The Whales Gathered in a Heart Shape. Experts Feared What Would Come Next.

The pod of pilot whales huddled together for nearly a full day in the shallow waters off a remote beach in Western Australia. At times it made the shape of a circle, or spread into a line, or even, for a few moments, formed a loose heart shape.

Some residents were thrilled by the unusual sight earlier this week. But authorities and researchers, baffled by the behavior, feared that a mass beaching was imminent.

The behavior was “really unusual,” said Kate Sprogis, a marine mammal ecologist at the University of Western Australia. “Healthy pilot whales don’t generally behave like this, and when you see it, you think there’s something odd going on.”

On Tuesday afternoon, researchers’ fears were confirmed. The pod of almost 100 long-finned pilot whales rushed to the shore, stranding themselves on Cheynes Beach near Albany, in southern Western Australia.

Rescuers raced against the clock to save them. Once a pilot whale — which can grow up to 24 feet in length and weigh up to 6,600 pounds — is out of the water, its organs can be gradually crushed under its own weight. Even when whales are successfully returned to sea, they will often strand themselves again.

By Wednesday, 52 of the whales had died, the authorities said. A team of local volunteer and conservation officials managed to move the remaining 45 back into the water and attempted to herd them back out to sea, using boats and kayaks to guide them.

However, that afternoon, the whales re-stranded themselves further along the beach, the authorities said. Officials later said that the survivors had been euthanized. Peter Hartley, of the Parks and Wildlife Service of Western Australia, said on Thursday that it was “one of the hardest decisions in my 34 years in wildlife management.”

Before the attempt to return the whales to sea, he had said that the animals would be released as a group, but if there were whales in that group that were weak or hadn’t fully recovered, “we stand the risk that that will drag the other animals back to the beach.” Each whale would be assessed to determine if it was ready for release, he added.

Researchers do not know exactly why mass strandings occur. One theory is that they happen when the matriarch of the pod falls ill and swims into shallow water, and the other members of the pod follow, given their tight-knit social bonds, said Dr. Sprogis. Another reason could be that they were disoriented by a loud offshore underwater noise, she said.

It was equally unclear why the whales had huddled in the shallow water before beaching themselves, Dr. Sprogis said, adding that pilot whales generally do not display behavior to indicate that a stranding is imminent.

Mass strandings in Australia are not uncommon. The country’s deadliest such event occurred in 2020, when 470 whales were beached on a coastline in Tasmania, with most of them dying. Two years to the day, another 230 washed up along roughly the same stretch of coast.

A similar stranding event occurred in Scotland a week ago, when 54 whales died on a beach on the Isle of Lewis. By the time they were found, most were already dead, and rescue teams decided to euthanize the surviving animals after determining that the rough waves and shallow beach conditions made it unsafe to refloat them.

Mr. Hartley said at a Thursday news conference that the stranding in Australia had at least provided researchers with a rare opportunity. Scientists from around the world have requested footage of the whales’ huddling behavior, he said, and local researchers will study the pod’s genetics.

“We’re undertaking a number of different samples to try and learn from this really horrible incident, and hopefully we get some really good learnings,” he said.