Researchers say a perfect storm of factors caused the death of more than 40 critically endangered sawfish on a WA cattle station owned by businesswoman Gina Rinehart.

Key points: By the time a rescue mission arrived to save the sawfish, only two were still alive

By the time a rescue mission arrived to save the sawfish, only two were still alive The Fitzroy River catchment is considered the world's last stronghold for the species

The Fitzroy River catchment is considered the world's last stronghold for the species A long-time researcher says the fish died from a combination of stifling heat and a severe lack of rainfall

The fish died in drying pools on Mrs Rinehart's Liveringa Station in December, but the fish kill has only just been revealed, with a full report tabled this week.

Associate Professor, David Morgan, from Murdoch University led a rescue mission to try to save the fish, but when the team reached the isolated floodplain in Western Australia's Kimberley region only two of the fish were still alive.

He said the fish kill was the biggest he had seen in 18 years of monitoring and tagging the fish in the Fitzroy River system, which is regarded as the world's last stronghold for the species.

Murdoch researcher Karissa Lear with Travis Fazeldean placing a sawfish into a fish transporter provided by Broome TAFE. ( Supplied: David Morgan )

He believed the fish had died due to a combination of stifling heat and a severe lack of rainfall during the Kimberley's very poor wet season.

"The water temperature [37–40 degrees Celsius] was above what they can survive in," he said.

"They literally looked like they had been in a slow cooker for a long time."

He said, with the low water levels, the sawfish were also more vulnerable to predators such as freshwater crocodiles.

Floods in 2017, he said, led to a boost in the sawfish population and might also explain why the creatures had spread out onto the floodplain.

Sawfish are critically endangered and the Fitzroy River is considered one of the most important habitats for the species globally. ( Supplied: Michael Lawrence Taylor )

'Gina's water rescue'

The fish kill comes amid reports that Mrs Rinehart's company, Hancock Agriculture, is angling to tap into the Fitzroy to grow fodder crops for a bigger cattle herd.

Mrs Rinehart bought a 50 per cent stake in Liveringa and Nerrima Stations in 2014 and the following year purchased Fossil Downs, north-east of Fitzroy Crossing.

According to a recent media report, the billionaire wants to harvest 325 gigalitres of surface water when the river is in flood to "supercharge" the region's cattle industry.

The project would create "105 direct jobs" and allow "20,000 more cattle" to be run year round on Hancock's stations, the article said.

Hancock Prospecting would not comment on the sawfish deaths, or the water proposal, despite posting the article on its website under the title 'Gina's Water Rescue'.

The article on Mrs Rinehart's proposal for the Fitzroy River is posted on Hancock Prospecting's website. ( ABC Kimberley )

'Once-in-a-lifetime' opportunity

The proposal has been discussed between Hancock and WA Government agencies.

The Pastoralists and Graziers Association's Tony Seabrook is right behind the idea.

"A staggering amount of water comes down those rivers in a normal wet season," Mr Seabrook said.

"Their [Hancock's] proposal is to build an off-take tank to one side of the river to take water only during the years when it can be taken in such a way that won't affect the rivers downstream and then to use that to irrigate.

"This is something that is a once-in-a-lifetime.

"Government and bureaucracy should recognise that that's exactly what it is, and run with it."

A CSIRO report published last year calculated the Fitzroy's mean discharge into the ocean at about 6,600 gigalitres a year.

The paper estimated that harvesting surface water could potentially support 160,000 hectares of irrigated crops in 85 per cent of years.

Dr Anne Poelina is a Nyikina woman from the west Kimberley. ( Supplied: Dr Anne Poelina )

Water 'not being wasted'

But the proposal has come as a surprise to native title holders along the river who have been negotiating with the WA Government and major players in the cattle industry over a promised national park for the area.

Chair of the Martuwarra Fitzroy River Council, Anne Poelina, said from the traditional owners' point of view the water from the Fitzroy was not being wasted.

"When we move through the landscape, what we see is that the water is already being used," Dr Poelina said.

"One of the things we are concerned about is the cumulative impacts of development.

"We're very concerned that there are gaps in the science."

WA's Water Minister, Dave Kelly, said there had been no formal application for a water licence from Hancock Agriculture.

"We have committed to a national park in the Fitzroy and part of that is a water allocation plan, which will bring together the hopes and aspirations of all the stakeholders in the Fitzroy," Mr Kelly said.

"Once that is in place, then water licences can be considered."

Sawfish facing extinction

The environment group, Environs Kimberley, said the recent sawfish kill showed that the critically endangered animal was already under extreme stress and any further extraction of water from the Fitzroy would have grave consequences for the species.

The group warned of a "Murray Darling-type disaster" if the WA Government agreed to Mrs Rinehart's proposal.

Associate Professor Morgan said he could not comment on the plan until he had seen the detail of what was being proposed.

In his report on the Sawfish deaths, tabled in parliament on 4th June, Associate Professor Morgan said that there had reportedly been at least two other occasions when Sawfish had become trapped in the pools on the floodplain since 2008 with staff from the station conducting their own relocation mission.

It said the station had also provided logistics and accommodation for this latest rescue.

David Morgan from Murdoch University says, with the low water levels, the sawfish were also more vulnerable to predators such as freshwater crocodiles. ( Supplied: Murdoch University )

The report said the fish deaths had happened several kilometres away from the area that was subject to the station's water licensing requirements, but that the pools should be monitored to assess the likelihood of future fish kills.

"What we need to do is be pro-active, each year, maybe in early spring, to determine if and how many sawfish are stuck in these pools … are they likely to dry up?" Associate Professor Morgan told the ABC.

"And get in early and not wait until December when it's too late.

"Let's get in and move them."

He said climate change was the biggest threat to the survival of the species in the long term.