The dead fish, mainly native species such as bony bream, Murray cod, and golden and silver perch, were found along a 50-kilometre stretch of the river.
It was first thought ‘just’ 300 dead perch were found Menindee on Saturday, but a NSW District Fisheries officer put the count at 10,000 dead fish one day later.
Local property owner Wayne Marsden made the initial discovery.
The Department of Primary Industries Fisheries have put the fish kill down to a lack of oxygen in the water from the blue-green algae outbreak.
“The prolonged dry period has resulted in poor water quality along much of the Darling River,” a DPI spokesperson said.
“Algal alerts have been in place for several weeks in the Menindee region and linked to this, low dissolved oxygen levels are likely to occur within slow flowing or still sections of the river.”
CSIRO environmental scientist Tim Malthus said blue-green algae posed a serious threat to marine life.
“Blue-green algae really deteriorates the water quality, plus it has the ability, when in those high concentrations, to release toxins, which are capable of killing fish like we’ve seen,” Dr Malthus said.
He said algal blooms were driven by high temperatures, low rainfall and a build-up of nutrients from agricultural run-off.
No respite for lower Darling water users
WaterNSW spokesman Tony Webber said the lower Darling could expect more outbreaks of blue-green algae as summer continued.
“There’s certainly going to be ideal conditions for blue-green algae right across New South Wales, as we go into the hotter weather and the dry conditions continue,” he said.
With WaterNSW ceasing flows from below Weir 32 in Menindee to the lower Darling at the end of December, only water users with access to block banks will have access to water.
It’s a dire situation for irrigators and graziers reliant on the Darling River for water.
Some towns, like Menindee, have already been carting water as the river supply became unfit for human consumption.
Swimming in water affected by blue-green algae can also cause skin irritations and gastroenteritis.
WaterNSW advised property owners not to water their stock with water from the Darling River.
Mr Marsden said he was at a loss as to what to do about the water quality.
“The government’s got to supply more fresh water to households for animals, or I don’t know … I really, honestly don’t know what to do,” he said.
“I’ve got 12 race horses on my property, and I’m afraid to give them this water. I don’t know how it’s going to affect them.”
Ngiyampaa Elder Beryl Carmichael said the government had completely ignored the interests and opinions of First Nations Peoples.
Fixing the river’s health won’t be easy
The only real remedy for the deteriorating health of the Darling River is water flows.
Dr Malthus said one option for remediating the oxygen-sucking quality of blue-green algae was water aeration. The DPI has rejected this suggestion.
“The difficulty we have is scale,” senior fisheries manager Cameron Lay said.
“Aeration makes a very small impact in a localised area and, as was evidenced by the fish kill, we had poor water quality over a 40-kilometre stretch of the Darling River.
“It’s not logistically possible for us to have any meaningful impact on the oxygen levels through aeration.”
With both the DPI and WaterNSW predicting more fish kills, it raised serious concerns for the survivability of native species.
Mr Lay said native fish populations were very good at withstanding poor water quality and long periods of no flow.
“Localised fish kills like this, while very unfortunate, are not uncommon in prolonged periods of drought,” he said.
Water NSW’s Tony Webber said a solution could only come from a break in the weather.
“We’re optimistic the Queensland storm system might deliver that solution, but we’ll have to wait and see,” he said.
With no end in sight to the drought, it’s a stark reminder that the health of Australia’s biggest river systems are reliant on summer rains.
Source: ABC 2018-12