If you're a Carp swimming in the murky waters of Lake Burley Griffin this festive season, should you be worried that you might not live to see another Christmas? Presumably, you may have heard about the oncoming Carpageddon. However, as we wait for the National Carp Control Plan to present its recommendations to the Government, the Carp must be wondering if there is any truth to Carpageddon, or just end up being just another failed doomsday prophecy.

If Carpageddon is the prophecy, then the prophet is Barnaby Joyce, whose response to a Dorothy Dixer from fellow National Party MP Kevin Hogan MP resulted in one of the more colourful speeches seen in Parliament:

We are afflicted with these disgusting, mud-sucking creatures—bottom-dwelling, mud-sucking creatures. The only form of control is a version of herpes; it is the only thing that will get rid of these disgusting, mud-sucking creatures. We will move forward on this because we believe that we should be getting rid of these disgusting, mud-sucking creatures in order to support some of the better animals of our waterways—the silver perch, the yellowbelly, the Murray cod, the Eastern cod and the catfish. We do not want to deal with carp; we have to get rid of the carp. When we have dealt with this virus, we are going to have between 500,000 and two million tonnes of carp.

It would be interesting to unpack this entire statement, but let's focus on three key claims:

  • the CyHV3 virus is the only option to get rid of the Carp
  • we will have 500,000 to 2 million tonnes of dead Carp, and
  • the waterways will better support other animals including native fish.

The last claim assumes 2 million tonnes of Carp suddenly dropping dead won't hurt our native fish.


1. About this article

2. The CyHV3 virus is the only option to get rid of the Carp?

Are there no other ways to reduce the Carp population?

Will the CyHV3 virus kill the Carp?

3. Will we have 500,000 to 2 million tonnes of Carp suddenly dropping dead?

4. The waterways will better support other animals including native fish

Dead Carp and water quality

Cleaning up the dead Carp

Once the Carp are gone, will the native fish come back?

5. Conclusion

1. About this article (return to Contents)

Before proceeding further, this article is based on the scientific papers and issues papers published by the National Carp Control Plan (NCCP).  In writing this article, I downloaded and printed 70 articles, progress reports, issues papers and scientific papers, weighing 2.7 kg. These were accessed from two websites: www.carp.gov.au and yoursay.carp.gov.au. At the time of publishing, the Yoursay website was down. I had planned to include links to the papers I downloaded from that site. 

This is not an independent review of the 70 papers I downloaded and printed, but a summary. However, it is my understanding that the issues papers and scientific papers I relied on for this article were peer reviewed before being published on the NCCP website.

I previously published a Q & A on the CyHV3 virus back in 2016. Much of the content is still current. Therefore, I have not revisited this. For those who are interested, I have also published a brief article on a stakeholder workshop about the National Carp Control Plan I attended on 19 February 2018. 

2. The CyHV3 virus is the only option to get rid of the Carp (return to Contents)

To examine this claim, we need to look at other methods to control Carp, and how effective is the CyHV3 virus is in killing Carp. 

Are there no other ways to reduce the Carp population?

Currently, Carp occupy 80% to 90% of the biomass in many waterways, and consume much of its resources. This leaves the Carp population with little opportunity to grow further.

Commercial harvesting or events like the Canberra Carp Out only reduces the population enough to free up resources that the harvested Carp otherwise would have consumed. After one or two spawning seasons, the population rebounds rapidly back to previous levels.

Carp out 2011 147

Events like the Canberra Carp-Out are great for raising community awareness about Carp, and for raising money for fish stocking. Unfortunately, they don't put much of a dent in the local Carp population. 

To prevent this, the population needs to be drastically reduced to the point where there are too few adult Carp to ensure consistently successful spawning.

Genetic biocontrol techniques such as the 'daughterless Carp' gene, show great promise in drastically reducing the Carp population to such levels. But this may take up to 50 years given Carp live up to 30 years and produce up to 3 million eggs each year.

On the other hand, the CyHV3 virus shows promise in achieving drastic reductions in the Carp population over a shorter time frame.

Will the CyHV3 virus kill the Carp?

The prediction of 'Carpageddon' was based on outbreaks of the CyHV3 virus overseas, resulting in the death of 70% - 100% of Carp populations in affected waterways.

Accordingly, it was hoped that releasing the CyHV3 virus in Australia would lead to self propagating outbreaks seeing millions of Carp fall like dominoes.

However, further research on the virus has since found that this is unlikely under Australian conditions. This is because the water temperatures required to trigger an outbreak will be limited to Spring and early Summer.

The CyHV3 virus is most effective in killing Carp within a temperature range of 22°C to 26°C. Some studies cite a more narrower temperature range of 22°C to 24°C.

Most carp exposed to the virus below 16°C or above 28°C will survive and become resistant to the CyHV3 virus. Above 30°C, the virus can no longer replicate and becomes harmless to Carp. Infected Carp will seek out warmer waters in an attempt to deactivate the virus.

In addition to water temperature, Carp need to be schooling in close physical proximity to allow transmission of the virus from infected Carp to uninfected Carp. The virus is spread through physical contact with infected Carp or shed mucous or other secretions from infected Carp.

Such schooling occurs when Carp migrate for spawning during Spring and early summer, which happens to also be when water temperatures are ideal for triggering an outbreak. In addition, the Carp's immune system tends to be weakest after winter and during spawning.

Given both the regional and seasonal variability of water temperatures across Australia, self-propagating outbreaks of the virus are unlikely, and outbreaks will be geographically limited.

3. Will we have 500,000 to 2 million tonnes of Carp suddenly dropping dead? (return to Contents)

The short answer is no.

The release of the CyHV3 virus will need to be carefully timed to coincide with ideal water temperatures and Carp spawning for each waterway. This means the virus will need to be strategically deployed in a series of surgical strikes across different waterways, possibly over a five to ten year period.

In addition, the latest biomass estimate of Carp in our waterways is a lot less than originally thought. The Arthur Rylah Institute in Victoria has calculated the Carp biomass as at May 2018 to be 205,744 tonnes with a lower limit of 117,532 tonnes, and an upper limit of 356,482 tonnes. During successive years of flooding, it estimates that the Carp population will peak at 1.2 million tonnes.

The Arthur Rylah Institute worked with fisheries research agencies in five States and Territories to calculate the biomass using fish survey data from over 150 studies and 4831 sites over 24 years, and using the levels of Carp DNA detected in the water (Environmental DNA or eDNA). Techniques for estimating biomass using eDNA were refined by comparing the estimates with actual Carp left in several waterways after they were drained.

While the estimated total Carp biomass is far less than originally expected, Carp densities in our waterways is still more than enough to cause environmental impacts on our waterways. Previous research indicates that this occurs at Carp densities of 80 kg to 100 kg per hectare.

The Carp biomass study also calculated and mapped Carp densities in the various river systems. Carp densities of 200 – 400 kg per hectare were recorded in Australia's southern river systems. The highest densities were recorded in lower system wetlands in the Murray, Murrumbidgee and Lachlan catchments.

As a result, Carp density mapping has identified hotspots in waterways where the Carp biomass is high enough to cause environmental damage, where Carp are likely to school or aggregate, and where clean up activities should be focused.

4. The waterways will better support other animals including native fish (return to Contents)

 Dead Carp and water quality

Concerns about the impact to water quality have been front and centre on the minds of most anglers, even those who support the release of the CyHV3 virus.

The NCCP acknowledged from day one the need to address those concerns. The NCCP has researched both the water quality risks and strategies to manage the disposal of dead Carp.

Research has identified the following water quality issues that may occur caused by dead Carp after an outbreak of the CyHV3 virus:

  •  hypoxia (low dissolved oxygen levels) or anoxia (total absence of dissolved oxygen) due to decomposition of dead Carp
  •  increased nutrients (such as phosphorus, Nitrogen, and Ammonia) being liberated from decomposing Carp
  •  increased risk of outbreaks of blue-green algae due to increased nutrient levels
  •  water treatment for human consumption becoming ineffective, and
  •  risk of secondary diseases such as botulism outbreaks

Blue green algae

Water quality following the release of the CyHV3 virus remains front and centre of the concerns held by many anglers.

Research by the University of Western Australia and University of Adelaide has examined how much dead Carp is required to cause water quality issues under different scenarios and different types of waterways. The research concluded that the expected numbers of decomposing Carp do not pose unmanageable risks to dissolved oxygen, nutrient levels and glue-green algae blooms.

At the risk of oversimplifying the complexities involved, it appears low dissolved oxygen levels, high nutrient levels and potential for blue-green algae blooms will present themselves where Carp densities exceed 300 kg per hectare. However, the extent to which those risks presents themselves in a particular waterway is influenced by the quantity of dead carp and the ability of the waterway to flush out the dead carp. Other local factors will play a role, such as wind, wave action and water temperature.

For example, shallow wetlands with no direct connection to water flow in the main river channel would face a higher risk of low dissolved oxygen levels, elevated nutrient levels, and blue-green algae blooms. River flows flushing out dead Carp in one section may result in dead Carp to accumulate at another section further downstream.

While oxygen levels can return to normal levels, and blue-green algae blooms can dissipate, nutrient levels are more likely to remain in the environment. These 'legacy' nutrients could potentially fuel future blue-green algae blooms.

The researchers also looked at how much decomposing dead Carp need to be in the water before it affected the capacity of water treatment plants to ensure drinking water remained within Australian Drinking Water Guidelines.

Water treatment can occur with minimal or no changes at Carp densities up 400 kg per hectare. Additional treatment such as the use of activated carbon will be needed for Carp densities up to 2000 kg per hectare. Water treatment is unviable at 4000 kg per hectare.

In the event that a massive accumulation of Carp carcasses occurs, an outbreak of botulism may occur due to serious oxygen depletion in the water and large volumes of nutrients to feed the Clostridium botulinum bacteria. The only way to mitigate this risk is to remove the dead Carp to prevent serious depletion of oxygen and reduce the amount nutrients to feed the bacteria.

Cleaning up the dead Carp

The NCCP refers to this as 'carcass management', because a full scale clean up is not on the cards. The aim of 'carcass management' is to ensure dead Carp do not pose risks to water quality, ecologically sensitive areas, social amenity (such as tourism and recreation), and infrastructure.

As noted earlier, studies indicated that the risk of low dissolved oxygen, nutrients from dead Carp and blue-green algae outbreaks occurs at Carp densities of 300 kg per hectare or higher. Accordingly, carcass management will focus on ensuring Carp biomass remains below 150 kg per hectare.

The NCCP has identified a range of carcass management strategies. I won't go into the mechanics of each strategy. But these are grouped into four broad categories:

  • herding live uninfected Carp through manipulation of water flows or water levels prior to the release of the CyHV3 virus so that they school in low risk locations
  • herding infected live Carp away from sections of waterway more prone to water quality impacts and into lower risk locations
  • regulating the flow of water to transport dead Carp and nutrients away from sensitive areas to lower risk locations or locations where carcasses can be intercepted and removed, and
  • physical removal of the carcasses from waterways.

A complete clean-up is only planned if there are risks to social amenity or to ecologically sensitive waterways. Otherwise, the aim of the clean-up will be to reduce the Carp biomass to less than 150 kg per hectare.

Once the Carp are gone, will the native fish come back?

Like Australia, the United States is also afflicted by European Carp. However, a collapse of the Carp population in the Mississippi River System has seen an explosion in the Bluegill population. Could we see a similar resurgence of our native fish population after the release of the CyHV3 virus?

To find out, researchers from the University of Canberra consulted a wide range of experts from different scientific disciplines and conducted a review of the scientific literature to examine the medium to long term (5-10 years) ecological responses to a 'do nothing' scenario, 25%, 70% and 100% reduction in the Carp population.

All experts confidently predicted that ecosystems would continue to degrade under a 'do nothing' scenario. A 70% reduction would be needed before any significant improvements could be realised. They assessed the likelihood of medium term recovery to be quite low, but much greater in the longer term.

They predicted a medium likelihood of long term recovery of water plants, invertebrates and water quality, and a low likelihood of recovery in native fish. Water birds that feed on fish were expected to decline in the medium term, and their recovery in the long term depended on how well the native fish population recovered.

The experts noted that Carp are just part of the wider environmental problems affecting our waterways. Other environmental problems include land use, changes to the flow of our river systems, other introduced species (such as Redfin Perch, Weather Loach, cattle, and horses), and climate change.

Accordingly, Carp reductions need to be accompanied by other actions to facilitate ecological recovery, including habitat restoration and fish stocking activities. Otherwise, there is a risk that Carp reductions leads to an altered ecosystem. For example, the biomass vacancy left by the Carp may get filled by Redfin Perch instead of Native Fish.

5. Conclusion (return to Contents)

The National Carp Control Plan is about to make recommendations to the Australian Government about ways to control the Carp population, including possible use of the CyHV3 virus.

It was initially thought that releasing the virus would result in a self-propagating outbreak of Carp deaths. However, we now know that is an unlikely scenario.

The CyHV3 virus will only result in major fish kills under a narrow set of circumstances: during Spring and early Summer, when water temperatures are within 22°C-24°C and Carp are schooling to spawn.

The large numbers of dead and decomposing Carp following a release of the CyHV3 virus does pose risks to water quality. However, those risks are manageable, and complete removal of Carp won't be required for all waterways. In most cases, a clean-up will aim to reduce the amount of dead Carp to 150 kg per hectare.

There needs to be a 70% reduction in the Carp population before any medium to long term positive environmental impacts can occur.

However, there is a risk that reduction of carp alone will not result in a reversal of the environmental degradation experienced in our waterways but the creation of an altered aquatic ecosystem (for example, Redfin filling up the vacant biomass left by Carp).

Accordingly, release of the virus must be accompanied by other actions such as habitat restoration and the restocking of Native Fish.

Share this article

Submit to FacebookSubmit to Google PlusSubmit to TwitterSubmit to LinkedIn