Scottish Natural Heritage found that four times as many basking sharks had been recorded per hour in Gunna Sound, between Coll and Tiree, and the sea around the islands of Canna and Hyskeir than anywhere else in the UK.

They are believed to be particularly important areas for mating.

However, before the few bathers who venture into the cold waters swim for shore it should be pointed out that they have no appetite for humans – they feed off plankton.

The west coast of Scotland has become a key area for breeding basking sharks.

The species, which has been known to live up to 50 years, can grow up to 11m in length and weigh seven tonnes. It is the second-largest type of fish in the world and has an average length of a double-decker bus. They feed on fish by filtering out seawater using comb-like structures, with adults on average filtering enough water in their gills in an hour to fill a 50m Olympic-sized swimming pool.

The SNH report interpreted data collected by The Wildlife Trusts’ Basking Shark Project along the west coast of Scotland between 2002 and 2006. In July 2006, 83 sharks were recorded round Canna, with the highest daily tally round Coll in August 2005, with 94 sightings.

Frequent displays of courtship-like behaviour, including breaching – where the shark leaps clear of the water – suggests that both areas are important for sharks looking for a mate.

Suzanne Henderson, marine advisory officer for SNH, said: “It is very exciting to find out that the west coast of Scotland is one of the best places to spot these majestic animals. The figures show how important these sites are nationally, and possibly globally.”

Colin Speedie, the shark expert who carried out the research for SNH, said: “Basking sharks are fascinating but we’ve still got much to learn about them. They are huge but they feed entirely on plankton – tiny animals that drift through the water.

“These minute creatures pass through their enormous gaping mouth and are filtered out by their comb-like gills.

“Basking sharks are most often seen in coastal areas in the summer and autumn when plankton are abundant at the surface and this is how they get their name – from apparently ‘basking’ at the surface in calm, sunny weather.”

Often confused with whales, the basking shark is a fish, although a slow moving one, so is consequently very vulnerable to human disturbance.

For generations they were hunted for the high oil content of their large livers.

More recently they were hunted in European waters for their colossal fins, which reportedly sell in east Asian markets for an average price of £31,400.

Fishy tales

Basking sharks are most often seen around coastal areas in the summer and autumn, when plankton are in abundance on the water’s surface.

The fish get their name from the way they seem to bask close to the surface in calm, sunny weather as they feed.

The genus name of the species, Cetorhinus Maximus, is derived from Greek words meaning “marine monster” and “nose”, and the Latin word for greatest.

Basking sharks can grow to 11m long and weigh 7 tonnes. They can also live up to 50 years. They travel thousands of kilometres in the winter months in search of food.

The largest basking shark was found in the Bay of Fundy in Nova Scotia, Canada, in 1851. It was 40.3ft long and weighed 17 tonnes. They normally measure 20-26ft, but large-scale specimens are rare due to decades of fishing.