Concerned communities are wondering whether it is, at last, time for Australia to unleash the carp herpes virus to control populations — but the conversation among scientists, conservationists, communities and government bodies is only just beginning.
Globally, the carp virus has been detected in more than 30 countries but never in Australia. There are valid concerns to any future Australian release, including cleaning up dead carp and potential significant reductions of water quality and native fish.
Let’s weigh the benefits of releasing the virus against the risks.
A house of horrors for rivers
Carp were first introduced to Australia in the 1800s, but it was only with ‘the Boolarra strain’ that populations exploded in the Murray-Darling Basin in the early 1970s.
Assisted by flooding in the 1970s, carp have since invaded 92 per cent of all rivers and wetlands in their present geographic range. There have been estimates of up to 357 million fish during flood conditions. This year, this estimate may even be exceeded.
Carp are super-abundant right now because floods give them access to floodplain habitats. There, each large female can spawn millions of eggs and young have high survival rates.
While numbers will decline as the floods subside, the number of juveniles presently entering back into rivers will be stupendous and may last years.
The impacts of carp are like a house of horrors for our rivers — they cause massive degradation of aquatic plants, riverbanks and riverbeds when they feed; and they alter the habitat critical for small native fish, such as southern pygmy perch.
Most strikingly, this feeding behaviour contributes to turbid rivers, reducing sunlight penetration and productivity for native plants, fish and broader aquatic communities.
The carp herpes virus
The carp virus (Cyprinid herpesvirus 3) represents one of the only landscape-scale carp control options, although there are some exciting genetic modification technologies also emerging.
Mathematical modelling suggests the carp virus could cause a 40 to 60 per cent knockdown for at least 10 years, which may help tip the balance in favour of native fish.
The risks and benefits of a potential Australian release of a carp virus are transparently addressed under the Federal Government’s National Carp Control Plan, released last year.
This plan provides some sorely needed leadership in the carp management space.
Risks the plan identifies include:
- major logistic challenges in cleaning up dead carp;
- potentially serious short-term deterioration in water quality; and
- potential native fish deaths due to poor water quality.
As carp continue to destroy Australia’s riverine heritage, it’s time to lay our cards on the table and have a serious conversation about the carp virus.
How else can we manage carp?
If we truly want to reduce carp numbers and impacts in the long-term then we need to examine all the roles humans play supporting them.
For example, the series of weir pools in the lower Murray create perfect conditions for carp because they give fish access to floodplains year-round.
Strategically lowering and removing weir pools to re-create flowing water habitats would be one solution to help Murray cod and other flowing water specialists, such as silver perch, river snails and Murray crays.
Also, floodplain structures (which create artificial ‘floods’) generate static, warm-bathtub conditions that carp, being from Central Asia, prefer — contributing to huge numbers especially in dry years. Few medium or large native fish benefit from these conditions.
Now the floods have returned, we need to move away from local decisions at the site-scale and instead manage ecosystems across the entire Murray-Darling Basin.
The present flooding also reminds us of the huge potential increases in the numbers of golden perch, frogs, yabbies and waterbirds. Animals that eat carp (Murray cod, golden perch, pelicans and cormorants) should all be as fat as can be.
Looking beyond carp
The huge numbers of carp is a big wake-up call on the poor state of rivers in the Murray-Darling Basin and how we’re managing them.
Perhaps what has been missing from the whole conversation is a vision for what our rivers should look like in 10 or 20 years’ time.
River health is an issue all Australians, country and city, need to engage with.
If we don’t identify a common purpose, then we will likely continue to remain in lock-step with the great armies of carp and rivers of fish kills for generations to come.
This is an edited version of an article that first appeared on The Conversation.
