Carcasses can explode if left to decompose on land and should be towed back to sea ‘where they belong’, experts say

Two mass strandings in Tasmanian waters in a week has left about 200 pilot whales and 14 sperm whales dead.

On Monday, 14 juvenile sperm whales died and washed ashore at King Island, in Bass Strait. Approximately 230 pilot whales became stranded on Ocean Beach, west of the Tasmanian town of Strahan on Wednesday.

Tasmanian authorities said on Thursday that they would be transitioning to “carcass recovery and disposal operations” in the coming days. But how do you safely dispose of the massive beasts?

If cetaceans are left onshore where they have stranded and died, their decomposition can pose a biohazard risk, said Dr Olaf Meynecke, of Griffith University’s coastal and marine research centre. “The removal of the animals is a major issue and something that we kind of forget once a rescue mission is over.”

In warmer climates, the internal decomposition of dead whales can result in spontaneous explosions. Gut bacteria in the whales can multiply quickly, producing large quantities of methane gas. “If the rest of the body is still intact – if the outer layer, the blubber, is still intact and not broken up – then it can lead to an explosion,” Meynecke said.

In 2004, the decomposing carcass of a 60-tonne, 17-metre sperm whale exploded on a busy street in the Taiwanese city of Tainan, “showering cars and shops with blood and organs and stopping traffic for hours”.

Researchers would probably carry out checks on the recently stranded animals, including necropsies to look at gut content, and assessing overall health indicators such as the thickness of their blubber layer, Meynecke said.

Usually necropsies cannot be conducted more than after a few days after a whale dies, due to the risk of explosion, he says. “It’s actually part of the risk assessment … the animal has to be assessed beforehand and if there are signs of swelling in the gut area, the pressure needs to be released ahead [of the necropsy].”

“If there’s anything of benefit, it’s that the dead individuals will be an opportunity to contribute to science,” said Dr Vanessa Pirotta, a wildlife scientist affiliated with Macquarie University, who described it as the silver lining of a sad situation.

“We can learn more about their diet, their genetics, how similar these individuals were to the population that stranded before,” she says, referring to a 2020 mass stranding event at the same location, in which 350 pilot whales died.

Cetaceans that die onshore after stranding should be towed out to the ocean, Meynecke said. “They should be returned to sea – that’s where they belong.”

Sam Gerrity, of Southwest Expeditions, has been involved in the logistical effort after both the most recent and 2020 mass strandings near Strahan. He said the disposal involved a “pretty confronting” process of towing dozens of carcasses out to sea.

Open decomposition and burial were both trialled after the 2020 pilot whale stranding, but authorities have said they are not the preferred methods for the most recent stranding. “Our first option will be long-lining the carcasses out to deep ocean,” the incident controller Brendon Clark, said at a press conference on Thursday.

But the logistics for larger whale species are far trickier than pilot whales, which weigh up to three tonnes. “[For a sperm whale] we’re looking at probably over 15 tonnes or more. Once they’re not in water any more, they become too heavy to drag with normal equipment,” Meynecke said.

Sign up to receive an email with the top stories from Guardian Australia every morning

Burying whales should be avoided, he said. “Disposing of a sea animal on land is generally not a good idea. The animals will decompose a lot slower once they’re buried … it will take months and it’s a very slow process.”

In 2017, a New South Wales council buried an 18-tonne humpback whale at Port Macquarie’s Nobbys beach and then excavated it a week later, due to community concerns about heightened shark activity.

“If you’ve got a connection to the water table there’s a chance that it does leak out into the ocean – it could potentially attract predators but … that’s not fully proven,” Meynecke said.

An infamous whale disposal case occurred in the US in 1970, when the Oregon Highway Division tried to get rid of a decaying sperm whale by blowing it up with dynamite.

“The humour of the entire situation suddenly gave way to a run for survival as huge chunks of whale blubber fell everywhere,” said a reporter in a now-viral TV story.

Meynecke called the incident “proof of human stupidity. We laugh about it, but it’s the same thing as burying something – only because we can’t see it, it doesn’t mean it’s gone, and only because we blow it up it doesn’t mean it’s gone – it’s just distributed in smaller pieces and it creates more problems.”

Why mass whale strandings occur is still not entirely clear. Pilot whales – misnamed as they are actually a large oceanic dolphin – are known as the most susceptible species to mass strandings, because they are highly social and form pods of several hundred.

“They end up in these big groups, but they don’t know each other very well,” Meynecke said. “If one of them starts to panic … there is a lot of miscommunication, because they don’t actually know each other and the calls don’t make sense to them.” He likened it to panic among humans at a concert or other crowd. “There is that emotional stress that actually drives them to continuously restrand as well.”

Sperm whales, however, do not usually mass strand, and the deaths of more than a dozen at King Island was concerning, Meynecke said.

“It probably isn’t a coincidence that these two species stranded at similar times, because they might have been looking for prey closer to the islands,” he said. “We do have drastic changes in the marine environment related to climate change. That’s also what was related to the stranding of sperm whales in Europe in 2016.”

That incident was linked to changes in water temperatures and the movement of food sources into shallower waters in the North Sea. “We might see more of these strandings in future,” Meynecke said.