loch ness monster
The Loch Ness monster is probably a giant catfish says expert Getty Images

The legendary Loch Ness monster, which has drawn visitors to the beautiful Scottish lake for centuries, is just a giant catfish, says an expert who has spent a quarter-century in search of "Nessie."

Steve Feltham left his girlfriend and gave up his job to move to the shores of Loch Ness in 1991, earning himself a place in the Guinness Book of Records for the longest continuous vigil in search of the monster.

Various explanations have been given for the sightings and sonar images over the years, from shoals of fish or logs to even a plesiosaur, though how one could have survived there for 65 million years is even more of a mystery.

Loch Ness Monster
Some theories believe that the Loch Ness Monster is a plesiosaur which survived the Jurassic period. Photo: hum-24.blogspot.com

Some of the sonar readings are of moving objects the size of a car, far larger than anything which is known to exist there - including salmon and seals.

Now however, Feltham, 52, says most sightings are probably of Wells catfish, which were released into Scottish lochs by the aristocracy for sport. The catfish can grow up to 15 feet long, weigh as much as 300kg and live for decades.

"Looking at all the evidence, speaking to eyewitnesses, the most likely solution is a Wels catfish," Feltham told Sky News.

However, he added, "I'm not saying the mystery's solved. I'm still looking for a better explanation than that (Wels catfish). This is an explainable phenomena. There is something to be explained in Loch Ness."

First mentioned by Irish monk Saint Colomba in the sixth century, the legend of Nessie has increased since the advent of photography, though many of the best-known images were later revealed as fakes.

With Nessie-hunters estimated to bring in up to £25 million to the economy around the shores of the lake every year, many locals will be hoping Feltham has got it wrong - and that the mystery endures for many years to come.