An air and sea search was launched off Port Kennedy, 30km south of Perth, early today after reports that a snorkeller had been taken by a shark.

Port Kennedy beach is adjacent to the popular Safety Bay area used for Windsurfing & Kitesurfing.

Witnesses who saw the the man in trouble off the dog beach near Cote D'Azur Gardens, Port Kennedy believe he was attacked at about 7.15am.

The 51 year old father and his 24 year old son were snorkling for crabs when the incident happened. The son swam safely to shore to raise the alarm but his father is still missing at sea.

Three rescue helicopters, four wheel drives, water police, sea rescue boats, volunteers and life guards searched the ocean and long span of beach for clues.

The missing man was described as an avid fisherman who loved the water and had great respect for its marine life.

Police spokesman Mark Valentine said witnesses had reported seeing "something pretty violent'" happening in the water and there was blood also colouring the water.

"Something very traumatic has happened there and we are treating it as a probable shark attack, there was lots of talk among witnesses at the incident location about seeing fins in the water but we can't yet say whether there was definitely a shark out there."

The beach is a popular swimming location and there were many people out because of warm weather conditions, Mr Valentine said.

About two dozen other beach users, joggers and walkers were also watching the drama unfold.