As a fish ecologist who lives on the northern shoreline of Lake Cargelligo in western New South Wales, I’ve been fortunate to have had a front-row seat to the ever-changing drama of all aspects of the Murray-Darling Basin over the last 30 years.
It’s never been more dramatic than this past spring and summer.
As the water rose in September and October – during what people like me call the “rising limb” of the flood – anglers flocked to our remote corner of the Lachlan River catchment and successfully pelted lures at the big native fish – Murray cod and yellowbelly – as they mobilised and bred following the ancient cues buried in their DNA.
These fish have behaved like this for millennia: when the slow-flowing western rivers gradually become the riverine equivalent of freeways the fish take full advantage in order to ensure their progeny have the best chance of survival.
But it all changed as the flood peaked and then began to subside, and the weather shifted from a wet spring to a more typical hot summer.
The flood – though obviously beneficial to all the native animals and plants that have evolved west of the Great Dividing Range – also provided ideal conditions for a massive and prolonged breeding event for one of our most ubiquitous feral species: carp.
Originally introduced in the 1800s, carp spread throughout the basin during the big floods of the 1950s and 1970s. Like all successful invasive species, carp are tough as nails, capable of living in harsh conditions and able to feed on pretty-much anything. The wetlands that are sustained by our big inland rivers suit them perfectly.
We know that carp cause damage to the banks and vegetation due to their feeding behaviour, and we’re fairly certain they have a devastating impact on native fish – if not through direct predation then through increased competition for space and resources.
In early 2023 one thing is for sure – there’s definitely not much space left for native fish. Carp of all sizes have swamped almost every fish sample I’ve taken since Christmas. Videos of swarming hordes of carp on social media also bear witness to the overwhelming abundance of the species across the inland rivers.
So what is being done, and what can be done about such an obvious and serious environmental problem?
The simple answer is, unfortunately, not much.
River managers can’t really release a flow of water for native fish that won’t also benefit carp. And fishing them out is futile except in a few select (and small) areas.
In the early 2000s a technique based on removing carp at barriers such as weirs using separation cages was fashionable, but it hasn’t really caught on in a big way.
Then came the hi-tech options – first the idea of breeding them to extinction using “daughterless” genetic manipulation, and then the Barnaby Joyce-endorsed herpes virus.
Of the two, the virus was the method that made the news and attracted millions of dollars in government cash. But although a fair amount of research was conducted and a final report handed to the government, everything’s been noticeably quiet on the herpes virus front for the last couple of years. The fact that a human virus came along and frightened everyone probably didn’t help.
So nothing’s really being done about carp, despite good intentions. And no matter what is done, I would bet a large amount that long after I’m pushing up weeds in the Lake Cargelligo cemetery, carp will still be sucking and slurping around the shoreline.
Following the changes in the ecosystem will be interesting in the wake of the great flood of late 2022. There is a chance that the carp explosion will persist, especially if 2023 stays wet enough to maintain water throughout the more marginal areas of the landscape.
Alternatively, if the Murray-Darling Basin enters a dry phase or a drought, the vast majority of carp may disappear as quickly as they materialised.
None of this alters the persistent problem, because we now have enough evidence to know that carp will multiply in their billions every time suitable conditions exist.
From my box seat I’ve watched all the changes: from freezing winters to 47C in January three years ago, from the big lake going completely dry to the recent flood. But I’ve never seen carp numbers as out-of-control as this summer.
And it’s the same in the Murray, the Darling, the Murrumbidgee, the Macquarie – everywhere.
There’s no magic bullet and no easy solution, but it’s time to start getting serious about dealing with this problem.
