Basking sharks made a dramatic comeback off Scotland's west coast this summer.

Sightings of Britain's biggest fish more than doubled on last year - which was one of the worst on record.

The Hebridean Whale and Dolphin Trust has revealed it had 108 reports of basking sharks this year - compared to just 48 in all of 2014.

The return of the world's second biggest fish to Scottish waters is a huge relief to scientists - and tourism boat operators - after such a poor previous year for sightings of the species.

This year's total logged 321 individuals - compared to just 142 in 2014.

Dr Conor Ryan, sightings officer with the trust, said it was "fantastic news" after the baffling disappearance of the animal from Scottish waters.

The HWDT's best year for seeing the world's second biggest fish was 2010 when there were 162 records. There were 73 in 2011, followed by 130 the next year and 143 in 2013.

But Dr Ryan said since 2007 the average group size reported had fallen from seven to just three.

Of this year's records, the biggest single group was of 40 two miles south of Eriskay in the Outer Hebrides spotted by Terry Robinson on June 12.

"This has been a much better year for sightings of basking sharks. This summer has been poor, limiting the numbers people from getting out on the water and seeing them - so there may have been many more sightings that have gone unrecorded," he said.

"By contrast last year was a good summer weather wise but it had few sightings - one of the worst on record.

"We need to know what affects their distribution and numbers of basking sharks to help better protect them. We need to know why their habitat changes and how they react to those changes."

August is the peak month for sightings, which tail off and usually end during October.

Despite their size, surprisingly little is known about the life history of basking sharks, partly because of the difficulty of conducting field research on an animal which spends most of its life out of sight, deep underwater. However, from May until late October they appear along the west coast of the UK.

Scientists from Scottish Natural Heritage and the University of Exeter have attached satellite tracking tags to 27 sharks in the Inner Hebrides, as part of a £147,000 project to find out more about their life cycle.

The tagging project, which began in 2012, was set up to find out more about the life cycle of the large numbers of sharks that gather around the islands of Coll, Tiree and Canna every summer. One was tracked from Scotland to the Canaries.

Basking sharks can grow up to 36 feet in length - the length of a double decker bus - and seven tonnes in weight but they feed entirely on plankton, tiny animals that drift through the water. The plankton pass through their enormous gaping mouth and are filtered out by their comb-like gills.

They are long lived, with some surviving as long as 50 years. Because they are slow moving, slow to mature and long lived, they are very vulnerable to human disturbance and impacts.

Basking sharks were once hunted by Ring of Bright Water writer Maxwell around Skye for their oil and meat.

Maxwell set-up a basking shark fishery off the west coast and wrote his first book - Harpoon At A Venture - about how he disastrously hunted the great fish.