A snorkeller is believed to have been snatched by a shark as he swam last weekend with his son at a beach south of Perth. Witnesses reported seeing a dorsal fin and thrashing in the water before the sea turned red and the man vanished at the scene on Australia's west coast.

Father-of three Brian Guest, 51, who had campaigned for many years for the protection of sharks, had been looking for crabs on yesterday morning with his 24-year-old son when he was attacked about 30 metres from the shore.

Luke Tubbs told how a witness ran to his house in shock and screaming for help: "He just saw a big splash and then the shark roll over in the water with the guy and then [he saw] no body or anything."

Daniel Guest, who was swimming six metres away at the Port Kennedy beach, heard his father's screams but did not see the attack. He raced to shore when he saw blood in the water. His father's shredded wetsuit was found later, and aerial searches spotted a five-metre great white shark swimming in the area.

In Sydney, some hours later, a kayaker survived a terrifying ordeal after being knocked from his craft by a great white shark. That encounter, endured by 29-year-old Steve Kulcsar, occurred less than a mile off Australia's east coast, at Long Reef, and was filmed by a fisherman in a small boat nearby. The video footage apparently showed the shark circling Kulcsar after bumping him off his kayak.

Kulcsar said: "I knew it was there, but my first thought was to just get back in the kayak as quick as possible."

For 10 minutes, the shark is seen lurking around him and two other kayakers before losing interest and swimming away.

The two great white encounters sparked panic and closed a number of beaches across Australia.

Yesterday Bondi beach was closed yesterday after a shark was spotted close to shore. Swimmers were also evacuated from a Queensland holiday island after sharks were spotted in waist-deep water.

During an aerial search on the West ­Australian coastline for Guest's body, sharks were spotted, forcing the closure of two beaches.

The attack on Guest was the second by sharks in Australia this year. A 16-year-old surfer, Peter Edmonds, died after being attacked by a shark near Byron bay, on the New South Wales coast, in April.

On average there are 15 shark attacks a year in Australia, with about one death each year. Swimmers are more likely to die of a lightning strike than be taken by a great white shark.

Yesterday Daniel Guest recalled how his father had spoken of the risks of sharks and how he had "loved and respected the ocean environment".

Brian Guest wrote on the Western Angler website forum in 2004: "I have always had an understanding with my wife that if a shark or ocean accident caused my death then so be it, at least it was doing what I wanted. Every surfer, fisherman and diver has far more chance of being killed by bees, drunk drivers, teenage car thieves and lightning. Every death is a tragedy – regardless of the cause – but we have no greater claim to use of this earth than any of the other creatures [we] share it with."

His son said he viewed his father's death as a random event that should not make people fear the water, and said he would eventually go back into the sea. "When I'm ready … I'll do that."