SIGHTINGS of porpoises off the west coast of Scotland were up by a quarter last year, but there was a dramatic drop in the number of basking sharks spotted, wildlife experts said.

Hebridean Whale and Dolphin Trust saw 574 groups of harbour porpoises during marine research expeditions last year, up 25 per cent on 2013, possibly due to better weather and calm seas during trips, making it easier to spot them.

But the number of basking sharks seen by researchers fell by a third (33 per cent) to 16 encounters, which the Trust said was part of a trend in fewer sightings of the world's second largest shark.

The experts said the drop in sightings did not necessarily mean the population was shrinking, as it could be down to their food source, plankton, shifting further offshore or deeper into the water so they are not feeding at the surface where they can be seen.

The research also revealed evidence of a shark attack on a member of an endangered group of killer whales, thought to be the UK's only resident population of orcas.

One of the males, known as John Coe, was seen with a large area of his tail missing, which experts said was almost certainly the result of a shark attack.

The killer whale pod, made up of five males and four females, has not produced young since studies began, prompting fears the group will die out.

In total, the surveys carried out by teams of marine scientists and volunteers on the trust's research yacht Silurian between May and October in the Hebrides recorded more than 1,400 encounters with whales, dolphins, porpoises and basking sharks.

Encounters included a rare approach to the vessel by a minke whale which was entangled in blue plastic strapping, which the team on board was not able to remove.

Good weather allowed twice as many opportunities for photo identification - which enables experts to recognise individuals by their distinctive markings and shed light on populations and how groups function - as in 2013.

Kerry Froud, biodiversity officer at Hebridean Whale and Dolphin Trust, said: "Our findings add to knowledge about cetacean distribution, abundance and habitat use in Scotland's western seas - and allow us to make informed recommendations to protect these remarkable species.

"This pioneering research is vital for effectively conserving the world-class biodiversity of these waters - and for securing the long-term future of spectacular cetaceans and iconic basking sharks."

Hebridean Whale and Dolphin Trust said it had previously discovered the Hebrides was home to one of Europe's highest densities of the harbour porpoise, but warned Scotland still had no protected area for the species.