Busting The River Rabbit Myth

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Busting the River Rabbit myth

(This is a transcript of an interview on the ABC AM Program this morning.)

TONY EASTLEY: It's not just the lack of water that's a problem for the Murray-Darling Basin.

The Murray River has been listed in the world's top 10 most endangered waterways because of invasive pests such as the European carp and the mosquito fish.

Until now, carp have been regarded as something of a cunning feral pest.

But the recent draining of a wetland in South Australia's Riverland has prompted scientists to change their minds.

Sabra Lane reports.

SABRA LANE: Fishermen loathe the European Carp. Since it was introduced into the Murray last century it's become an invasive pest. Some even call them "river rabbits".

BEN SMITH: If anyone has seen them feeding in the mud, they're really ... they really are quite destructive. They suck in mud as they feed, and they sort out the food and spit everything else out, making the water more turbid.

And they certainly dig up submerged aquatic plants and you know, little animals that are living within the mud. So, they are very destructive, especially given their number in the river at the moment.

SABRA LANE: That's carp researcher, Dr Ben Smith from the South Australian Research and Development Institute.

Last month, he was part of a research team, which monitored the draining of the Banrock Station wetland in South Australia's Riverland. The team made a fascinating observation.

BEN SMITH: We netted the outlet during the draining event and we found that the native fish left the wetland quite quickly, whereas the carp remained within the wetland. They even swam further into the wetland, so thereby trapping themselves.

SABRA LANE: Okay, so the native fish seemed to sense that trouble was happening and they had to get out whereas the carp really didn't sense that at all.

BEN SMITH: Yeah exactly, there wasn't actually much water within the wetlands to begin with, it's been evaporating over the summer months. So, the native fish were probably already sensing that something was up and they had to get out.

So, when they were given that opportunity, they certainly escaped fairly fast within the first six hours, but we rarely saw carp moving further into the wetland. It was actually quite surprising.

SABRA LANE: The researchers estimate about 60 tonnes of carp are now dead or dying in the wetland, underlining the point about these called so-called cunning fish, that they may not be so smart after all and may even become part of the Australian vernacular.

Could they replace the goldfish as being you know, how people say "I've got a memory like a goldfish" and "as dumb as a carp"?

BEN SMITH: Dumb as a carp. I think they are smart, but their biology leads them to be smart in many ways, but this behavioural trait is certainly a weakness and it's one that we're looking to exploit.

SABRA LANE: And that's exactly what excites the researchers.

With estimates that European carp cost Australia $16-million each year in economic cost and environmental impact, they believe they might be able to exploit this weakness to reduce their numbers.

Ben Smith.

BEN SMITH: There's no one silver bullet that's going to rid carp ... you know, the Murray of carp. But certainly if we can learn more about their biology, find out what they're doing, when they're doing it and why they're actually doing the things that they do, then certainly we can ... like when they move into wetlands we can cut them off at the pass.

There's often discreet channels that they have to swim through every year to spawn in wetlands, which they prefer to do. And conversely, after spawning, they go back into the main river channel over winter.

So, as they're passing through the Murray wetland inlets for instance, we can trap them on their way.

TONY EASTLEY: Carp researcher Dr Ben Smith ending that report by Sabra Lane.