While authorities around the world are trying to keep the deadly virus KHV out of their waters, scientists in Australia have started a study to investigate whether they should introduce it.
Researchers at Australia's Commonwealth Scientific and Industrial Research Organisation Australian Animal Health Laboratory (CSIRO AAHL) are investigating the impact of introducing Koi Herpes Virus - or Cyprinid Herpes Virus-3 (CyHV3) as it is formally known - into the wild to help control the rapidly growing populations of introduced carp.
Carp were first introduced into Australia for sport fishing in the 1850s. They caused few problems until one particular form - an aquacultured carp known as the Boolara strain - was introduced in the 1960s. Although the fish were kept on farms, severe floods in the 1960s and 70s led to the strain escaping to Australian waterways, where it has been a problem ever since.
River rabbitsDr Ken McColl of CSIRO-AAHL told Practical Fishkeeping: "There have been few, if any, systematic surveys of the distribution of carp in Australia, but it is known that they occur throughout the Murray-Darling system, and all of south-eastern Australia. They have also been found in Tasmania and Western Australia. In these areas, they can be found in virtually all aquatic environments (even slightly saline waters) except free-flowing rivers and mountain streams.
"The effect of carp on the environment has caused some debate. They are known to cause increased water turbidity by their mode of feeding (sucking mud from the bottom, and then spitting it out as they sieve-out the food); they are associated with a reduction in the amount of submerged vegetation (directly, by uprooting plants, and, indirectly, by increased turbidity resulting in less light for plant growth); and they apparently are associated with increased algal blooms.
"Whether they can be blamed for a decrease in the number of various native fish is, however, debatable. Some experts feel that carp are both a cause, and an effect, of the degraded nature of many of our inland waters. It appears that many of our native species were in decline before carp became a problem, possibly due to the poor state of many of our waterways. Carp may then exacerbate this decline in numbers of native fish (by hastening the degradation of the rivers), while, at the same time, they themselves thrive in the degraded waters."
WipeoutMcColl and project leader Dr Mark Crane are investigating whether they can introduce the virus into the wild carp populations in an attempt to kill-off the alien carp. KHV has a very limited host range, and it is currently only known to kill carp. Not only that, but Australia also has no native members of the carp family, Cyprinidae, making the likelihood of the virus jumping to a new host much less likely.
Crane said: "If the laboratory studies show promise, the next step will be extensive government, public and industry consultation to determine the best course of action to control carp, while protecting and restoring Australia's valuable waterways. It is anticipated that if these technologies are proven to be effective and safe, they will be applied on-ground in an integrated pest fish control program for the Murray-Darling Basin."
The concerns are that infected carp which survive a KHV infection can remain carriers, but survive further infection, even when water temperatures are high.
CarriersMcColl told Practical Fishkeeping: "The issue of carriers is an interesting one, and one where we lack a great deal of information. In experimental and natural infections of carp with KHV, there appears to be a very high mortality rate (70-100%). However, there has been little work done on fish that survive infection.
"In order to protect their carp industry, one of the strategies that the Israelis use is to infect their young carp with KHV at about 23C, and then, after a few days, hold the infected fish at 30C for about a month. After this, the fish are returned to 23C. This strategy apparently results in a marked reduction in mortality (presumably the fish can survive at the higher temperature, but the virus does not do well at that temp), and the fish effectively become vaccinated.
"Whether the "immunity" of these "vaccinated" fish subsequently wanes, and they again become susceptible is not clear. Whether there are any implications for the potential use of KHV as a biological control agent is also unclear. We expect that we will be in a better position to address these issues at the end of the current project at AAHL."
"Based on overseas work, it is currently felt that KHV is specific for common and koi carp. The susceptibility of about a dozen other species of fish have been tested, but there is no evidence of the virus causing disease in these species. We aim to test the susceptibility of a number of Australian native species in the work to be conducted at AAHL, but that will not be until much later. It is worth noting that there are no native cyprinids in Australia (cyprinids being the Family of fish that includes carp)."
Other methods also being investigated to control the carp populations include the production of carp-specific biocidal chemicals, pheromones and sensory attractments and the production of "daughterless" fish.