above 30 degrees Celsius will block infection. So if infected but resistant or immune or genetically polymorphic carp seek out warm water and reproduce, the consequences could be very significant for the proposed program.
In addition, we don’t know yet if koi herpes virus is absent from all Australian waters. If it isn’t, and it’s already present, it could reflect a situation found in Japan in 2004, where a high prevalence of infection existed, but without clear ill effects.
In view of the above, it is not in the least surprising to find that Belgian, English and Australian scientists have recently called on the Australian government to reconsider its approach to the carp problem. According to a letter written by several scientists that was published in Science magazine in February, the virus release program would be ineffective and could pose a threat to ecosystems.
The scientists suggest that, instead, the authorities introduce a limited testing program to assess accurately if the virus can, indeed, control the carp without damaging ecosystems. They maintain that this would be a sensible approach, because, once the full—and costly—release takes place, it would be irreversible. They also advocate an alternative approach, supported by stakeholders, consisting of the release of daughterless koi as a means of long-term control of the carp.
Meanwhile, the Australian koi sector is monitoring the situation, not just with great interest, but also with great apprehension.
John Dawes is an international ornamental aquatic industry consultant. He has written and/or edited more than 50 books and has contributed more than 4,000 articles to hobby, trade and academic publications. He is the editor of the OFI Journal and a consultant to AquaRealm, the new trade show that took place June 2017 in Singapore.