Tragedy in Tasmania: what are pilot whales, and why do they strand themselves?

Tragedy in Tasmania: what are pilot whales, and why do they strand themselves?

About 380 pilot whales have died in Tasmania in one of the biggest mass strandings of the marine mammals on record.

Rescuers saved at least 70 of about 470 animals that got stuck on sandbanks and beaches mostly inside Macquarie Harbour on the state’s remote western coast.

But as a rescue mission turns to retrieving and carting hundreds of dead mammals into the open ocean, some weighing more than a tonne, many are asking how such a tragedy could unfold.

What are pilot whales, and are they really whales?

Long-finned pilot whales are known for stranding in mass groups. But these “whales” are misnamed.

“The name doesn’t fit them – they’re actually a large dolphin,” says Dr Emma Betty, of the Cetacean Ecology Research Group at Massey University in New Zealand.

“All their life history and evolution makes them more related to a bottlenose dolphin than to a humpback whale.”

Betty is an expert on the life history of long-finned pilot whales and stranding events. Much of what is known about the species, she says, comes from stranding events.

Pilot whales spend almost all of their time in the deep ocean, making them hard to study.

Long-finned pilot whales feed on squid and fish below 200m, but can dive to 1,000m and hold their breath for half an hour.

Betty says they spend time in pods of 20 or 30, but sometimes form temporary “super pods” of up to 1,000 individuals – a behaviour she says is probably to help them feed en masse or to avoid inter-breeding.

David Hocking, a marine mammal scientist at Monash University, says these mammals push air through their nasal cavity and then through a bulge on their skull – known as a melon – that allows them to focus their clicks.

“It means they can hunt in the dark and catch fast-moving fish and squid,” he says.

Evidence suggests female pilot whales can live to 40 years or more – about 10 years longer than males. They form tight-knit groups, with some individuals staying in the same pod for life.

Pods are usually led by dominant females and they can use their echolocation to communicate. They can alert the group to a food source, potential danger, or that one of them is in distress.

But it is these traits – the close bond and the ability to communicate with echolocation – that can be their undoing if they find themselves too close to shore.

Why would so many get stuck in that spot?

Macquarie Harbour is a known hotspot for pilot whale strandings, which frequently occur in shallow, sloping and sandy areas.

Betty says: “What continues to be the strongest factor is the topography that forms natural whale traps. The whales are unfamiliar with this topography and also probably how the water rushes in and out with tides.

“They have trouble with the echolocation. Because of the shallow slope they don’t get a clear picture. They’re failing to detect the proximity of the shore until it’s too late, whereas coastal species don’t have that problem.”

Kris Carlyon, a wildlife biologist with the Tasmanian government’s marine conservation program, has said the pod may have come close to the shore to feed, although the exact reason may never be known.

Once the pod was close to shore, Betty says, it’s likely they became disorientated. If a few took a turn into the harbour, the rest may have followed.

If some of those whales got stuck on a sandbank they could have sent out a distress call, with more and more of the pod following in.

Was the Tasmania stranding unusual?

Pilot whales are the most susceptible of any of the cetaceans – that’s whales, dolphins and porpoises – to mass strandings.

The five largest stranding events in Tasmania have all involved pilot whales. Australia’s previous biggest stranding event also involved pilot whales – 320 of them got stuck in Western Australia in 1996.

There is no global database of cetacean strandings, but Betty says the 470 at Macquarie Harbour may be the third largest on record.

Still considered to be the biggest was a 1918 stranding of about 1,000 pilot whales at Chatham Islands in New Zealand.

In 2017, about 600 long-finned pilot whales stranded in Golden Bay on New Zealand’s South Island.

Could there have been a human cause?

There is no evidence the Macquarie Harbour stranding was anything more than anatural tragedy.

While there is evidence that seismic testing and sonar can interfere with some marine mammals, Betty says the location and the similarity to past events all point to a natural phenomenon.

The 1918 stranding off New Zealand occurred before sonar or seismic exploration was being used.

What is it like to witness a whale stranding?

Betty has attended several mass pilot whale strandings and, while they’re widely accepted to be natural events, she says they are still a tragedy.

“They are horrible things to have to deal with, but we do know that if you can refloat them, then there’s a chance of success. If they’re healthy, there’s a chance to save them.”

She says some of the “vocalising” that rescuers hear will include distress calls, or mothers calling out for their calves.

This also makes saving the mammals a challenge.

On the first day of the Macquarie Harbour rescue, at least two whales that were led into the open ocean returned to their stricken pod mates – such is the strength of the instinct to be together.

Betty says: “They are communicating as a group. They do have an ability to communicate different things.”

There is no reliable estimate of how many pilot whales there are in the ocean or if their numbers are going up or down.

“We don’t know how much of an impact this [mass stranding] could have on the population,” Betty says.

“We don’t know the impacts of losing numbers in large events like this.”

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