Dolphins are prospering in the seas off Scotland's west coast with record numbers documented for three species, but scientists can't explain yet why the populations are growing.

So they are looking for more volunteers to help continue their research as 'citizen scientists'.

The Mull-based conservation charity the Hebridean Whale and Dolphin Trust today publishes data from its marine research expeditions in 2016.

From the trust’s specialised research yacht Silurian, volunteers and scientists recorded 2,303 individual common dolphins, 42 bottlenose dolphins and 94 Risso’s dolphins. The figures for all three species were the highest ever recorded in its annual survey seasons.

These compare to average annual figures over the previous 14 years of 463 individual common dolphins, 14 bottlenose dolphins and 12 Risso’s dolphins. In two of the earlier surveys, no common dolphins were spotted at all.

Dr. Lauren Hartny-Mills, the trust's Science Officer of Hebridean Whale and Dolphin Trust, said: “The reasons for the high number of sightings of these charismatic dolphin species, and the broader effects on the marine environment and other species, remain unclear. But the intriguing findings highlight the importance of on-going monitoring and research, to strengthen our understanding of what is taking place in Hebridean waters, and to ensure well-informed conservation action.”

It is all part of the trust’s unique long-term "citizen science project" monitoring whales, dolphins and porpoises, collectively known as cetaceans, as well as basking sharks in the Hebrides.

It depends on paying volunteers, and 71 were welcomed aboard the Silurian in 2016. They worked with marine scientists on visual surveys and acoustic monitoring with underwater microphones or hydrophones; and identifying individual cetaceans through photography.

The trust now holds data from more than 95,000km of survey effort at its Tobermory HQ. It aims to pass the 100,000km milestone during 2017, and it is currently recruiting volunteers to support this by working as citizen scientists onboard Silurian for periods of almost two weeks from April to September.

Alison Lomax, Director of Hebridean Whale and Dolphin Trust, said: “Long-term scientific studies of this globally-important habitat and its inhabitants are crucial if we are to ensure a secure future for the Hebrides’ spectacular cetaceans.”

During 2016, Silurian, previously used in filming of the BBC’s The Blue Planet series, covered more than 5,000 nautical miles, compared to an average of almost 4,000 miles annually over the previous 14 years. Its crew documented more than 1,300 cetaceans and basking sharks, and recorded almost 700 hours of underwater detections of cetaceans using specialist listening equipment.

Notable highlights included an encounter with a humpback whale in the northern Minch. "An hour was spent with the massive creature lunge feeding, tail slapping and swimming under Silurian, alongside a large group of common dolphins."