Murray choked: 14.4m carp infest South Australia’s Lower Lakes

South Australia’s Lower Lakes are choked with at least 14.4 million carp — the Murray Darling Basin’s rabbits of the river.

River rabbits: Carp schooling at Lock 1 at Blanchetown in South Australia. Picture: Garry Warrick

SOUTH  Australia’s shallow Lower Lakes are choked with at least 14.4 million carp, weighing an average 2kg and more than 15cm long, according to a national study into the pest’s biomass.

While drought has curbed carp breeding across much of the Murray Darling Basin, the South Australian Government has maintained Lakes Alexandrina’s and Albert’s salinity and water levels at ideal levels for carp breeding.

Given the lakes cover 82,000ha, the Arthur Rylah Institute researchers estimate the carp density at 350kg per hectare, yielding 27,800 tonnes of the noxious pest.

The estimate does not include juvenile carp under 15cm in length.

The ARI biomass study found the lakes and South Australian Lower Murray “had some of the highest modelled biomasses (550 kg/ha), reflective of the regulated series of slow-flowing weir pools and permanent adjacent wetlands, which provide optimal habitats for carp”.

“As such the entire SA section of the Murray River (including the shallow Lower Lakes) should be considered a carp hotspot,” the study found.

“The South Australian section of the Murray River contains 250 wetland complexes comprising 1100 wetlands. As a result of river regulation (i.e. locks, weirs, flood plain levees), approximately 70 per cent of these wetlands are permanently inundated.

“These wetlands are predominantly shallow, well-vegetated, slow-flowing habitat which is characteristic of areas that carp actively seek for spawning and nursery sites. Indeed, up to 98 per cent of carp recruits are produced in shallow off-channel wetlands.”

The ARI biomass study found carp densities in many locations were well above the 80-100kg/ha that “at which detrimental ecological impacts may occur”.

Further upstream, environmental watering appears to have exacerbated the carp problem, with the ARI team reporting other hotspots in the Barmah-Millewa Forest, Gunbower, lower Ovens floodplains, as well as the Lachlan catchment, Macquarie Marshes and Gwydir Wetlands.

The Weekly Times has previously reported carp populations are booming in the wake of artificial environmental river flows that spill out on to the Murray Darling Basin’s floodplains, creating hot spots for Australia’s rabbits of the river to produce millions of offspring.

ARI principal research scientist Jarod Lyon said post-drought it would only take three wet years to drive the population up to a million tonnes.