Basking shark: the biggest animal you will ever meet in Irish water, and it has no interest in you

The classic sighting is two fins moving slowly through calm water with a surprising gap between them — dorsal fin in front, tail behind. People assume they are looking at two animals. They are looking at one, and it is very large.

The animal

Cetorhinus maximus, the basking shark — the second largest fish in the world after the whale shark. Adults commonly run to seven or eight metres and the largest reliably measured animals were considerably bigger. Grey-brown, with gill slits so long they very nearly meet around the throat.

It feeds with its mouth open, filtering plankton from the water it swims through — no chasing, no biting, nothing that involves you. The teeth are tiny and functionally irrelevant. A basking shark is about as dangerous as a cow, with the same caveat: it is very big, and you should respect what a tail that size can do without meaning to.

What the Irish name tells you

The Irish name is liámhán gréine — roughly, the sun-fish: the animal that lies up at the surface in the sun. English does the same thing, calling it a basking shark. Both names describe a habit rather than the animal, and that habit is exactly what made them easy to find, and easy to kill.

Ireland ran a substantial basking shark fishery, most famously off Achill, where thousands were taken through the middle of the twentieth century for the oil in their enormous livers. The fishery collapsed because the sharks did. They are slow-growing and slow-breeding, and populations recover over decades, not years.

They are now a protected species in Irish waters. Seeing one off the Clare coast is a sighting of an animal whose population is still climbing back.

When and where

Spring and early summer are the classic season along the west coast, when plankton blooms bring them to the surface inshore. They follow the food — fronts, tide lines, headlands where water is being pushed together. Calm days with a visible slick or line on the water are worth watching.

But the spring feeding season is not the whole story on this coast, and in recent years the end of the summer has produced something far stranger.

The circles: late August and September off Clare

In late summer, basking sharks off this coast stop feeding and start circling. Groups of them gather and swim slowly round in a ring at the surface, with more animals turning in the same circle below — not a flat ring but a three-dimensional doughnut of sharks. Researchers call it a torus.

This is not a rumour or a local story. Between 2016 and 2021, scientists from the Marine Biological Association, the Irish Basking Shark Group and colleagues recorded 19 of these circling groups off the coast of County Clare, using drones and underwater cameras, and published the findings in 2022. Each torus held somewhere between six and 23 sharks.

What the footage showed is that this is courtship. The sharks in the circles were sexually mature, in roughly equal numbers of males and females, and they were not feeding — the mouths were shut. Females appeared paler than males, a colour change seen during courtship in other sharks. The animals touched fin to fin and fin to body, and rolled over to show their undersides to the shark behind them. The circles form at the end of the feeding season, when the shoals gathered at plankton-rich thermal fronts switch from eating to following one another.

The significance of that is hard to overstate: this stretch of the Irish coast is the first courtship habitat identified anywhere in the world for a species nobody has ever seen mate.

An endangered animal found across the North Atlantic, and the one place on earth it has been documented courting is the water off County Clare.

And the jumping

The other thing people report is breaching — a shark the length of a minibus coming clean out of the water and landing flat. It happens alongside the circling, and Loop Head has produced the highest reported breaching activity on the coast. In one three-day spell in September 2020, 93 separate breaches were recorded off County Clare. The numbers involved can be extraordinary: 125 basking sharks were counted in a single sighting at the Bridges of Ross on 11 August.

Why they jump is not settled. The leading idea is that it is part of the same courtship display as the circling — a signal, and possibly a demonstration of fitness. The other long-standing explanation is parasite removal: lampreys attach to basking sharks, often around the pelvic area, and a hard landing may be an attempt to dislodge them. The two are not mutually exclusive, and honest answer is that nobody knows for certain yet.

If you are anywhere near Loop Head or the Bridges of Ross on a calm day in late August or September, it is worth an hour with your eyes on the water. Look for a slick or a tide line, and for a patch of sea with an unusual number of fins in it. A torus from a clifftop reads as a slow, deliberate wheel of dark shapes that does not break up the way a feeding shoal does.

They remain unpredictable year to year. Some seasons produce numbers along a stretch of coast and the next produces almost nothing. Satellite tagging has shown they do not simply vanish in winter — they go deep and they travel a long way, including well south of Ireland.

If you are near one

This matters more than usual around a torus. These animals are courting, they are endangered, and there is nowhere else known on earth where they do this. A boat driven into the middle of a circle breaks up the one behaviour we are only beginning to understand.

• In a boat: hold a steady slow course, do not drive at them, and never cut across in front of a shark or into a circling group. Put the engine in neutral rather than manoeuvring around them.

• In the water: stay to the side, never in its path, and well clear of the tail. Do not touch it — the skin is covered in hard tooth-like denticles that will take the skin off you, and it is a protected animal.

• Remember a breaching animal weighs several tonnes and does not know where your boat is. Give a group that is breaching a very wide berth.

• No flash, no diving down on top of it, no herding. Let it hold its line.

• Report it. Sightings logged with the Irish Basking Shark Group and the Irish Whale and Dolphin Group are genuinely useful data — the Clare research was built on exactly this kind of reporting.

The animal is filtering plankton with its eyes half shut, or courting. Everything that goes wrong in these encounters is caused by the people, not the shark.

Conditions for Kilkee are live on our home page. Flat, bright spring days are good for the feeding season, and calm days at the end of August and into September are when to be watching for the circles — and if you see two fins with a gap between them, that is one animal, and it is worth stopping for.