Basking Sharks (Cetorhinus maximus) in Cornwall

Basking sharks have long been summer visitors to the shores of Devon and Cornwall. Sometimes reaching over ten metres in length and up to seven tonnes in weight, they are the largest wild animal regularly found in Britain's waters. Sometimes, on calm days, the nose, dorsal fin and tail fin are all visible at the same time, breaking the surface of the water, as the sharks feed in sheltered bays and off headlands.

Basking shark
photograph by Colin Speedie

Despite their massive size, these are gentle giants which pose no direct threat to man.

In fact, basking sharks have only minute teeth, and live on tiny zooplankton which they filter out with modified gill rakers from vast quantities of sea water passing through their enormous mouths.

Basking shark in Harlyn Bay near Padstow
photograph by Colin Speedie

Basking sharks are found in temperate oceans throughout the world, usually during the summer months. They normally arrive off the Cornwall coast in April, with the highest numbers appearing in May and June. Occasionally, large schools of over a hundred sharks have been sighted, sometimes very close to popular beaches, providing a fantastic spectacle for watchers ashore and afloat.

A basking shark known as Stumpy
photograph by Colin Speedie

It is thought that basking sharks come into our inshore waters not just to feed but to find partners for mating. The young are born live, at about 1.5-2m in length, and newborn sharks of this size are seen from time to time, although the average size of sharks recorded in recent years has been around five metres.

No clear understanding exists of where the sharks go in winter: whether they move offshore into deep water and feed at a reduced level, or whether they stop feeding altogether and allow their gill rakers to regenerate. Very rarely basking sharks are sighted at the surface in the winter, and equally rarely they have been caught in nets by trawlers operating in deeper waters.

Much remains unknown about these fascinating but enigmatic creatures, which makes their annual visits to the waters of the West Country all the more interesting and valuable for visitors and researchers alike.

Colin Speedie

Basking sharks are the second-largest fish in the world; the whale shark is the largest. These huge filter feeders swim with their mouths wide open. They do have teeth, in fact they have hundreds of teeth, but they are tiny and of little use. They feed by sieving small animals such as plankton, baby fish and fish eggs from the sea through gill rakers which are made up of thousands of bristles about ten centimeters long. They can process over 6,000 litres an hour, expelling the water through the five pairs of gill slits.

Coloration varies from greyish brown or slate grey to almost black on top and lighter or white underneath. The shark generally moves slowly - five kilometres per hour - but it can move at speed, moving its entire body from side to side, unlike many other sharks that just use their tails. Appropriately, this shark spends a lot of time "basking" at the surface, often with its dorsal fin high out of water. It has also been seen "taking the sun" on its side or back and, thanks to recent observations of the large numbers off the Cornish coast in 1998 and 1999, we now know that breaching is not infrequent.

It comes into Cornish seas in the late spring and summer months, sometimes singly and sometimes in groups of many dozens. There is some evidence that it is arriving earlier in the year. But where does this shark go in the winter? Unlike the whale shark, which can rely on a supply of plankton all year in warmer waters, the basking shark depends on the spring, summer and early autumn bursts of plankton. A few individuals have been found hibernating in deep water, having shed their gill rakers. Is this what happens to all our summer visitors?  Any cast up dead in the winter need to be notified quickly so that they can be examined.

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