Australia’s longest river, the Murray, gained international prominence last month through a report from the World Wildlife Fund International, but it’s not a good report card.
One of the world’s longest rivers and with one of the biggest catchment areas, the Murray-Darling system made it to the WWF’s top 10 of endangered rivers, thanks to reduced water flows, salinity and a proliferation of invasive fish and plant species.
Major changes in the flow of water and State governments’ failure to control or kill invasive exotic species such European carp and plague minnow have put huge pressure on native fish such as the silver perch and large Murray cod.
The drought is not to blame for the Murray-Darling’s greatly reduced flow over recent years, although it may have contributed to the decrease. The report said excessive extraction by irrigators and control of the river’s flow by weirs and dams had resulted in major reductions to the flow and produced conditions favourable to the invasive species, which were flourishing.
“When we regulate the flows we make the rivers more suitable as habitat for invasive species,” said the freshwater policy manager for WWF Australia, Averil Bones.
However, she said irrigation was no longer a key threat, because of recent government initiatives to change the amount of water used by farmers and irrigators.
One of the biggest problems for the river is this explosion in European carp numbers. Approximately 60-90 per cent of fish in the river are carp, with densities in some places as high as one carp per square metre. Carp are bottom feeders and stir up the mud in the water, making it harder for fish like silver perch to feed. Their sheer numbers are also crowding out the slower reproducing native species.
Carp and other introduced species can be linked back aquarium fish. “Since 1990, the number of exotic fish in Australia’s waters overall jumped from 22 to 34, and all except for one of these introduced species originated from the aquarium trade,” the report noted.
The aquarium trade is also partly responsible for the rapid growth of exotic plants, released from home aquariums and ponds, which crowd the river banks and choke their courses. The report also named lippia as a problem plant, introduced as an ornamental ‘no mow’ lawn.
“These plants reduce the natural productivity of the floodplain, river and other wetland habitats, further depleting natural wildlife populations,” Bones said.
Fiona McKenzie, Policy Analyst for the Wentworth Group of Concerned Scientists, said Australia has a long legacy of problems caused by invasive species, “many of which we are only just coming to terms with.
“However, biodiversity is one of many challenges confronting the Murray-Darling Basin. What we have learnt is that, at the end of the day, the system must be managed as a whole.”
A spokesperson for the NSW Department of Primary Industry said habitat protection scientists were not prepared to comment until they had read the full report.