A full-scale search is on for a man in Australia believed to have been taken by a shark while out snorkelling with his son.

Brian Guest, 51, was looking for crabs early on Saturday morning with his 24-year-old son off a beach in Port Kennedy, about 30 miles south of Perth, when he suddenly disappeared.

His son told police he saw a "commotion" in the water before his father vanished. He quickly swam to shore and sounded the alarm.

"It's believed he's been taken by a shark," Western Australia state police Sergeant Greg Lambert said.

Witnesses spoke of seeing flashes of fins in the area and the water was colored by blood.

The beach was immediately closed, and police on jet skis, several marine rescue patrols and at least three helicopters began scouring the waters for the missing father of three.

Searchers have found a piece of a wet suit believed to be that of the missing swimmer in the water, not far from where he vanished, Sgt Lambert said.

Aerial footage of the shark believed to have taken the missing man was shown on Australian television.

The 6-mile beach is a popular swimming location and there were many people out because of the early morning warm weather conditions.

If Guest has been taken by a shark, it would be the second such incident in Australia this year.

A shark in April killed a 16-year-old boy who was bodyboarding with a friend off Australia's eastern coast.

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