THEY are the world's second-largest fish, grow to the length of a double-decker bus and can weigh more than two cars – but very little is known about the basking shark.

Now scientists will begin to tag some of the endangered species in a bid to learn more about the secret life of large basking sharks visiting Scottish waters.

Marine biologists from Scottish Natural Heritage (SNH) and Exeter University will attach satellite tracking tags to 20 basking sharks in the seas around the Inner Hebrides, allowing their movements to be monitored on the SNH website in close-to-real time.

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The project is expected to provide information on the location and behaviour of the sharks during the summer when they can be seen feeding in large numbers, "basking" on the surface.

The researchers will also track the sharks for several months afterwards, in a bid to understand if they travel to deeper water around Scotland and further afield over the winter.

Project leader Dr Suz Henderson, from SNH, said: "We're really lucky to have the world's second-largest fish visit our waters every summer but we know very little about their movements throughout the rest of the year.

"We want to know how the sharks use the waters between Skye and Mull and how long they remain in the area. We would also like to find out how important this area is in the life cycle of the sharks, and if some areas are used more than others.

"In addition, an area between Skye and Mull has recently been identified as a place where it might be appropriate to develop a marine protected area (MPA) to protect basking sharks.

"The tagging work will help determine if an MPA is an appropriate way to safeguard these magnificent animals."

The tagging work will be done around Coll and Tiree, and the small island of Hyskeir, near Canna, after research showed these areas to be hotspots for basking sharks.

Displays of social and courtship behaviour, such as breaching and following each other nose-to-tail, have only been observed in these areas, suggesting they are important for key stages in the life cycle of the sharks.

Part of the study area also overlaps with the site of a controversial plan from ScottishPower Renewables for an offshore wind farm west of Tiree, which could have up to 500 turbines. The tagging project will provide information about the use of this area by the sharks to help SNH advise ministers on the environmental impact.

The tags, which will be attached to the sharks using titanium darts and darting poles, will record their movement, depths and water temperatures before detaching from the sharks after several months.

Dr Henderson said: "The tags might be washed up on beaches after being released from the sharks and if we are able to retrieve them they will provide us with additional information. We'll be asking the public to keep an eye open for them and help us recover as many as we can."

Dr Matthew Witt, from Exeter University, said: "Although they have captured the public imagination, we actually know relatively little about how basking sharks live."