The Orca or Killer Whale (Orcinus orca) is the largest species of the oceanic dolphin family (Delphinidae). It is the second-most widely distributed mammal on Earth (after humans) and is found in all the world's oceans, from the frigid Arctic and Antarctic regions to warm, tropical seas. Orcas are highly social and generally travel in stable, matrilineal family groups.Orcas are versatile predators, with some populations feeding mostly on fish and others on other marine mammals, including large whales.The three types of orcas are:Resident: These are the most commonly sighted of the three populations in the coastal waters of the northeast Pacific. The resident orcas' diet consists primarily of fish, and they live in complex and cohesive family groups known as pods. Female residents characteristically have a rounded dorsal fin tip that terminates in a sharp corner. They are known to visit certain areas consistently. Transient: The diet of these orcas consists almost exclusively of marine mammals. They do not eat fish. Transients generally travel in small groups, usually of two to six animals. Unlike residents, transients may not always stay together as a family unit. Female transients are characterized by dorsal fins that are more triangular and pointed than those of residents. Offshore: These orcas cruise the open oceans and feed primarily on fish, sharks and turtles. They have been seen traveling in groups of up to 60 animals. Currently there is little known about the habits of this population, but they can be distinguished genetically from the residents and transients. Female offshores are characterized by dorsal fin tips that are continuously rounded.Orca photos taken by professional nature and wildlife stock photographer Christina Craft

Lerwick based consultant anaesthetist Catriona Barr had a very close encounter with an orca while she was returning from her swim to the point of the Knab on Wednesday morning.

In what is quite possibly the closest anyone has go to an orca in the sea around Shetland, Dr Barr was within five or six feet of a huge bull male that swam beneath her for an inspection.

An orca can make short work of a 500lb grey seal or even a great white shark, so Dr Barr, clad in a black wetsuit, was understandably terrified when she saw the powerful, beautiful and menacing animal go past her in the water, its stark black and white colouring a sign that it is top of the food chain and has nothing to fear in the ocean.

Keeping her cool, she swam to the nearest rocks and came out of the water, much to the relief of a crowd of concerned onlookers who witnessed the whole drama.

She said yesterday: “I know in my mind they do not go for swimmers but it was just the most amazing creature – huge, powerful, fast and terrifying to look at under the water.

“It could have got me any time and there was nothing I could do, but it chose not to.” She said it was one thing to see such a creature from the shore and another to see it glide past a few feet away from you in the sea.

“At one point it was between me and the shore and I think there was a seal in the Waarie Geo where they often haul up.”

The thing that really stood out was the vividness of the orca’s markings in an environment where things tend to merge into greys and browns.

Dr Barr, a frequent and very experienced “wild swimmer”, admitted she was greatly relieved to come ashore and was also concerned for the onlookers who had been obviously very worried to see what was happening.

She was wearing a yellow hood and towing a red float to warn boats that she was in the sea. She added: “This was a good lesson for me. I am used to being in the sea and very careful about things like the weather. But this shows you the sea is a wild place and things can happen that you do not expect.”

She said that it appeared there was another orca investigating the shoreline with the huge bull, but the pair lost interest and headed out into the harbour again. “I was amazed at how close to the shore they came in,” she added.

Overcoming her trepidation about going back in the sea, Dr Barr was in for a swim again later that evening and again yesterday morning, when she was accompanied by Ryan Leith and also managed to spot a dolphin in Braewick Bay.

A video taken by Erik Isbister and widely shared on Facebook shows the whole encounter.