Basking sharks are not sharks at all in the ‘Jaws’ sense, so there is no need for panic when the huge form of this gentle browser of the coastal waters is seen just beneath surface off the Pembrokeshire coast.
The sight of a basking shark is one of the bonuses of a boat trip to the bird sanctuary island of Skomer, and, if one can get close enough to appreciate the bulk of this marine creature, the second largest ‘fish’ in the sea, it might be possible to discern, just below the surface, the huge gape of its mouth as it filter feeds on the plankton which is its staple diet.
Its Latin name Cetorhinus maximus means ‘greatest monsternose’ or ‘greatest whalenose,’ but it is not a whale, which is a mammal, but a species of shark related to the dogfish, tope and other sharks, and the second largest fish after its cousin the whale shark. It is called a basking shark because observers thought its presence just under the surface of the water, with its pointed dorsal fin protruding, indicated that it was basking in the sun. But it is near the surface that the plankton it survives on is concentrated as it rises in upwelling currents from deep depressions on the seabed. Basking sharks swim slowly along filtering up to 2,000 tons of seawater an hour through their huge mouths and gills to extract the zooplankton they exist on.
They are huge fish measuring an average of 20 to 26-feet and weighing around six tons, although a giant caught in a herring net in the Bay of Fundy, Canada, in 1851 measured 40-feet and weighed 21 tons. They are typically shark-shaped and have been mistaken for great whites, but unlike the great white’s dagger-like teeth the basking shark’s are tiny, and another distinction is the huge gape of the metre-wide mouth and the gill slits which almost encircle the head. Despite their normally slow-moving progress at around two knots through the water they can breach clear of the surface, in what may be attempts to shake off parasites or merely displays of size and strength for the benefit of their few predators, which appear to include only killer whales and great white sharks. The basking shark, which has a life-span of about 50 years, is hunted in some parts of the world for the leather from its thick hide, its large, oily liver and, by Chinese and Japanese fishermen, for its fins, used to make the supposedly aphrodisiac delicacy shark-fin soup. Satellite tagging has revealed that basking sharks move thousands of miles during the winter as they follow plankton blooms. Small schools have been observed in the Hebrides swimming nose to tail in circles in what may be a mating ritual. They can submerge as deep as 3,000 feet. They are not curious about boats or divers, and it goes without saying that they are harmless to humans.