Circles in the sea: the mystery of basking sharks

At this time of year basking sharks are first reported off Ireland’s coasts. They feed on plankton, following blooms of the tiny critters into warm summer waters. Second only in size to the whale-shark, these 5 tonne fish are typically solitary creatures roaming across the north Atlantic from Europe to North America. But in 2022 Dr Simon Berrow was surrounded by sharks on a small boat off Co Clare.

“Everywhere we went, I could see fins,” says Berrow, a founding member of the Irish Basking Shark Group. They broke the surface, dark shapes thrashing and churning the sea.

To make sense of the chaos, Berrow, who is also a lecturer at Ireland’s Atlantic Technological University, launched a drone. As it went skyward, the distinctive geometry of the sharks’ movement was revealed. They were circling, some 12 sharks following each other in a tight radius.

The fact that basking sharks often travel alone makes such spellbinding behaviour a startling thing to observe. But these formations are not unknown; they were first spotted in Ireland by filmmaker Ken O’Sullivan in 2016. Scientists describe the gyrating sharks as a torus, understood to be a courtship behaviour, a sort of slow-paced shark speed-dating.

Election campaigns well under way as Ming pips the first televised debate

---

They say ‘Dublin 1 is for the far-right’ and threaten to kill a politician, yet Ireland ignores the threat

---

Your top stories on Tuesday: Gardaí to use lorry to see into cars and catch distracted motorists; plan to extend Dart to Drogheda

---

From the surface the torus is visible only as a mass of writhing fins, but aerial or underwater perspectives give insight to what is happening. In Nova Scotia aggregates of up to 1,300 sharks have been observed from aircraft; and divers off the coast of Donegal have spotted more than 30 basking sharks circling at a depth of 20m.

Within sight of Co Clare, Berrow moved from one torus to the next, barely able to keep up with the unimaginable numbers of sharks. “I’m absolutely convinced there were hundreds, if not thousands of sharks. When you see multiple toruses at once, you get a sense of something mysterious.”

Indeed, circles are enigmatic shapes. They are also relatively common and perhaps the most recognisable pattern to appear in nature. They are in the spots of a butterfly’s wing, the spiral of a snail’s shell, the sun and moon. Earth itself is a circle, as is the eye that observes it. The eminent nature writer Ralph Waldo Emerson described the circle as “the first of forms…the highest emblem in the cipher of the world”.

Why would all the sharks in the north Atlantic come for the coast of Ireland? We don’t know, but it’s significant

The circle is simultaneously one of the oldest and most powerful symbols of geometry and spirituality. It embraces depth and height, writes Irish poet and philosopher John O’Donohue: “The circle never gives itself completely to the eye or to the mind but offers a trusting hospitality to that which is complex and mysterious.”

The mystery of the circle is more than a metaphor when it comes to basking sharks. While scientists can be quite certain about the courtship function of the torus, the sharks’ reasons for gathering in such huge numbers around Ireland are not yet known.

[ Bluebells: native flower under threat from larger, invasive species ]

[ Calls for larger deer culls after record 55,000 shot dead last year ]

“Why would all the sharks in the north Atlantic come for the coast of Ireland?” muses Berrow. “We don’t know, but it’s significant.”

After years of research, the fact that Berrow is so full of questions when observing a torus is something that worries him. If we are still to understand aspects of this particular mystery, what secrets might still be held in Ireland’s depths? His concern is that knowledge about the sea won’t be able to keep up with the rate of decisions made about it, from deep-sea mining to marine protected areas.

Last year basking sharks were granted status as a “protected wild animal” under Ireland’s Wildlife Act. Nevertheless, Berrow says they deserve better recognition as an emblematic species: “Wouldn’t it be wonderful if basking sharks were ingrained in our psyche?”

Their Irish name, ainmhí sheoil, translates as “the beast with the sail”, alluding to the dark fins that beat at the surface. These fins are described in folklore as serpents twisting and writhing along the sea.

Basking sharks have enchanted our coasts for 30 million years and the work of Berrow and other researchers is beginning to decipher their mystery. With a small grasp on these peculiar circles of the sea, he moves to the next question: why do basking sharks gather in the hundreds, possibly thousands, off the coast of Ireland?