Conservationists are to attempt to find the key areas of importance to the impressive basking shark, the largest fish to visit UK waters.
By Alex Kirby
BBC News Online environment correspondent
The crew of a yacht will spend six months surveying hundreds of km of coastline, from Cornwall to Scotland.
The sharks, which are harmless, are still rare around the UK, and are under threat from several human activities.
They grow up to 12m (38 ft) in length, can weigh seven tonnes, and have been known to live for 50 years.
But they feed on plankton, and the only real risk they pose to people is if they collide with boats.
The survey, a repeat of a similar voyage in 2003, is being organised by the Wildlife Trusts.
The Trusts' project, supported by the National Express group and the Heritage Lottery Fund, involves the yacht Forever Changes sailing on 1 May to monitor likely shark sites from the western Channel to the Firth of Clyde, Northern Ireland and the Western Isles of Scotland.
The survey is due to end in early October. It aims to judge what threats the basking shark still faces, to identify individuals, and to help to establish population size and structure.
Basking sharks are already known to travel considerable distances in search of food. One of the main aims of the survey is to find out which areas are important feeding and breeding grounds for the sharks and if they return to the same places each year.
Photo-identification and video recording carried out in previous surveys allows the team to track the movements and behaviour of individual sharks over long periods.
"Hot spots" already identified include the area around Land's End, the Lizard Peninsula and the waters off Plymouth in the south west; the Isle of Man; Strangford Lough; the isle of Arran; and the Hebrides.
The Trusts say recent studies by Cefas (the UK's Centre for Environment, Fisheries and Aquaculture Science) using satellite tags have shown the sharks tend to move out to deeper water outside the summer months, when they are most likely to be seen close inshore.
The survey team is keen to identify not only where the sharks congregate to feed, but also where they mate: the frequent courtship behaviour observed off the Lizard suggests this may be an important breeding area.
The sharks are protected within UK territorial waters, though some of the legislation does not cover Scottish and Northern Irish waters. Last year's survey covered 3,695 km (2,295 miles) and sighted 108 sharks.