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The mammal was photographed by Scots paramedic Danny Kerr off the west coast, between Islay and Gigha, as he was flying to pick up a patient.

He later sent the image to the marine charity Sea Watch, which enlisted the help of two American academics.

All three independently concluded that it was a Sei whale.

Adult Sei whales can weigh between 20 and 30 tonnes and grow up to 19.5m in length.

Crew excitement

The mammals can live up to 65 years and swim up to 55km per hour.

Mr Kerr spotted the whale while the aircraft, from the Clyde Heliport in Glasgow, was flying at about 500ft.

"There is always excitement on board when the team spots a school of dolphins or other marine animals," he said.

"This time I was lucky, there were no patients on board and I had a camera to hand and was able to get some shots.

"When I got home I looked at the pictures and my first impression was a humpback because I thought the pectoral fins were white but due to its body shape and broad tail, I then thought it looked more like a fin whale which is when I contacted Sea Watch."

The charity's research director Dr Peter Evans was also puzzled and sought independent views from two American experts in species identification.

He sent the image to Dr Phil Clapham, co-author of Sea Mammals of the World, and Dr Tom Jefferson, co-author of another identification guide, Marine Mammals of the World.

Unusual sighting

Dr Evans said: "We ruled out a humpback whale because of the slim body shape and smooth back but I didn't understand how its pectoral fins could be white until Tom mentioned that from above these can appear very light.

"We then considered whether it could have been a Fin whale, which would itself have been unusual for the waters, but the picture showed that the dorsal fin curved backwards and was relatively large, two features that fit Sei whale but not Fin whale.

"The three of us independently concluded it was most likely to be a Sei whale."

Sei whales are protected internationally and are usually spotted singly or in pairs and feed on schooling fishes, small crustaceans, other invertebrates, including squid.

There have been only eight confirmed sightings in Scotland in the past 50 years.

It is thought there may be only about 13,500 Sei whales in all of the North Atlantic - their numbers having been severely depleted by whaling and fishing.

From BBC News