In an unusual case of déjà vu, a pod of some 230 pilot whales stranded on Tasmania's western coast on Wednesday (Sept. 21), exactly two years to the day after a different pod of pilot whales stranded on and near the same beach.
The eerie timing of the stranding could well be a coincidence, Rob Deaville, project manager of the Zoological Society of London's Cetacean Strandings Investigation Programme (CSIP), told Live Science. A number of factors can drive cetaceans — marine mammals such as whales and dolphins — to strand, and at this point, it's unclear which factors played into this particular incident, he said.
"These events are obviously terribly unfortunate and terribly sad, but there is a historic precedent for them," Deaville added. "There's a long history, unfortunately, for pilot whales stranding and mass stranding, globally," and Australia happens to be a hotspot for such events, he said.
About 200 pilot whales (Globicephala) so far have died in the most recent Tasmania stranding, and their carcasses may offer clues as to why the animals ended up on shore. The Department of Natural Resources and Environment Tasmania (NRE Tas) will conduct post-mortem investigations on the deceased animals, but for now, "what caused the whale stranding is unknown," the department said in a statement. Even with the post-mortem analysis, the cause of the stranding "may not be able to be determined," the department said.
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"Unfortunately, we do have a high mortality rate on this particular stranding," as most of the animals stranded on Ocean Beach, a long stretch of coastline exposed to the pounding surf, Brendon Clark, an NRE Tas staff member and incident controller for the stranding event, told reporters at a local press briefing on Thursday (Sept. 22). Only a few of the animals were found stranded within the nearby Macquarie Harbor, on a sand flat, NRE Tas reported.
During the previous mass stranding in 2020, many of the pilot whales beached themselves within the estuary of Macquarie Harbor, "so they were actually stranded in much more sheltered waters," Clark said.
That previous stranding involved about 470 long-finned pilot whales (Globicephala melas), which are actually a large species of dolphin, Live Science previously reported. About 30 of the animals stranded on Ocean Beach and more than 200 more were discovered on sandbars located just inside Macquarie Harbor's entrance — a shallow, narrow channel known as "Hell's Gates." About 200 more pilot whales were found further south, in bays within the harbor, The Sydney Morning Herald reported at the time.
In 2020, rescue teams were able to "re-float" and release about 111 of the pilot whales back into deep water, NBC reported. In this week's stranding, rescuers have so far been able to release 32 pilot whales back into the ocean and they aim to rescue three more on Friday morning (Sept. 23), according to NRE Tas. Those remaining three whales were initially inaccessible due to the tides.
Ocean Beach remains closed to the public as the stranding response team continues their rescue efforts, examines the deceased whales and preps the carcasses for disposal, which will involve working with local aquaculture companies to pull the dead pilot whales offshore and deposit them in deep water.
As the response team deals with the immediate effects of the stranding, the question of why the pilot whales beached themselves still looms unanswered.
Generally, "it tends to be the most social species that mass strand the most frequently," Deaville said. Pilot whales — named "pilots" because they follow one another in a line — are known to form strong social bonds, he said. Evidence suggests that, in some cases, mass strandings might occur because one member of the pod becomes ill or injured and ends up stranding and the rest of the pod follows suit. In these cases, the injured or sick cetacean can end up in the shallows after becoming disoriented or drifting ashore with the current.
Related: Nearly 30 pilot whales die after mass stranding in New Zealand
Evidence also suggests that whales' and dolphins' echolocation doesn't work as well in shallow, near-shore waters as it does in the depths of the ocean, Deaville said. So if these animals venture into shallow waters while chasing prey or following another pod member, they may be unable to navigate back to the safety of deeper waters.
Tasmania's western coastline happens to lie near a "shelf edge," beyond which the water deepens and marine mammals often forage, Kris Carlyon, a wildlife biologist with the NRE Tas Marine Conservation Program, told ABC News. Examining the deceased pilot whales' stomach contents may reveal if the whales were hunting near-shore prey prior to stranding, Carlyon said at a press briefing.
The post-mortem examinations could also reveal if any whales were ill, injured or exposed to some sort of toxin before stranding, but "we're not expecting to find anything like that, to be honest," Carlyon said. "These mass stranding events are typically the result of accidental coming to shore, and that's for a whole host of reasons."
Sometimes, those reasons are related to human activity, Deaville said. For example, noise pollution, including that caused by sonar and seismic surveys, can disrupt cetaceans' ability to navigate, frighten the mammals or even injure them, potentially raising the risk of strandings, according to National Geographic.
Given all the factors that can lead to strandings, is there any way of preventing such events? "That's sort of [a] million dollar question — what can we do to stop this happening in the future, given that we know this is a mass stranding hot spot?" Carlyon said at the briefing. For now, that question doesn't have a satisfying answer, he said.
"We'll continue to look at that question, and whether some new technology or new, novel ideas might help out in that regard, but at the moment we simply don't have that option up our sleeve," he said.
Originally published on Live Science.
