Shining new light on sperm whales and the importance of Irish waters to them

On a Saturday evening a few weeks ago, locals walking along a remote beach near Glencolmcille on Donegal’s southwest coast spotted a 70-foot male sperm whale. The largest toothed predator on Earth, capable of diving to depths of 3,000-plus metres, this particular creature had been washed up dead, its 25-tonne body already decomposing as it lay on the sand.

These occasional strandings, as tragic as they are, offer us a close-up look at the magnificent whales and remind us of the extraordinary richness of our deep waters. For cetologists – researchers who spend their professional lives studying whales – they are a rare chance for physical contact.

In remote environments, tracking what’s happening thousands of feet underwater is tricky. Sperm whales will dive thousands of metres underwater to hunt for prey, such as squid, and can stay there for up to 90 minutes. It’s rare to see them on the surface, and even when they appear above water to breathe, the most you’ll see is a sighting of their dorsal fins. Researching them off our coast is expensive, weather-dependent and time-consuming. While drones have been of enormous help to cetologists, they’re still insufficient to surmount these immense challenges, and the resulting data is relatively scant.

And so, a dead whale on a beach can be a gift to science and further our knowledge. In December 2019, for example, a sperm whale was discovered washed up on a beach on the Isle of Harris in the Scottish Hebrides. It had died from starvation, and scientists who dissected it soon found out why: when they looked into its stomach, they discovered that it was full of plastic waste, such as fishing nets and debris.

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Cetologists don’t yet know why the Glencolmcille sperm whale perished, but commonly, stranded whales will die at sea. In 2022, two sperm whales were washed up in Donegal, and both were in an advanced state of decomposition. In January 2001, a 30-tonne, 40-foot fin whale washed up in Dingle Harbour but had died in the mid-Atlantic about a month previously, according to local marine experts at that time. The cause of death is frequently from infectious diseases, such as stomach parasites, or starvation.

[ Ireland follows a global trend for increased whale and dolphin beachings ]

Very little is known about the life of sperm whales. It’s known that they breed in the Azores, where adult females, juveniles, and calves are predominantly recorded. In contrast, primarily males are found in the feeding grounds of northern Norway and Iceland. It was long believed that most sperm whales in Irish offshore waters were large males, just like the male animal washed up on the Donegal beach a few weeks ago.

These assumptions have been backed up by historic whaling logbooks and data from strandings, which all indicate that when it comes to Irish waters and sperm whales, it is – mainly – a male-dominated zone.

However, a study published in January sheds new light on their population here. It suggests that a far higher number of younger whales are found, possibly including females and their calves. The paper’s authors, Joanne O’Brien and Cynthia Barile from Atlantic Technological University, Galway, and Simon Barrow from the Irish Whale and Dolphin Ground, looked to technology and acoustic data to help them understand the population structure of sperm whales off our western coast.

Sperm whales are very vocal. They have massive brains contained in their bulging heads, and it’s here that they make their characteristic “click click click” sounds, which enable them to communicate over distances of thousands of miles as well as locate potential prey using echolocation. The structure of each click is based on the size of their head, which grows as they age, and males have larger heads than females. By analysing these clicks, researchers can derive an individual’s size and estimate the structure of the population – their age and sex – in Irish waters.

The researchers put listening devices, made of glass spheres in plastic helmets, at 2,000 metres depth at different locations in the Atlantic. Over two years, they gathered 10,000 hours of sound recordings and then used machine learning to analyse the clicks. From this they were able to categorise the estimated length and sex of the animals.

What they found surprised them. They expected to see a higher percentage of larger males, but in fact adult females and juvenile males were “predominant”, suggesting – for the first time – evidence of a mixture of males and females of various age groups in Irish waters.

All of this adds to our understanding of the importance of Irish waters for cetaceans such as sperm whales, and helps scientists and policymakers find a more effective and targeted way of managing their populations, something which is of urgent importance given their sensitivity to rapid climate warming. Thanks to machine learning, scientists are increasingly mining a wealth of information previously hidden within acoustic data to reveal new understandings of the life of sperm whales in the depths of our seas.

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