WHEN marine researcher Scott Sheehan saw an object launch itself out of the water, he assumed it was a dolphin feasting on a school of baitfish.

After motoring over to the spot off Jervis Bay, camera at the ready, Mr Sheehan hoped for a second glimpse.

"We motored over to where it had first jumped and it jumped again," he said.

media_camera Thresher shark ... Pic: Scott Sheehan / Marine Mammal Research.

"There were at least 50 dolphins and 10 to 20 seals which had forced the yellowtail baitfish into a baitball but it wasn't a dolphin, that's for sure.

media_camera Thresher shark ... Pic: Scott Sheehan / Marine Mammal Research.

"We saw this crazy-looking shark explode from the water, it was at least 2m in the air before it smashed down on to the surface in this massive belly flop.

media_camera Thresher shark ... Pic: Scott Sheehan / Marine Mammal Research.

"What Mr Sheehan, who said the shark was between 3.5 and 4m long, had photographed last Saturday was a thresher shark, one of the most enigmatic of the ocean's prime predators.

"Threshers are the only sharks that use their tails to herd and stun their prey in a unique predatory behaviour," Dr Vic Peddemors, head of the shark research section of the NSW Department of Primary Industries, said. "Half of its length was the top lobe of its tail, they don't look like any other shark."

Mr Sheehan, project co-ordinator for the independent research group marinemammalresearch.com had been aboard a boat looking for humpback whales when he came across the feeding frenzy.

media_camera Thresher shark ... Pic: Scott Sheehan / Marine Mammal Research.

The acrobatic shark was a common thresher - there are also pelagic and bigeye thresher varieties - which favour temperate waters, grow to more than 5m and, being a fish and squid eater, are not considered to be dangerous to humans.

Dr Peddemors said threshers were "not rare" but sightings were infrequent and "not much is known about thresher sharks off the east coast of Australia".

Originally published as Prime predator's acrobatic stunts