The ‘monstrous shark’

Drifting lazily through warm summer waters gave the appearance of basking in the sun, thus earning these sharks their common name. Orcadian fishermen, however, called them the ‘Homer’ or ‘Hoe-mother’ because they resembled the much smaller and more plentiful dogfish (Scyliorhinus sp). In Scotland, basking shark have the Gaelic name Caerban. Their scientific name Cetorhinus maximus comes from ancient Greek words meaning ‘sea monster’ (or whale) and ‘shark’, plus the Latin ‘maximus’ meaning ‘greatest’.

Identification

These very large sharks have a typical lamniform body plan, making them relatives of large predatory sharks such as great white, thresher, tigershark and extinct Megalodon. As with most sharks, they have rough skin. They are dark-coloured and possess very large gill slits and an enormous mouth. The eyes are comparatively small and located towards the tip of the small, protruding snout. They are most often seen swimming slowly at the surface, typically showing a large, floppy dorsal fin with the tip of the tail fin visible just behind. In addition, their small, blunt snout sometimes appears at the front.

Diet and habitat of basking sharks

The basking shark is one of only 3 living species of filter feeding sharks (the others being the megamouth and whale shark). Because they share a similar feeding style, these 3 species exhibit similar characteristics, namely large body, small eyes, tiny teeth and a huge mouth.

These enormous filter feeders swallow plankton and shoals of small fish. But in order to get enough food, they must filter up to 450 tons of water per hour (equal to the volume of an average swimming pool). To achieve this, they swim slowly at about 2 mph (approx 2kn) with their meter-wide mouth open, through concentrations of krill, copepods, shrimp, etc.

Although they swim slowly when feeding they occassionally jump completely clear of the water (‘breaching’). This behaviour may be a social display or to help dislodge parasites.

In their search for plankton, basking sharks travel long distances. Scientists have recorded them near the continental shelf at about 910m depth (2990 ft) and very close inshore, even in brackish water. They have a global boreal-temperate distribution, so they prefer cooler waters and only transit through warmer equatorial seas.

Mating occurs in early summer, with the birth occuring at least a year later during the following summer. Like many shark species basking sharks give birth to fully-developed live young, which obtain nutrition from their mothers via a yolk sac instead of a placenta.

Basking sharks reproduce slowly and may live for up to 50 years. However, our knowledge of this species is still limited by the great depths and distances over which they travel. Their populations have certainly faced huge pressures in the past. In the northeast Atlantic (including Scotland) basking sharks have been hunted for nearly 200 years, mainly using harpoons. Remote island communities depended on the meat and ‘squalene’ oil, the latter becoming especially lucrative for lighting, cosmetics and medicines. Unfortunately, this led to large declines in basking shark populations, with corresponding crashes in the market. Eventually, basking shark was included on the IUCN Red List of endangered species and the postwar UK fishery stopped in 1998. They are now legally protected by the UK Wildlife and Countryside Act (and by similar laws elsewhere) and populations of these magnificent, mysterious fish are slowly recovering.

🦦 Gavin Maxwell ran a short-lived basking shark fishery based from the island of Soay, near Skye in Scotland. He was also author of one of Britain’s most famous nature books, Ring of Bright Water.