BBC Wildlife Magazine
The second largest fish in the world frequents UK waters in warmer months but we know relatively little about it. Scientific studies are now delving deeper into the mysteries of the basking shark.
Somewhere in the wilderness of the Atlantic Ocean, four of us bob around like corks, kept afloat by buoyant wetsuits. The sea surrounds us, glittering under the June sun. There is no land for miles – in fact, facing west, nothing lies between us and the USA. Beneath our fins stretches a vast expanse of turquoise, eerily quiet and seemingly empty, made more daunting by the knowledge that it is anything but. Our boat is just behind us – far enough to limit disturbance, but close enough to scoop us up in a heartbeat – and yet there is a sense of vulnerability. We are, quite literally, out of our depth. This world belongs to cryptic giants, who have roamed this territory long before we even existed. One of which is heading straight towards us.
I motion for my group to stay still. Just like we practised, they lay flat on the surface, fins up and heads down. Their eyes are wide with a mixture of excitement and nerves, which is quite understandable. The fin that is currently advancing towards us stands almost 1m tall and belongs to the second biggest species of shark in the world – the basking shark. This situation was no accident. We were there to encounter these animals in their natural environment. But despite their enormous size – they average 8–10m in length – the basking shark poses no threat to humans. They are filter feeders, perfectly adapted to a diet consisting exclusively of tiny zooplankton. Good news for us, but a bit bizarre when you consider that an individual shark can weigh up to five tonnes, yet subsists on organisms millimetres in size – that’s like us existing solely on poppyseeds.
I wait for just the right moment, to ensure the shark won’t divert at the last second
(they have a tricky habit of doing so), and tell my companions to look down into the blue. For a split second, there is nothing except a murky plankton soup. But suddenly, an immense shadow begins to approach from the gloom. A cavernous mouth, almost 1m wide, is heading in our direction, the gill rakers starkly white – these are comb-like structures that act as a kind of sieve, trapping zooplankton as seawater passes over the gills. Basking sharks are known as passive filter feeders, meaning that they rely on forward movement to push water into that gaping mouth and out through the gills, which completely encircle the head.
Basking sharks are often found congregated at tidelines, using the force of an incoming or outgoing tide to shovel as much food in as possible, with minimum effort. To maintain such a huge size, these animals are actual feeding machines, capable of filtering almost 1 million litres of seawater per hour. With such a low-energy food source, they move slowly and deliberately and, without the need for complex hunting strategies, have a brain of about just 10cm. They truly are gentle giants, thinking of little but where the next buffet is. For this reason, with the right training, care and respect for these animals, we can get close enough to observe them in their natural environment.
I am lucky enough to spend my summers as a guide for Basking Shark Scotland, a wildlife tourism operation based on the west coast. We have a dual purpose. We take very small groups to encounter sharks in the wild, to educate and raise awareness about the species and the wider marine ecosystem. But we also use this opportunity to gather vital data, such as sex, size and any markings that may help us to identify individuals returning to the same area. One of the most alluring things about basking sharks is that we know relatively little about them. Much of their life is shrouded in mystery.
For example, we know that from May to September basking sharks return to coastal surface waters after spending winter at deeper depths offshore. It is this sun-seeking habit that gives the species its common name and is driven by an abundance of food, following a chain of events. In Spring, oceanic and weather cycles create ideal conditions for phytoplankton – microscopic organisms that photosynthesise, and thus rely on sunlight to produce energy – to inhabit surface waters and explode in vast numbers. Shortly after, animal-based zooplankton – which includes everything
Basking shark migrations are a little like extended family having a reunion at their favourite restaurant.
from tiny shrimp-like creatures to jellyfish – also appear in great quantities to feast on this phytoplankton bloom. And that in turn lures the planktivores – filter-feeding sharks, baleen whales, turtles and bony fish (and their predators).
This magical sequence of events happens in nutrient-rich waters, and we are lucky that Scotland has some of the richest waters in the world, thanks to the ocean currents. We find aggregations of basking sharks in ‘hotspots’ around the west coast, where currents draw dense, cooler, nutrient-rich water towards the surface in a process known as upwelling, and support plankton blooms. Some of these hotspots are concentrated around the Hebrides, particularly Coll and Tiree, and form the sites for our expeditions.
Scientific studies have provided fascinating insights into the life of the basking shark. A team of scientists from across the world
– led by the University of Aberdeen – used genetic tags to study shark movements in UK waters, and found evidence that individuals return to the same site year after year – some even at approximately the same date. Furthermore, sharks revisiting the same sites were more related than expected, suggesting basking shark migrations are a little like extended family having a reunion at their favourite restaurant. Basking sharks are highly migratory, with satellite tagging and telemetry data showing evidence of trans-Atlantic and even transequatorial movements, of up to thousands of kilometres.
Sharks have travelled from as far as Newfoundland, the Bay of Biscay and the Azores and, during winter, plunge to depths of over 1,000m. What they do during this time, however, remains a mystery. Given the offshore locations and inaccessible depths to which they descend, scientists can, at
present, only speculate. Some scientists believe they follow plankton, and in effect feed year-round. But we don’t know for sure. The question remains… what on earth are basking sharks up to after summer?
One huge enigma is mating and reproduction. These parts of the life-cycle are hidden from us and have never been scientifically recorded. There is some anecdotal evidence of sharks ‘thrashing’ around in pairs at the surface, including observations from our own boat. We have also witnessed sharks closely following one another almost nose-to-tail. This has been interpreted as possible pre-mating behaviour, though a 2019 study led by marine biologist Mauvis Gore concluded it was more likely to be feeding-related, as individuals took advantage of hydrodynamics.
Another mysterious behaviour is breaching, where individuals launch themselves clean out the water – quite a phenomenal sight to behold. The reason why is unclear. A popular theory is parasite removal; sharks are often seen with parasitic lampreys attached to their underside. Yet this is quite an exhaustive way of shaking off irritants. Other breaching species – such as humpback whales – are thought to use this
Basking sharks were once extensively hunted for their liver.
behaviour to convey information. Possibly, it could be a signal to other sharks in the area that they are available for mating, or a display of dominance by males. Other observations from Donegal in Ireland seem to suggest breaching is potentially stimulated by environmental factors, such as sea surface temperature. Certainly, more research is needed to study this behaviour in relation to sex and other potential triggers.
Another secret is where, and how, basking sharks give birth. From very limited evidence, it appears they are ovoviviparous, meaning that the new-born pup breaks out of the egg while still inside the mother, and is effectively born already hatched.
The only existing eye-witness account of pupping comes from two Norwegian fishermen, Hans Goksoyr and Jonas Sordal, in 1936. Basking sharks were once extensively hunted for their large and oily liver – used in lamps, cosmetics, and lubricants. Large fisheries existed in Scotland, Ireland and Scandinavia, and exploitation almost drove the species to the brink of extinction.
While towing a harpooned shark back to the fishery, the Norwegian farmers described a live shark being birthed and swimming away, closely followed by four others, with one other dead inside the mother. This tiny snippet of information implies that basking sharks have litters of up to six and are possibly K-selected – investing more in smaller, but more developed, young. Juvenile sharks have been sighted – conspicuous for their more pronounced, twisted noses, which straighten out over time – however, their entrance into this world is yet to be scientifically recorded.
Learning more about this mysterious species is vital to its protection. Distinguishing breeding, mating and feeding sites can help us identify potential overlap with human activities, informing protective measures in these areas. The dense aggregations we observe around Coll and Tiree – as well as the possible mating activity – suggest that this is a highly important area for the species. A Marine Protected Area (MPA) has been proposed for the Hebrides, to protect the basking shark and other mobile species that depend on these waters, such as minke whales and Risso’s dolphins. Though MPA designations are challenging – especially for migratory species – this could provide vital safeguarding for an area key to the basking shark’s survival.
Back in the gently undulating waters of the Atlantic, we watch the shark swim past, so close you can look her in the eye. For one so big, she moves with ease through the water, her size almost too much for the human brain to comprehend. The girl next to me grabs my arm and squeezes it tight. I turn to see her beaming face and slightly fogged-up mask; the others over her shoulder look just as exhilarated. The encounter was just a few seconds, but nothing compares to a 9m shark just casually passing you by.