The causes of mass strandings remain unknown despite scientists studying the phenomenon for decades
Australian rescuers were forced Thursday to begin euthanizing some surviving whales from a mass stranding that has already killed 380 members of the giant pod.
While 88 pilot whales have been saved since the pod was discovered beached on Tasmania's rugged western seaboard four days ago, the death toll is expected to rise as the window for rescue closes.
"We still have a few more live animals that we think are going to be viable to move," said Tasmania's Parks and Wildlife Service manager Nic Deka, praising the hard "yakka" (work) of rescuers who will continue until nightfall and into Friday.
"There is a likelihood that we'll be continuing the rescue effort tomorrow... our focus has been on those that appear the most viable and have the most chance of success," he said.
A crew of around 60 conservationists and expert volunteers have spent days wading in the chilly waters of Macquarie Harbour, surrounded by the anguishing cries of dying whales.
"It is emotional," said rescuer Sam Thalmann.
"There are animals swimming around, they are vocalizing. We can see the bonds and the pairings within them."
Pilot whales -- which can grow up to six metres long and weigh a tonne -- are highly social.
Some animals have resisted rescue or tried to return to the pod after being freed, becoming beached for a second time.
Such is the level of distress that authorities said they had to carry out mercy killings of at least four whales.
"Those four whales were euthanized earlier today," using firearms and specialist ammunition, said Tasmanian environment department marine biologist Kris Carlyon.
"We've got a few others that we currently giving veterinary assessment."
"That's based purely on animal welfare grounds," he said.
The crews are now focusing efforts on a group of 20-25 partially submerged whales, using boats fitted with special slings to guide them back to the open ocean.
But, increasingly, attention is turning to how to safely dispose of the carcases of the nearly 400 whales that have already died.
"Our preference is for disposal at sea. We're still taking expert advice about where exactly the drop-off point may be," said Deka.
Left where they are, the whales would "bloat and float", causing a navigation hazard, polluting the bay and potentially attracting sharks and other predators, Deka said.
"The decomposition of such a large number of animals could actually affect oxygen levels in parts of the harbour, which could affect the marine life in those places."
'Little we can do'
A resident and cruise-boat worker who gave her name only as Monique said the local community has been devastated by witnessing scenes of such anguish.
"You could see that they were obviously suffering," she told AFP.
"On the beach they were still... puffing, flipping about and you couldn't really do much to help them."
The causes of mass strandings remain unknown despite scientists studying the phenomenon for decades.
Some researchers have suggested the pilot whales may have gone off track after being attracted by food close to the shoreline, or by following one or two members of the pod that strayed.
Marine biologist Carlyon said it was a "natural event" with strandings of the species regularly occurring in both southern Australia and neighbouring New Zealand.
"We do step in and respond in these situations, but as far as being able to prevent these occurring in the future, there's really little that we can do," he said.