Freshwater, Native, Stuff we've learnt

Carp herpes virus: risks and rewards

The virus will work in the medium term, but in the long term other control strategies will need to be developed. As fun as it is, fly fishing probably isn't one of them
The virus will work in the medium term, but in the long term other control strategies will need to be developed. As fun as it is, fly fishing probably isn’t one of them

The fishing media is currently awash with articles about ‘carpageddon‘ and wild claims from all sides. In this post, we want to answer a few of the questions that have been raised about the release of the virus and the potential habitat and ecosystems benefits that may occur, but also some of the risks of the release.

What is it? 

Apart from being something for people to joke about, carp herpesvirus (CyHV-3) is a carp bio-control agent that is scheduled to be released into Australian rivers. The virus first originated in Israel in 1998 and has since spread rapidly in many regions of the world including Japan, North America, Europe and Indonesia. When it first spreads within populations, mortality is between 70-100%, in all age groups of carp. For a number of years now the CSIRO has been testing the viruses suitability for release, which has included testing the susceptibility of native fish and other vertebrates to the virus and evaluating the potential risks of release. No native species have been found to be susceptible to the virus and the release of the virus is be highly likely to be safe for everything except the carp, hence its approval for release.

Does this mean carp are controlled forevermore? 

No. Much like myxomatosis and rabbit caliciviruses, over time carp will evolve resistance to the virus (as well as the virus evolving to be less virulent/deadly). Over time carp populations will begin to bounce back (although like rabbits, probably not to the plague proportions they were before the release of the viruses) and other control measures will be needed. These might be new strains of the virus or genetic technologies such as daughterless carp. Follow up strategies and funding for those strategies will be required if large long term environmental gains are going to be realised from the release of carp herpesvirus in Australia.

What are the benefits?

Firstly, carp make up a very large proportion of the biomass in the Murray Darling basin, roughly 80%, but as high as 90% in some areas. Reduction in carp populations is likely to lead to less competition between native fish and carp, as well as freeing up large amounts of resources (aquatic invertebrates, zooplankton etc) that can be utilised by native species. This isn’t going to be the only positive though. Carp are well known to be ecosystem engineers. That is, they not only exist within habitats, their presence modifies them. Their benthic feeding habits re-suspend sediments increasing turbidity as well as uprooting aquatic plants. Increased turbidity then leads to less aquatic plant growth. This is a problem because aquatic plants provide vital habitat for fish, crustaceans and all manner of invertebrates and play a major role in nitrogen uptake. In all this leads to a reduction in habitat (aquatic plants), reduced nitrogen uptake and trophic transfer and ultimately reduced diversity and abundance of macroinvertebrate communities. Similar results have been seen in a range of ecosystems, including Australian ecosystems (Vilizzi et al 2014). In short, reducing carp populations will not only reduce competition it will improve habitat. It will mean less turbid water, improved nitrogen cycles, more aquatic plants, more abundant and diverse macroinvertebrate communities and in general better habitat. This should mean more fish, more birds and better ecosystem function overall. So while its true that carp are by no means the only process which has degraded habitat in the Murray Darling basin, and their reduction wont solve all the problems within the MDB, their reduction will play an important role in improving habitat within the basin.

Importantly, any release of carp herpesvirus should ideally be complimented by ongoing habitat restoration efforts, including stock exclusion, riparian revegetation, resnagging and control and/or removal of pests and weeds.

What will Murray cod, Golden perch and water birds eat when there are no carp?

The argument that carp are an important food source for predatory species including popular angling targets such as Murray cod and Golden perch and charismatic fauna such as water birds has been made by a lot of people since the news of the carp herpes virus release. The insinuation is that without carp, these species will starve for lack of food. While there haven’t been a huge number of studies done on on the effect of removing carp on food webs and small native forage fish, the work that has been done suggest that the outlook is far from bleak. Work done as part of the Native fish strategy, funded by the FRDC has shown that once carp are removed, small native fish populations quickly bounce back and flourish. In the study, four lagoons were chosen, in two carp were controlled (removing 43% and 33% of carp biomass, 34 and 26 kg per hectare respectively), in the other two lagoons nothing was done. In the two lagoons where carp were controlled, native fish biomass increased by 90 kg per hectare, roughly three times the biomass of carp removed. Added to this, large zooplankton populations (e.g. Boekella and Daphnia) increased 10 times and populations of aquatic insects and crustaceans also boomed. In the two lagoons where nothing was done, populations of native fish, zooplankton, aquatic insects or crustaceans did not change. What this work suggests is that native fish a much more efficient in their use of food resources than carp (producing three times the biomass) and that removing carp will likely increase the food available for predatory fish and waterbirds, not decrease it. This should ultimately lead to bigger, healthier populations of popular native angling species and waterbirds.

Will it drastically change turbidity in the Murray darling basin?

At the top we mentioned that carp are ecosystem engineers and that one of the things they do is increase turbidity. While this is indeed true, carp aren’t the ONLY thing that causes the turbidity in the MDB. So, will removing carp lead to crystal clear rivers as has been suggested in some articles about the carp herpes virus? The likely answer is no. It will lead to some improvements in turbidity, both through less fish re-suspending bottom sediments and increases in aquatic plant life (which may act as sediment traps), but its unlikely to lead to clear rivers. For example, in the research discussed above, while removal of carp led to huge increases in native fish, it was not associated with changes in turbidity. Instead, turbidity changes were mainly associated with water level changes, local run off and surrounding land use practices.

To be clear, carp do can have large effects on turbidity. Vilizzi et al (2014), showed that transparency was strongly negatively correlated with carp biomass. More carp equals more turbidity and muddier waters. So carp do have an effect and their removal/reduction in populations will lead to improvements. It’s just that land use, riparian vegetation and other factors also lead to turbidity, so the removal of carp isn’t likely to completely fix turbidity in the MDB. It will improve it but if we want to have a “clear” Murray Darling basin a lot more will need to be done repairing riparian vegetation and changing land use practices.

What about all the dead fish?

A major concern, whether real or manufactured, is the problem of what we are going to do with all of the dead fish. Anyone who has been downwind of a dead carp knows that it’s not pleasant, and much of the discussion about the release of the virus has been around the clean-up. While this might be important from an aesthetic and amenity perspective and clean ups around population centres obviously make sense, the money allocated to cleaning up dead fish in other more remote parts of the basin could perhaps be spent more effectively in tackling some of the other issues related to the current degradation of the MDB more broadly.

Nature is incredibly good at cleaning up dead things, so it’s likely that within a few weeks or months, most of the dead carp will be gone. This, however, may have interesting ecosystem ‘shocks’, with potential risks of fish kills caused by all the rotting fish. On the flip side, it would provide an abundance of food for invertebrates like yabbies, but also a plentiful food source for undesirable pests like feral pigs and foxes.

What are some of the other risks?

It’s the ‘unknowns’ that are the main risks. We can guess but will never really know what the ecosystem shocks will lead to until they occur, but even degraded ecosystems have an impressive ability to absorb one off shocks in the short to medium term and will generally find new a equilibrium. It seems clear that any new equilibrium will be ‘better’ in the absence of carp, but there are concerns that some other undesirable fish species (like redfin or tilapia) may end up filling the ecological niches that carp currently occupy.

Will it fix the Murray Darling Basin?

The short answer is no. Its a great step, but as mentioned above, there are many other factors that have contributed to and continue to contribute to the degradation of the basin. To reiterate, the release of carp herpesvirus should ideally be done in conjunction with efforts to revegetate degraded riparian zones, resnag rivers, remove fish movement and reproductive barriers (including cold water pollution), exclude livestock from rivers and control other feral pests and weeds. This is likely to take a lot longer and require far more resources than have currently been allocated. In saying this, the main risks to the success of releasing carp herpesvirus virus will probably be politics, funding and inadequate planning.




Note: If you want to know about the history of carp in Australia we wrote a pretty in depth post on it a while ago here. Matt Barwick from the NSW DPI talks about the virus and its effects in detail in this series of videos which are well worth a watch.



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