On a steamy night in tropical far north Queensland, Barbara Wueringer and her team climb aboard their 4.5-metre boat on the Norman River.

Key points: Four species of sawfish used to range around the top half of Australia's coastline in ocean and estuarine environments

Four species of sawfish used to range around the top half of Australia's coastline in ocean and estuarine environments Numbers have been dropping since around the 60s, but there is a lack of data

Numbers have been dropping since around the 60s, but there is a lack of data Early gillnetters would cut off the sawfish rostrum and leave the animal to die. These days dams and water diversion threaten sawfish, as do increasing crocodile numbers

They string gill nets across the murky waters and wait in the dark, hoping to trap a sawfish before the tide begins running too fast and their sampling window closes.

Ripples from a buoy keeping the net afloat are cause for excitement, but again tonight they go home without seeing any sawfish.

It's the same story every night on the 14-day survey, and they're forced to concede that this trip won't yield any precious data on one of Australia's most unique and threatened species.

They're disappointed but not too surprised to come up empty handed, according to Nicole Weller who was also on the trip.

"I can't say it was unexpected," the aquarist from Sea Life Sydney Aquarium said.

"The sawfish population has been declining for a long time."

Four species of sawfish used to range around the top half of Australia's coastline, all the way down to Sydney Harbour in the east and as far south as Cape Naturaliste, south of Perth in the west.

But their numbers seem to have dropped off a cliff.

"Green sawfish used to be all the way down to Sydney," Dr Wueringer from Sharks and Rays Australia said.

Lack of good historical data means no-one is really sure when they began to decline, or exactly why.

To help fill in some of that historical data, Dr Wueringer is involved in a program encouraging people to report sightings of saws or sawfish.

"Even if they see one in their grandfather's photo album, absolutely anything," she said.

She believes people taking the saws as trophies was a big factor in their initial decline.

Saws chopped off rather than untangled from fishing nets

Sawfish rostrum get entangled in fishing nets where they may be cut off or abandoned. ( Supplied: WWF )

But habitat change in the south and gillnet fishing in the north are also primary culprits, sawfish expert Peter Kyne from Charles Darwin University said.

"The decline sort of happened around the '60s and '70s and '80s, and it corresponded with a time of increased commercial fishing activity in northern Australia," Dr Kyne said.

"Primarily that was the beginnings of gillnets and trawl nets which [sawfish] easily get entangled in."

The sawfish "saw" or rostrum is lined with sharp spines that look like teeth, but are actually modified scales which they use to kill prey — mostly small fish, prawns, and crabs.

They were seen as a hazard for early fishermen who would cut the saws off the sawfish's face, rather than untangling them from their nets.

"Back in the day, before they were protected, when they were caught they'd often be killed and the rostra would be removed as a trophy or souvenir," Dr Kyne said.

"In some places they were so common they were used as fence posts."

By the time sawfish became protected in 2009, numbers had plummeted, and their range had contracted to the tropical north of Western Australia, and around to north of Cairns in north Queensland.

Old crocodiles 'remember' the nets

Crocodiles make sampling for sawfish hazardous. ( Supplied: Wade Kelly )

While we know that sawfish numbers are low, it's very difficult to accurately gauge how many are left.

Sawfish like to inhabit murky waters, so visual spotting isn't a possibility, and entering the water is off limits.

"[Crocodiles] mean we can't get in the water, so we're doing everything from boats, but that's sort of standard up here," Dr Kyne said.

Instead, researchers have to use gillnets themselves.

Although commercial gillnets have been banned in rivers in the Northern Territory for decades, Dr Kyne said some of the bigger, older crocodiles remember them, and recognise that they're a potential food source.

"They see a gillnet there, and they're curious. We've had [crocodiles] come along, and they stick their snout on the top of the net, on the top of the line," he said.

"And they wait and sense movement in [the net]. When that happens we grab the net and go somewhere else."

But in north Queensland, commercial gillnetting is still legal.

A passer-by reportedly found this sawfish still alive in the Wenlock River in north Queensland, with a gillnet operator fishing nearby. ( Supplied: WWF )

Conservation group WWF Australia have called for the banning of gillnets in northern Queensland rivers where sawfish occur.

They point to recent examples of animals found with their rostra hacked off, and others found washed up dead on beaches near gillnet operations as proof that gillnetting can't coexist with sawfish.

"Sawfish are one of those species that are vulnerable to a bunch of threats — habitat destruction being a key one, change of river flows, pollution, all that kind of stuff," WWF's Head of Oceans spokesperson Richard Leck said.

"The removal of gillnets in that area of the very far north of Queensland is something that could be relatively easily implemented, could potentially make a big difference to sawfish populations and also benefit a whole range of other species."

Damming the north a bigger threat than netting

Some massive sawfish have been caught in north Queensland, like this one from the Mulgrave River in 1938. ( Supplied: Queensland State Library )

But gillnetting isn't having the impact on sawfish that it did in the bad old days, according to Dr Kyne.

Fishers are better equipped to handle and release sawfish, and trophies aren't really looked on as favourably as they once were.

"On occasion there have been instances where they have been caught without their rostrum — it's been cut off but the animal is still alive, but looking like it's not been feeding, it's very thin," he said.

This animal was reportedly found with bullet holes and missing saw in the Archer River in north Queensland. ( Supplied: WWF )

"But that happens probably rarely. Compliance across the commercial industry is pretty high with observers and enforcement and awareness."

Dr Wueringer said that improving release methods is the best way forward for the fishing industry.

"We work with fishermen, so I don't really see the nets themselves as a threat," she said.

"The initial capture is not the problem, the problem is releasing them."

Instead, the big threats to sawfish are changing water flow regimes — dams and water diversion, according to Dr Kyne.

Particularly the largetooth sawfish, which has a complex life cycle in which the females enter estuary systems to "pup" — give birth.

The young then swim further upstream and spend the first four to five years of life in freshwater, before heading out into coastal waters.

Research from CSIRO last year identified several rivers in the north that could be dammed to support irrigation schemes, as part of the Australian Government's Agricultural Competitiveness White Paper.

All three catchments they focussed on are critical habitat for sawfish, Dr Kyne said.

"There are certain rivers— the Adelaide River, the Daly River in the Northern Territory, the Fitzroy River in the Kimberley — they're certainly key spots where there are still persistent populations of largetooth sawfish," he said.

Knocking out those populations would mean they're gone forever. Largetooth sawfish are like turtles, in that they return to the same place to give birth.

Crocodile conservation could be impacting sawfish numbers

Gudjuda Land and Sea rangers found six sawfish and two dugong washed up at Wunjunga last year. ( Supplied: Gudjuda Reference Group )

Ironically, another threat to sawfish could be the conservation success of the saltwater crocodile.

Especially where dams trap sawfish, or where they become stranded in billabongs after the wet season, they're easy prey for crocodiles and bullsharks.

To help support numbers, Dr Kyne works with the Malak Malak rangers on the Daly River south of Darwin.

The rangers do regular patrols of the channels that drain the masses of water off the floodplains after the wet season.

Towards the end of the dry, they rescue animals from drying ponds.

"The water's so shallow you can see the sawfish rostra breaking the surface. So that's an annual patrol we do with the traditional owners out there," he said.

"We've rescued 60 largetooth sawfish doing these patrols."

He fears climate change could put even more pressure on numbers, as wet seasons become less reliable.

"On a global scale these species are on the brink of extinction outside of Australia," Dr Kyne said.

"So Australia and to a lesser extent New Guinea, are sort of a global refuge now. We call it "lifeboat Australia" — for these species where they're either extinct or on the verge of extinction outside of Australia."

Dr Wueringer is also concerned about the impacts climate change will have on the remaining systems that support sawfish.

But despite not trapping any in the Norman River, she isn't declaring them gone from that system yet.

They're complex animals that we still don't fully understand, she said.

And she thinks there are probably environmental factors at play that could explain their failure to find any this year.

"With every river system we work in there are always different factors that come into it," she said.

"The Norman River was part of that really big flooding event in Queensland that killed 500,000 cattle. So the Norman River was one out of three river systems that basically merged.

"[And] on this trip we had extremely low water temperatures which could also mean that, you know, they're not really active during these times."

Dr Kyne agrees. But they're getting harder to find, especially the big ones, and he thinks that's a bad sign.

"That's a bit of a worry. There are certain rivers that are really important," he said.

"There are a lot of factors we still don't understand about seasonality and movement, but with that amount of [sampling] effort, you historically would have caught many sawfish."