A whale has been euthanased while 10 others appear to be heading out to sea after a mass stranding in Western Australia’s south.

A group of 11 long-finned pilot whales were among a group of 87 whales and five dolphins that washed up on the beach at Hamelin Bay before sunrise on Monday.

Department of Environment and Conservation Incident Controller Greg Mair said the condition of one of the surviving 11 whales had deteriorated throughout the day before environment officers shot it with a high-powered weapon on Tuesday afternoon.

“The whale had sustained significant trauma as a result of the stranding and was in a very poor condition and a highly distressed state, becoming separated from the main pod,” Mr Mair said in a statement.

“In addition to the importance of ensuring the most humane outcome for the individual animal, one of the most important considerations was reducing the likelihood of negatively affecting the rest of the pod.”

Mr Mair said wildlife officers had been closely monitoring the whales while encouraging them into deeper waters since they were transported from Hamelin Bay earlier on Tuesday afternoon.

“At the last sighting, the other 10 whales appeared to be moving into deeper waters,” he said.

“Aerial surveillance to monitor progress of the other 10 whales and to check beaches for other possible strandings will continue tomorrow.”

The mammoth rescue effort involving up to 180 volunteers, wildlife officers and veterinarians began when 87 whales and five dolphins were discovered by early beach walkers at the idyllic holiday community, south of Margaret River.

More than 70 died, some went back out to sea and 11 others were on Tuesday trucked to nearby Flinders Bay, on the eastern side of Cape Leeuwin, to be released in calmer waters.

The whales, some measuring up to six metres long and weighing 3.5 tonnes, were watered down during their road journey.

They were then returned to the sea using harnesses and strapping attached to the vehicles.

Volunteers kept them corralled in shallow water until they were ready to be released together.

The whales initially resisted their rescuers’ efforts, with at least two starting to swim back to shore.

The DEC has closed the beach from Hamelin Bay to Boranup North Point while the whale carcasses are cleared by local authorities.

There have been 21 mass strandings of whales and dolphins along the West Australian coast since 1984, according to the DEC.

bron: Sydney Morning Herald