Darling River oxygenation trial hopes to avoid future mass fish deaths like Menindee

A trial to inject oxygen into the Darling River in a bid to improve conditions for marine life has begun in outback New South Wales. 

Last March, tens of millions of native fish died on the Darling River from suffocation after oxygen levels plummeted.

The results of the state government trial to see whether pumped pure oxygen can help prevent or reduce the severity of future mass kills won’t be known for months, but it’s a step in the right direction according to state Water Minister Rose Jackson.

“We’ve seen far too many catastrophic fish deaths in recent years,” she said.

“We’re not prepared to just shrug our shoulders and say, ‘Oh well we’ll just see how it goes'”.

Not a silver bullet

While designed for emergency situations, water quality scientist Joe Pera with Water NSW is optimistic about the tank’s role in improving river health but warns that it is no silver bullet.

“It draws in water with no oxygen, mixes it, super saturates it with oxygen and then releases water with 100-per-cent saturated oxygen,” he said.

The unit is portable and can be deployed up and down the river “reasonably quickly” and is designed for “emergency situations” according to Water NSW.

The system has been successfully trialled in Western Australia, and while there are several differences between the two locations the trial will help to identify what does and doesn’t work.

“In WA they didn’t have such a flowing river as we do here. It’s a little bit shallower here, [the tank] works better in deeper systems,” Mr Pera said.

“It doesn’t say that it doesn’t work here, but that’s one of the limitations.”

The trial will also assess whether there’s any danger the oxygenated water could attract large volumes of fish to a small section of river and what impact that would have. 

The technology is commonly used in aquaculture.

According to University of Sydney professor and head of the school of civil engineering, Stuart Khan, it remained to be seen whether the trial could be effective over a larger scale. 

“The concept itself makes sense, it’s really a question of how large an area can you re-oxygenate and how well can you hold that off against the pressures that are producing the deoxygenation,” he said.

Acting on report recommendation

In late 2018 and early 2019, a series of fish kills around Menindee highlighted the river’s health, its management and the affect of drought.

In March last year, following a flood, the situation culminated in the death of a large numbers of native species.

A report by the state’s chief scientist’s office followed, with recommendations handed down in September 2023.

One of the many ideas floated was a trial where tanked oxygen would be pumped into the river to improve the overall quality of water and fish health.

Ms Jackson believes the technology is worth trying.

“The whole point was to give it a go, to give everything a go,” she said.

“To show that we were listening to the local community, to the experts, to the chief scientist.”

Several reviews took place following 2018–19 events, but little had been done, according to Ms Jackson.

“Those reports have sat gathering dust in someone’s bottom draw and I was determined that the chief scientist’s report was not that,” she said.

Big picture management needed

The NSW government has begun adopting short term (0–12 month) recommendations from the chief scientist’s report, but according to locals like Graeme McCrabb there was still a long way to go.

Community distrust with water agencies has been brewing for years, and Mr McCrabb believes tank oxygenation could result in more fish deaths.

“Because there’s oxygen there that will draw a lot of fish in there, which is counterproductive because that increases the oxygen draw on that water in that zone,” he said.

Ms Jackson confirmed the organisation was looking to fund “fish ladders, fishways and other structural support”, which were medium-term (1–5 year) recommendations by the chief scientist.

While Professor Khan welcomes the oxygenation trial as an immediate fix, he doesn’t want people to lose sight of the long-term goals.

“The only real negative is if people do start to see this as a silver bullet solution and forget about all of the other big picture water management practices,” he said. 

The removal of fish, particularly carp from the Menindee Lakes and Darling River was also highlighted by the chief scientist. Whether this happens is something the locals are still waiting for word on. 

The oxygenation trial is among a few strategies employed by the state government to improve river health, others include the employment of more local water agency staff and small releases or “pulses” of water from the nearby Menindee Lakes.