Sawfish exposed as the ultimate stealth hunter

March 23, 2017

Sawfish rostrum

The hydrodynamic shape of the rostra causes little turbulence in the water (Pic: University of Newcastle)

New research has revealed the lethal side swipes of sawfishes’ distinctive rostra are barely detectable to their prey.

Associate Professor David Morgan from the Centre for Fish and Fisheries Research at Murdoch University, was part of a research team which simulated sawfish movements finding the hydrodynamic shape of the rostra caused little turbulence while moving through the water.

The stealthy sawfish use lateral swipe movements to impale prey such as fish and crustaceans on their teeth-lined snouts, which, thanks to being covered by electrosensitive pores, also help them to detect prey.

The data indicate the rostra are unlikely to be used to stir up river and sea beds to uncover prey, as has been suspected by scientists for many years.

Sawfish, a type of ray found in both salt and fresh water, are identified by their uniquely flattened bodies and snouts (rostrum), which are lined with razor-like teeth. Largely found in the Kimberley region of WA, limited research into this endangered species mean its behavioural habits are not fully understood.

Professor Morgan worked with researchers from the University of Newcastle, James Cook University and Sharks and Rays Australia, to come up with the latest findings which were recently published in the Journal of Fish Biology. The paper can be read here.

Utilising CT technology, rostra specimens of three sawfish species were scanned to create 3D replicas of the sawfish head. Computational fluid dynamics (CFD), or a virtual wind tunnel, was then applied to imitate their movement in water.

Lead researcher Associate Professor Philip Clausen from the University of Newcastle said the results proved sawfish were the ultimate stealth hunter.

“The hydrodynamic nature of their rostra makes any movement barely detectable in water," he said.

“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.

Sawfish 3D model

The researchers applied computational fluid dynamics to imitate the movement of a sawfish head in the water (Image: University of Newcastle)

“Our work also shows moving the rostrum a few centimetres above the ground creates almost no disturbance at all, which would make stirring the river or sea bed difficult."

Due to their rarity and preference to feed in murky water, sawfish feeding habits have seldom been sighted in the wild, leaving researchers to rely on their observations in captivity.

Co-author and Director of Sharks and Rays Australia, Dr Barbara Wueringer, has studied sawfish habits closely and said she was delighted to have found another piece of the puzzle.

“The shape of the rostrum is likely to reduce noise in the water, increasing the ability of sawfishes to detect minute vibrations caused by prey during their lateral swipes.

“The results of our work will help us gain a better understanding of their habits and ultimately help us improve our conservation efforts,” she said

A seemingly unlikely collaboration between engineers and biologists, the project was sparked by an episode of the TV show River Monsters. Professor Morgan was the featured sawfish expert on the program, which prompted the University of Newcastle engineering researchers to reach out and initiate the partnership.

Professor Morgan leads Team Sawfish at Murdoch University, who since 2001, have tagged hundreds of critically endangered sawfish in the Fitzroy River in the Kimberley. They work closely with Indigenous Ranger Groups in the area with the aim of uncovering sawfish secrets to aid their management and conservation.

He said: “We have encountered instances of hunters removing the rostra as a kind of trophy.

“The findings of our study show just how instrumental sawfish rostra are for their survival and we would urge the few remaining human population centres that have sawfishes inhabiting their local waters address this destructive phenomenon,” Professor Morgan said.

To watch how sawfish use their stealthy rostra to catch prey, click here.

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