How to solve a mass stranding: what caused 77 healthy whales to die on a Scottish beach?

A mass stranding last week that led to the deaths of 77 pilot whales on the Orkney island of Sanday was the largest ever recorded of the species on British shores. Initially, 12 of the animals at Tresness beach were still alive – but sadly did not survive.

The event occurred almost exactly a year after the stranding of 55 pilot whales on Tolsta beach on the Isle of Lewis in the Hebrides on 16 July 2023. All but one of those whales died. According to Dr Andrew Brownlow, director of the Scottish Marine Animal Stranding Scheme (SMASS) at Glasgow University, this may not be a coincidence.

“The Orkney stranding last week has so many parallels with what happened a year ago, in terms of number of animals involved and their behaviour,” he says.

Brownlow’s research proposes a drastic scenario: that mass strandings are increasing exponentially – in the numbers of animals and events. There have been about 13 mass strandings of pilot whales since SMASS was started in 1992, 10 of which have been in the last decade. Evidence indicates that the situation is only going to get worse.

Long-finned pilot whales can reach more than 7 metres in length and are found in temperate waters around the world. These sleek black animals are named after their apparent propensity for following a lead or “pilot” whale – hence their near-suicidal urge to accompany an ailing individual ashore. Often found together in great numbers, pilots are among the most likely cetaceans to become stranded .

Blame in the past has been placed on severe weather conditions, illness and solar storms disrupting the whales’ natural navigation system and deceiving them into swimming on to the shore. But are these the reasons for the most recent stranding and rise in events over the years?

A team of 22 scientists from SMASS and the Cetacean Strandings Investigation Programme from London’s Institute of Zoology is working at the Sanday site, along with strandings officers from Cornwall and Wales, in a race against time: a sort of “CSI whale”.

Many attended last year’s stranding, when they were unable to make full postmortem examinations of the animals due to delays and hot weather causing rapid decay.

However, Mariel ten Doeschate, one of the scientists, says this time they have been able to perform postmortems on 20 animals.

“With squid in their stomachs, it was clear that the animals had recently fed,” she says. Ruling out illness as a reason for the stranding, she explains that these were “very healthy whales”, comprised not of one pod, but a “mixed-clan aggregation … several different pods that come together to breed”.

Among them were two calves aged two or three weeks old, and at least one of the females was found to be pregnant.

If these waters are noisy, then that’s a hazard for animals that have a herd mentality, that are easily spooked

Dr Andrew Brownlow

Crucially, Doeschate notes that their position on the beach was “clustered round key animals in the group”. This strongly suggests they had been frightened into coming ashore as a “stress response”.

They might have been fleeing predators – orcas had been seen in the area, she says. But the scale of the Orkney stranding may prove long-held suspicions: that extremely loud sounds, caused by people, were responsible.

The evidence for such damage is found in tiny hair cells embedded within the organ of Corti, which converts sounds into electrical signals that can be transmitted to the brain through the auditory nerve. This is the holy grail of whale biologists, embedded in the animals’ walnut-sized earbones, or cochleas, which are themselves buried deep within the whale’s skull.

It is a paradox that the fate of these huge animals might come down to such tiny things. The hair cells, which can be scarred by severe sonic events, must be delicately extracted from the cochlea.

Unfortunately, the bone is so dense that samples will have to spend up to a year softening in a chemical solution in a laboratory before the hair cells can be examined. Doeschate says that the team have successfully retrieved six cochlea for analysis.

We’ve got to be really careful about what else we are doing in those waters

Dr Andrew Brownlow

Brownlow sees the Scottish strandings as a result of a fateful combination of factors. First, the warming waters around Scotland, which are bringing in new prey for the pilot whales and other cetaceans to feed on; striped dolphins, more used to Mediterranean temperatures, are now the most common oceanic dolphins seen off Scotland, says Brownlow.

Second, the pilot whales are taking advantage of those warmer waters to calve there. Third, and crucially, they are doing so off irregular coastlines with which they are perilously unfamiliar.

It is a plaintive notion. The whales are lured into waters by the promise of food and warm conditions in which to give birth, only to die en masse.

“In last year’s stranding there was a significant number of animals that were pregnant or in the process of giving birth,” says Brownlow. “So they’re using these waters as calving areas.

“But the problem with that is, if these waters are noisy, then that’s a dangerous hazard for animals that have a herd mentality, that are easily spooked, and you have some complex beaches that are difficult to navigate round and are opaque to their sonar,” he says.

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Ironically, a fourth factor in this “perfect storm” of conditions lies in our own success in halting most commercial whaling in 1982 and the subsequent rise in whale populations. Brownlow says: “While these whales were gone, we didn’t take our foot off the throttle. Now they’ve come back into a much more industrialised, more hazardous arena than the one they evolved in.”

Since the 1980s, researchers have cited the damaging effect of noise pollution on whales and dolphins, from seismic surveys for oil and gas to military sonar . But Brownlow counsels caution, suggesting that natural earthquakes may also have the same effect.

Whatever the reasons for the Orkney event, its consequences are serious, not only for cetaceans, but for the health of our seas. A measured and careful scientist, Brownlow nonetheless delivers a stark warning: “We’ve got to be really careful about what else we are doing in those waters.

“Otherwise,” he says, “this is going to become a horrifically common occurrence.”