SYDNEY (Reuters) – Rescuers were racing against the tide on Wednesday to free whales beached off the Australian coast, with more than half the estimated 470 mammals in the country’s biggest stranding on record already believed dead.

The pod of long-finned pilot whales was first spotted on a wide sandbank during an aerial reconnaissance of remote and rugged Macquarie Harbour in Tasmania state on Monday, launching a difficult rescue operation.

Around 25 whales were freed on Tuesday, but officials said some had 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,” said Kris Carlyon, a wildlife biologist with the state government conservation agency. “We’re still very hopeful.”

A team of about 65 state park workers, fishermen and volunteers were triaging the pilot whales, a species of oceanic dolphin that grow to 7 metres (23 ft) long and can weigh up to 3 tonnes, to identify those most likely to survive.

The refloating process involves as many as four or five people per whale wading waist-deep in freezing water, attaching slings to the animals so they can be guided out of the harbour by a boat. (Graphic: Whales stranded off the coast of Tasmania – https://graphics.reuters.com/AUSTRALIA-WHALES/azgvoakqwvd/AUSTRALIA-WHALES.jpg)

The stranding, about 200 kms (120 miles) northwest 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 Peter Harrison, a professor at 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 then reported to be the country’s biggest mass stranding. About 600 pilot whales beached in nearby 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 around 200 of the mammals 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, at a news conference.

(Reporting by Byron Kaye; Editing by Stephen Coates and Jane Wardell)

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