Surprisingly little is known about the largest fish in British waters, the basking shark, Cetorhinus maximus; for example, where does it go in winter? Even its name is misleading. When this 10-metre (33ft) giant is on the surface, it is not sunbathing but swimming gently along with its metre-wide mouth open, filtering tons of water to harvest the plankton.
These sharks used to be hunted for their large oil-filled livers, a quarter of their body weight, which kept them buoyant. The oil was used to fuel Victorian lamps. Shark numbers are much reduced and efforts are being made to track them, identify individuals and conserve them.
Sightings in UK waters are between May and October, usually along the west coast, but sharks have been seen this summer along the Channel as far east as Kent. They are thought to breed in UK waters, with a pregnancy lasting 14 months. The young, up to six at a time, are born ready to fend for themselves and are more than one metre long. Recent research shows that these sharks are not quite as placid as thought and can swim exceedingly fast and breach – ie throw themselves more than a metre clear of the water. Whether this is a mating display or to get rid of irritating parasites is still not clear.