Basking Shark

Very little information is known about the natural ecology and behaviour of the basking shark. It receives its common name from its feeding behaviour, when individuals appear to be 'basking' on the water's surface, swimming very slowly with their entire dorsal fin out of the water (2). These sharks feed passively (unlike the also plankton-feeding whale shark and megamouth shark (Megachasma pelagios) which can use its head muscles to suck water into the mouth), merely by swimming through the water with their mouths gaping (2). As water passes over the gills, plankton are retained; a fairly large shark can filter roughly 1,500 cubic metres of water an hour (6). These giant fish have occasionally been observed leaping out of the water (2), which is probably related to social behaviour (12). Basking sharks are usually solitary, although pairs and groups of up to 100 individuals have been seen (2). This species mysteriously disappears from coastal waters in the winter months and it was recently suggested that they 'hibernate' in the deep water. It is also thought that during this time of low food availability basking sharks shed and then replace the gill rakers (11). This suggestion has been refuted by scientific satellite tracking of sharks, revealing extensive migrations throughout all seasons (13). The only pregnant female ever caught gave birth to six live young; the prevailing view is that that these sharks are ovoviviparous (8), and it is likely that they only give birth every two to four years (9).

The basking shark is now protected in the territorial waters of some of the countries, including the UK, in which it occurs (2). In 2002, this species was accepted onto Appendix II of the Convention on International Trade in Endangered Species (CITES), thus reducing international trade in basking sharks (4). A three-year research programme in Britain is currently underway (9). The attraction of these large and appealing creatures for ecotourism may also benefit their conservation.

The basking shark is the second largest fish in the seas, after the whale shark (Rhincodon typus); its maximum size is thought to be at least 10 metres long (6). This creature is a gentle giant however, filter-feeding on plankton through the five massive gill slits that almost encircle the head (2). Thousands of fine, bristle-like 'gill rakers' adorn each of the gill arches within the slits (7). The basking shark has an extremely distinctive body shape with its conical, almost pointed snout and large dorsal and pectoral fins that can reach as long as two metres each (8), and a crescent-moon shaped tail (2). The body is a greyish brown colour, either all over or with a paler shade underneath, and is covered with a layer of mucus (9). The large mouth, which may gape one metre across (8), contains many small hooked teeth (2); more than any other shark (9). This species has a particularly large liver that can weigh up to 25 percent of the body weight and provides buoyancy for its oceanic life (2). Juveniles have a distinctive hook-like snout, which changes shape during the first year of life (2).

These sharks are found at the surface of coastal waters during the summer to feed on seasonally abundant copepods which bloom in frontal areas during spring and summer, but it is thought they migrate further offshore or to deeper waters during winter (11).

The basking shark is found throughout the world mainly in cool and temperate waters (7), although some sharks have recently been tagged, and found in tropical waters (10), although it was probably never very abundant (1). There has been some suggestion that the geographically isolated northern and southern hemisphere populations represent distinct species but this remains unclear (6).

Classified as Vulnerable (VU) on the IUCN Red List (1), and listed on Appendix II of CITES (4) and Appendices I and II of the Bonn Convention on Migratory Species (CMS) (5).

This species was traditionally hunted for its vast liver, which was sold as an aphrodisiac in Japan, and also used by fishermen for lighting in the UK, whilst the oil was used in the manufacture of cosmetics (2). Due to its long maturation time and slow reproductive rate this species is particularly vulnerable to over-fishing, and targeted populations are very slow to recover from targeted fisheries (1). Today the biggest threat comes from the demand for fins for shark fin soup in the Far East and from accidental by-catch in the fishing industry (14). Although exact population figures are difficult to assess, there has been a reported decline by as much as 80 percent since the 1950s (2).