Whales and dolphins from the Mediterranean are for the first time being regularly seen in northern Scottish waters.

Scientists say that the phenomenon may be the result of rising sea temperatures off the West Coast of Scotland.

The Hebridean Whale and Dolphin Trust has also recorded changes in the habits of minke whales. Until 2005, minkes arrived in the spring and stayed until October and November, but they are now leaving in July and August. The trust monitors whale and dolphin populations around the Argyll Islands, which are regarded as an important European cetacean habitat.

Nearly one third of the world’s population of whales, dolphins and porpoises have been recorded there.

Cally Fleming, the chief executive of the trust, said that warm-water species such as the striped dolphin and common dolphin, both of which are from the Mediterranean, were regularly sighted in Argyll.

Sightings of basking sharks and beaked whales, a warm-water species rarely seen in the Hebrides, were also becoming increasingly common.

There has been a corresponding decline in sightings of colder-water cetaceans such as the whitesided dolphin and the white-beaked dolphin.

Whales and dolphins are regarded as signal species for research into oceanic change because they are at the top of the food chain. Their distribution and wellbeing provide a good indication of the health of the entire ecosystem.

Jack Matthews, a professor of marine biology and chairman of the trust, said that temperatures on the West Coast had changed. “There is a need for long-term studies of species distribution to document the physical changes,” he said.

Professor Matthews is also worried that the North Atlantic Drift is weakening. Water in the North Sea needs to be cold to sink and cause the displacement that creates the Gulf Stream. Professor Matthews said that there had been a smaller increase in temperature on the West Coast of Scotland compared with the North Sea. “That fits in with the theory about the weakening of the Atlantic Drift,” he said.

This could ultimately lead to the switching-off of the world’s oceanic circulation system, something that has not happened for 11,000 years. Scotland, which is on on the same latitude as Labrador, could face a much colder climate and even permafrost.

Robin Harper, the leader of the Green Party in Scotland, said: “The research is good news in that there are still species to see; as others move out, new ones come in. People observing this are going to be acutely conscious of the effect of climate change.”

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