What about carp?
Our position on the release of the carp virus and managing invasive European carp
We strongly support efforts to reduce populations of the invasive European carp (Cyprinus carpio) and encourage the recovery of native fish species.
European carp is an invasive freshwater species that was brought to Australia in the 1800s. Their numbers were sparse until the 1960s, when river engineering transformed Australian waterways to suit irrigation, creating the perfect conditions for carp to thrive. Extensive networks of weirs and dams eliminated the highly variable flows of rivers, and turned seasonal rhythms upside down. The flows and flow-dependent habitat that native fish need were undermined – while engineering created the slow, warm pools that suit carp.
The explosive growth of carp has been catastrophic for waterways. Their feeding style and ability to rapidly increase in population contribute to significant environmental damage as they out-compete native fish and stir up sediment from the riverbed causing murky water (turbidity), damaging aquatic plants and degrading habitat.
The release of the Cyprinid herpesvirus 3 (‘carp virus’) is being considered as a biocontrol agent in Australia. The virus is present in overseas carp populations and causes a fatal disease.
Our position on the management of European carp
- The carp virus alone will not solve the carp problem
It is important to recognise that the carp virus alone will not solve the carp problem. The effectiveness of biocontrol measures diminishes over time as the virus loses virulence and/or the target species develops resistance to it. If the carp virus was introduced, its success would depend on other measures being implemented at the same time. This includes:
- Restoring a more natural flow regime that supports native fish movement, habitat and bank stability
- Dismantling outdated river engineering infrastructure
- Restocking native fish populations
- Restoring wetting and drying phases for wetlands to support flow-dependent habitat and suppress carp populations
- Ensuring environmental releases are delivered in a way that provide an advantage to native fish over carp
- Implementing additional carp control measures such as electrofishing and netting
- Investing in comprehensive monitoring and clean-up.
Critically, these measures need years of secure funding to ensure carp are removed at a rate higher than their reproduction rate, enabling gradual decline of this widespread species.
In a highly variable climate, long droughts will be interrupted by intense, unpredictable floods. Remembering that the floods of the mid-1970s contributed significantly to carp spreading throughout the Basin, carp control must be understood as a long-haul initiative that will require ongoing investment and flexibility to respond to natural events.
If the carp virus is ultimately approved by evidence-based risk analysis, we need to remember that the virus can only be introduced, for the first time, once. It is our job to reduce carp populations – and make flows less favourable to carp – before release of the virus, and to maintain an ongoing program of on-ground efforts to support the recovery of native fish species after the virus is released.
- Restoring a more natural flow regime will be critical for success
The Murray-Darling river system is predominantly managed to meet irrigation needs. The warm weir pools and constant low flows that this creates is enabling carp to flourish while reducing and degrading suitable habitat for native fish. Restoring a more natural flow regime with more variable flows will be vital for reducing carp numbers and supporting the recovery of native fish populations. Effectively reducing carp numbers will not be possible unless we alter the flow conditions that are enabling this species to dominate.
- More research is needed
It is clear that more research is needed before a decision is made on releasing the virus. For example, it is unclear whether the virus could impact other species and if herd immunity might emerge. It will also be important to better understand transmission rates: will the virus spread more quickly than anticipated, leading to more distributed die-offs, requiring broad monitoring and rapid cleanup to prevent water quality impacts?
Summary
Effective carp control will only be possible through a holistic approach of restoring a more natural flow regime; relaxing constraints to enable overbank flows; removing barriers to native fish passage and controlling carp populations through a virus and other direct controls.
The Murray cod, listed as vulnerable under national law, is one of the species that are negatively impacted by the abundance of invasive European carp.