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Encounters with common dolphins off the west of Scotland have more than doubled over a decade, according to experts.

And now research is under way to find out why, with scientists proposing that climate change may have caused the surge in numbers.

Common dolphins were once a rare sight in the Hebrides, preferring warmer waters found further south, leading experts to believe that global warming has led to pods moving north.

Encounters with common dolphins (a stock image is shown) off the west of Scotland have more than doubled over a decade, according to a leading research body. Monitoring by Hebridean Whale and Dolphin Trust teams has seen the number of encounters increase by 68 per cent over the past 12 years

Monitoring by Hebridean Whale and Dolphin Trust teams has seen the number of encounters with common dolphins increase by 68 per cent over the past 12 years.

The dolphins come to the Hebrides in spring to take advantage of seasonal food stocks, travelling in large groups and sometimes forming 'super-pods' of thousands of individuals.

The species, which are smaller than the area's resident bottlenose dolphins, often approach boats and play in the wake, the trust said.

While they were once drawn to warmer waters above 10°C south of the area, climate change is causing sea surface temperatures in the Hebrides to rise by around 0.5 °C a decade. 

And warmer water species appear to be colonising new areas further north or closer to shore, the trust said.

The dolphins are moving further north than they did decades ago, possibly because the water has become warmer. While the shift could create new feeding opportunities for the common dolphins, it could increase competition between other types of dolphin or seabirds. An Oystercatcher on the Isle of Mull is shown

The shift north could be creating new opportunities for the common dolphins to find food in new areas, but may mean the species is competing for fish with other types of dolphin or seabirds.

Colder water species such as the white beaked dolphin could be forced to retreat further north, and while the trust said it had found no evidence of the white beaked dolphin being displaced, continued monitoring of the situation is needed.

The charity is calling for volunteers to join its research vessel Silurian to help conduct visual surveys and acoustic monitoring and identification of cetaceans. 

This includes dolphins, whales and porpoises. 

Dr Conor Ryan, sightings and strandings officer at Hebridean Whale and Dolphin Trust said: 'An increase in common dolphins means that those wishing to encounter dolphins in the wild are in luck - but further research is needed to explain why this is happening, the extent to which it has been caused by human activity and the implications for other cetacean species.'

Kerry Froud, the trust's biodiversity officer, added: 'Our research expeditions depend on volunteers.

'In return they offer the opportunity of a lifetime to contribute to a better understanding of cetaceans and basking sharks, whilst enjoying the beautiful scenery of Scotland's west coast and experiencing exhilarating sailing. 

The charity is calling for volunteers to join its research vessel Silurian to help conduct visual surveys, acoustic monitoring and identification of cetaceans - which include dolphins, whales and porpoises - as well as helping to run the yacht. A stock image of the crew of the yacht is shown

Areas that will be covered in the forthcoming survey depend on the weather but will range from Mull of Kintyre in the south (shown with a red marker), Cape Wrath in the north and St Kilda in the west.