Hundreds of the animals, some distressed and other dead, were discovered Monday, and more were found around six miles away Tuesday.
For the 20 pilot whales still alive but trapped, survival chances diminished rapidly Thursday.
Rescuers and experts had on Wednesday warned some of the freed pilot whales may return to the shallow waters overnight and once again become stuck. Officials cited strong tides.
Pilot whales’ social bonds tend to pose a challenge for rescue operations, said Karen Stockin, a marine biology researcher at Massey University in nearby New Zealand, who has done extensive work on strandings.
“These animals are constantly communicating,” she said. “It’s like a distress call. So even when you got some animals successfully into deeper waters, it’s not uncommon for them to turn tail and come straight back in.”
It is difficult to “undertake any meaningful re-floatation when you’ve got so many animals that are literally coming in faster than you can get them out,” she said.
As a result, rescuers may have to prioritize animals with the best chances to survive the ordeal — but making that determination can be difficult because of potential internal injuries.
“As long as we have live animals with a chance and resources to shift them, then we will give it a go,” Tasmania government official Nic Deka told the Australian Broadcasting Corporation on Thursday.
Cold temperatures in the waters off Tasmania this week meant the stranded pilot whales may have survived longer than they would have during the southern hemisphere’s summer, as they were not at an immediate risk of overheating. But the cold conditions also hampered rescue operations, officials said. Each rescue attempt requires around five people, who have to pull the animals out of sandbanks with slings and guide them to deeper water, Reuters reported.
Pilot whales, which are in fact large oceanic dolphins, can grow to around 20 feet. Adults can weigh several tons, making it difficult to free them once they become stranded — and to move the bodies once they are dead.
Due to their weight, some stranded animals “are under compression to the point where you’ve got organ failure,” or they are suffering a stress response, Stockin said.
In other cases, the animals overheat as their insulated skin is exposed to the sun and cannot be cooled by water, she said.
In 2017, New Zealand saw one of its worst mass strandings in history in which 600 pilot whales were beached. Around 400 of them died. Many of the carcasses were buried in remote sand dunes — which Australian officials may now consider, as the carcasses can explode as they decompose, posing a threat to bystanders.
Pilot whales travel in large pods, which makes the species vulnerable to mass strandings. Some regions, including Tasmania, have a long history of such incidents.
The reasons whales and dolphins strand remain unknown.
Researchers said a number of behavioral factors, such as the pursuit of prey into shallow coastal waters, might play a role.
Stockin said it is unlikely researchers would be able to determine exactly why the pod stranded off Tasmania.
“You might have one or two compromised individuals in that whole group that do have an injury, or a debilitation, or a disease that’s caused some kind of navigational failure,” she said. But “it’s a little bit like looking for a needle in a haystack.”
This report has been updated.