Their size may be awesome, but they are quite harmless to people and other larger fish (though they swallow small fry along with the rest of the plankton). Because they are only visible for a few months of the year - and then only sporadically when they swim close to the surface - we really know very little about them. How many are there? Where do they go in winter - do they migrate hundreds of miles away, or stay dormant on the seabed? And is this magnificent, gentle giant under threat from humanity?
The Wildlife Trusts and other conservationists think they may be. The sharks were once caught mainly to provide liver oil, used to make cosmetics and specialist aviation oils. Until a few years ago there was one Scottish fisherman with a licence to harpoon basking sharks but he no longer pursues them. Now the main threat is thought to come from rising demand for their enormous dorsal fins for use in sharks fin soup among Chinese communities around the world. In Singapore the prized fins of the basking shark have been fetching over pounds 200 a kilogram. The nearest place to Britain where they are caught deliberately in large numbers is Norway, where some whaling boats now harpoon them.
The basking shark is a slow breeder which takes many years to reach sexual maturity and has fairly few young. That makes it highly vulnerable whenever humans decide to target it. There have been several examples of local basking shark numbers collapsing because of fishing, including one off Ireland's north-west coast in the 1950s.
Ken Watterson is fascinated by the huge fish, and has been running a surveying project on the Isle of Man for years. He is affiliated to the Manx Nature Conservation Trust, part of the Wildlife Trusts. Using a boat to sight them from, he has attached tags to the beasts so they can be individually identified each time they are sighted. He believes that in the space of a dozen years the numbers around the Isle of Man, a favourite haunt of the sharks visiting British waters, have fallen from thousands to hundreds. ''I've got 13 years of data now,'' he says. ''Last year the numbers round here were only 15 per cent of the peak numbers in 1989.''
Further north, the Scottish Wildlife Trust runs a database to log every basking shark sighting reported to it in Scottish waters. It is also helping the Government-run Scottish Natural Heritage and Durham University in their attempt to attach ingenious radio beacons to several sharks this summer off the Island of Arran. The electronics and batteries are packed into a large plastic fizzy drink bottle, which is then attached to the shark just behind the dorsal fin with a barbed hook - the job can be done with a long pole handled from a boat running beside the fish. The device records the water temperature and depth of the water the shark swims in, and when it surfaces - which hopefully it will do - the information is beamed up to satellites which also log its position. If it works, this should solve the mystery of where the sharks go in winter. An attempt was made to do this last summer, but it failed mainly because the number of sharks turning up was so low. Back on Manx, Ken Watterson also hopes to attach three different kinds of tracking and monitoring device to the great fish next month. He, however, will be catching them in a specially designed net and then strapping the electronic packages around their bodies.
The trusts were deeply disappointed earlier this year when Britain's Department of the Environment refused to take up a request from the island's own, largely autonomous government that the international trade in products made from basking sharks be regulated so as to guarantee their survival. The Manx government has already prohibited any fishing for the shark within its 12-mile territorial limit. It wanted the UK DoE to propose to the world's nations that the species be given this protection at the June meeting of the Convention for the International Trade in Endangered Species - Cites.
The proposal to Cites would - if accepted - not simply ban the international trade in basking shark products, but it would compel nations which have signed up to the Convention to monitor the trade and reduce catches if it was shown to be unsustainable. The proposal would probably have run into insurmountable opposition from South-East Asian countries which strongly oppose the idea of Cites covering any fish stocks. But, say the conservationists, merely getting it debated at top international level would have been well worthwhile.