Only six whales have survived a mass stranding of pilot whales on the coast of Western Australia.

About 150 of the animals were found beached at Hamelin Bay, about 300km (180 miles) south of Perth.

Their discovery by a local fisherman on Friday prompted a major rescue effort to return them to deeper waters.

However, by nightfall, more than 140 of the whales had died, with deteriorating weather conditions and the threat of frenzied sharks impeding efforts.

More than 100 volunteers, wildlife personnel and others came to the aid of the beached pilot whales, a species known to strand en masse.

“I’ve never seen anything like it, seen so many whales beached like this,” one tourist told the Associated Press news agency.

“Unfortunately, most of the whales beached themselves on dry land overnight [on Thursday] and have not survived,” Parks and Wildlife Service spokesperson Jeremy Chick said in a statement.

The rocky beach terrain, dead whales surrounding the survivors and rough seas were challenging factors in moving the surviving whales, officials said.

Mr Chick said there was a risk the surviving whales might come back into shore and re-strand.

“This has often been the case in previous mass strandings,” he added.

‘A mystery of nature’

Scientists do not know exactly what causes whales to beach themselves.

Experts have said stranding can occur when whales are sick or injured, or make navigational errors, particularly along gentle sloping beaches.

“It’s one of the mysteries of nature,” one of the rescue coordinators told the Sydney Morning Herald, adding that “once they come ashore like that they do deteriorate quite quickly.”

Sometimes beached animals can send out distress signals that attract other whales to become stranded.

In 1996, about 320 whales became beached in Western Australia’s largest stranding.