Killer whales pay a surprise visit to the Firth of Forth

Workmen repairing the bridge were the first to report seeing the distinctive black and white mammals, which weigh around two tonnes and can reach lengths of more than eight metres.

Specators who have flocked to the waterside have reported seeing four adults and two calves swimming up and down the firth, passing under the bridge at high tide.

The killer whales, or orcas - which featured in the Hollywood film Free Willy - are often seen off the Western Isles and Orkney, but marine specialists say the last time they were seen near the Forth Bridge was in the early 1990s.

It is believed the mild winter weather and easy access to seal pups, which they eat, could be behind the latest sightings.

Although cetacians sometimes end up in unusual places because they are ill or disoriented, experts say these orcas - the largest members of the dolphin family - seem to be healthy and happy in their new home.

Fife Police wildlife liaison officer Mark Maylin said: "The whales seem able to tell the difference between people and seals, and certainly don't actively seek out humans, so there shouldn't be a problem there.

"The potential for risk would come if people go out in small boats and one of the whales strikes the boat - that could easily overturn it." But he warned that the killer whales themselves could be at risk from larger boats, particularly if they were hit by a propeller.

William Black, a night watchman from Deep Sea World, was one of the first people to spot the killer whales. He said: "They are magnificent creatures.

We have seen them swimming around the Forth Bridge for about half an hour at a time."

Natalie Smart, spokeswoman for the Scottish SPCA, said: "

It is very unusual for these creatures to be spotted this far south in January, which could be due to the warm winter. We just hope that after feeding they'll move back out to sea."

MOTHER OF ALL WHALES

KILLER whales, Orcinus Orca, have no natural predators and can live for 50-80 years.

Orcas are technically dolphins and are the largest species of that family. Although they have been hunted they are not endangered: it takes 21 orcas to produce the same amount of oil as one sperm whale.

They form strongly matriarchal societies. Mothers, daughters and sons form groups, with offspring staying with their mothers even after they are fully grown.

They can be found in all oceans, but seem to prefer coastal waters and cooler regions. The female can grow up to 7.9 metres, with a dorsal fin of up to a metre. The male can reach 8.5 metres. They have approximately 48 to 52 conical teeth.

They have a distinctive appearance: a black body with white patches, usually over the eyes, under the jaw and on the belly.