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About 150 whales have become stranded on a beach in Australia, prompting a major rescue effort.

The animals were spotted by a fisherman at Hamelin Bay, about 300km (180 miles) south of Perth, early on Friday.

About half of the whales were already dead, according to authorities in Western Australian (WA).

Conservation officials said they were trying to save the surviving animals on the beach.

"The strength of the animals and the windy and possibly wet weather conditions will affect when and where we attempt to move them out to sea," said Jeremy Chick, from the Department of Biodiversity, Conservation and Attractions.

Officials said the stranding involved two species - short-finned and long-finned pilot whales.

The Australian Broadcasting Corporation reported that dozens of rescuers were at the beach.

Authorities have also issued a shark alert, warning people to stay away from the area.

"It is possible the dead and dying animals will act as an attractant, which could lead to sharks coming close into shore along this stretch of coast," the state's fisheries department said in a statement.

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

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

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

In 1996, about 320 long-finned whales became beached in Western Australia's largest stranding.