Researchers at the University of Newcastle have shown that the water dynamics of the sawfish's weapon make it one of nature's deadliest stealth predators.

Rather than use its 'sword' to sift for prey, it uses it to swipe its dinner with minimal disturbance in the water.

"Moving the rostrum [or snout] just a few centimetres above the sea floor creates almost no disturbance at all," said Associate Professor Phil Clausen from the university. "Our results show sawfish are an ultimate stealth hunter.”

The University of Newcastle engineers worked with fish experts at Murdoch University and Sharks and Rays Australia to better understand how the critically endangered sawfish hunts.

"The hydrodynamic nature of their rostra [snouts] makes any movement barely detectable in water," said Professor Clausen, lead investigator of the study published in The Journal of Fish Biology.

"We were surprised at how fast the motion was. Our modelling clearly shows that with a lateral swipe, by the time the sword reaches the prey, it's already too late.”

The collaboration for the study emerged after University of Newcastle PhD student Sam Evans watched a feature on TV show River Monsters featuring Associate Professor David Morgan from Murdoch University's centre for fish research.

The Newcastle University mechanical engineers in this study normally work on the the aerodynamics of wind turbine blades – “so I'm incredibly interested in the movement and efficiency of blades," said Mr Evans.

David Bradney and Associate Professor Clausen said understanding the wing dynamics of birds of prey to move silently is being applied to large wind turbines to reduce their noise.

 "We have the ability to apply engineering principles to something outside the engineering box," Professor Clausen said.

Professor Morgan leads "Team Sawfish" at Murdoch University, which works with Aboriginal rangers in the Fitzroy River area to conserve the fish. "We have encountered instances of hunters removing the rostra as a kind of trophy," he said.

Barbara Wueringer is director and principal scientist of Sharks and Rays Australia in Cairns and says the study builds on her research on the sawfish's feeding behaviour.

"Using computational fluid dynamics, we have found that the shape of the rostrum reduces noise in water, increasing the ability of saw to detect minute vibrations caused by prey in lateral swipes of the weapon," Dr Wueringer said.

Mark McGrouther at the Australian Museum said: "This research fits with our understanding the sawfish's rostrum is a slashing weapon to hunt for prey."

Mr McGrouther said in Australia the four species of sawfish were mainly found in northern tropical waters of Western Australia, Northern Territory and Queensland.

"There are some sightings in waters north of the Clarence River in NSW," he said. "There is even a historical sighting of the fish in the Parramatta River in 1851, but you won't find these fish in the harbour today."