Rescuers were racing against the tide on Wednesday to free whales beached off the Australian coast, with more than half of the estimated 470 mammals believed to be dead.
The pod of long-finned pilot whales was first spotted on a wide sandbank during aerial reconnaissance of the remote and rugged Macquarie Harbour in Tasmania on Monday, sparking a difficult rescue operation.
About 25 whales were freed on Tuesday, but officials said some beached themselves again when they were brought back in by the tide, creating an exhausting loop for rescuers in freezing waters.
"We're not at a point where we're considering euthanasia, but it is always something that we have at the back of our mind. We're still very hopeful," said Kris Carlyon, a marine biologist with the state government conservation agency.
A team of about 65 state park workers, fishermen and volunteers were triaging the pilot whales, a species that can grow to seven metres long and can weigh up to three tonnes, to identify those most likely to survive.
The refloating process involves about five people per whale wading waist-deep in freezing water and attaching slings to the animal so they can be guided out of the harbour by a boat.
The stranding, about 200 kilometres north-west of the state capital Hobart, is the biggest on record in modern Australia and one of the largest in the world, drawing attention to a natural phenomenon that remains a mystery to scientists.
"It's certainly a major event and of great concern when we potentially lose that many whales out of a stranding event," said Prof Peter Harrison of the Southern Cross University whale research group.
"Quite often we only get to really see them when there are bad outcomes, such as this stranding event. We absolutely need some more investment in research to understand these whales in Australian waters."
In 1996, 320 pilot whales washed up on the coast of Western Australia, in what was reported at the time to be the country's biggest mass stranding.
About 600 pilot whales beached in New Zealand in 2017.
The stranded pod in Tasmania was first believed to be about half the size, before further aerial searches spotted another group of about 200 whales nearby.
"They didn't look to be in a condition that would warrant rescue," said Nic Deka, a regional manager of Tasmania's Parks and Wildlife Service.
