But now Steve Feltham, who is recognised by the Guinness Book of Records for the longest continuous monster hunting vigil of the loch, believes Nessie is no plesiosaur but a giant catfish first introduced by Victorians.
And what is more, there may be only one ‘Nessie’ left.
Mr Feltham’s “Nessie no more” verdict - which he said had been “a long journey” of realisation - could be devastating to the Loch Ness Monster tourism industry, which is said to be worth around 25 million-a-year to the area.
More than 85 per cent of the estimated one million visitors are attracted by the phenomenon of the Loch Ness monster - many to visit Mr Feltham - and Nessie has her celebrity fans too.
Two years ago Hollywood hellraiser Charlie Sheen even went to look for Nessie - allegedly armed with a leg of lamb and a large hook - and promised to return with American ex chat show host Jay Leno.
But Mr Feltham, 52, said he was now convinced that Nessie was a Wels catfish - a giant fearsome fish that can grow as long as 13ft and up to 62 stone.
“It is known they were introduced into English lakes by the Victorians for sport. They are very long lived and it is entirely possible they were introduced by Victorians to the loch - which would explain why the main sightings of Nessie really started in the 1930s - just as the animals were reaching maturity,” said Mr Feltham, who gave up his girlfriend and home in Dorset to hunt for Nessie.
“There was a viable breeding population but I think the numbers have declined to the extent that there are now just one or two left. They also eat other catfish and may have eaten breeding females over time. Nessie is destined to be no more, I’m afraid.
“I’ve had to change my mind slowly over time, but what a lot of people have reported seeing would fit in with the description of the catfish with its long curved back.
“Its natural decline in numbers over time would also explain the tail off in sightings in recent years. I have to be honest. I just don’t think that Nessie is a prehistoric monster. However the monster mystery will last forever and will continue to attract people here - monster or not. I certainly don’t regret the last 24 years.”
The legend of the Loch Ness Monster has been around since the sixth century, when Irish monk Saint Columba witnessed locals burying a man who had been attacked by a ‘water beast.’
Sightings were scarce until the first modern newspaper report of a monster in the Northern Chronicle of 27 August 1930 which told of fishermen in a boat on Loch Ness being “disturbed” by a 18 feet long creature.
But it was the famous sighting in 1933, when George Spicer and his wife claimed they saw ‘a most extraordinary form of animal’ which was 4ft high and 25ft long crossing the road near the loch, that started Nessie mania.
The following year, Dr Robert Kenneth Wilson took a picture - the most famous picture of the monster - although it was later revealed to be a hoax made with a toy from Woolworth’s.
And in 1935, renowned big game hunter Marmaduke Wetherell found a footprint he said was the monster’s. It was later revealed to have been made by him using a dried hippo’s foot of the type used as umbrella stands at the time.
Nessie has also spawned countless books and films - among them Loch Ness, starring Ted Danson and Joely Richardson.
Mr Feltham, who has spent nearly quarter of a century looking for the elusive beast, has had few encounters himself. He said it showed up on the one day he didn’t have his camera on him.
Mr Feltham left his job fitting security cameras and his home in Dorset to move to the banks of Loch Ness and devote his time to searching for the legendary monster in 1991.
After two years of patiently scanning the loch, Mr Feltham said he finally caught a glimpse of Nessie, but he didn’t have his camera to hand.
Since then, he has kept a careful watch on the waters of the loch, but she has not shown herself again. However, he is now convinced Nessie is a giant catfish.
The Wels catfish, also called sheatfish, is native to wide areas of central, southern, and eastern Europe, and near the Baltic and Caspian Seas. It has been introduced to Western Europe and is now found from the UK all the way east to Kazakhstan and south to Greece. It is a scaleless fresh and brackish water fish recognizable by its broad, flat head and wide mouth. Wels catfish can live for at least 30 years and have very good hearing.
Their wide diet even includes other catfishes. The larger ones also eat frogs, mice, rats, and aquatic birds like ducks. They have also been observed lunging out of the water to grab pigeons on land.
Exceptionally large specimens are rumored to attack humans in rare instances, a claim investigated by extreme angler Jeremy Wade in an episode of the Animal Planet television series River Monsters following his capture of three fish, two of about 145lb and one of about 160lb, of which two attempted to attack him following their release.
A Wels catfish was reported in 2009 dragging a fisherman near Gyr, Hungary, under water by his right leg after the man attempted to grab the fish in a hold. He barely escaped with his life from the fish, which must have weighed over 220lb, according to the fisherman
Last year a fisherman has caught a giant 8ft 9in long catfish weighing 19 stone - which could be the biggest ever caught with the humble rod and line.
Dino Ferrari hooked the huge Wels catfish in the Po Delta in Italy.
Sightings of Nessie have become scarcer, but so far there have been three this year.
The last one logged on the official Loch Ness Monster Sightings Register was on April 25. Dee Bruce of Elgin was driving to Skye with her partner Les Stuart when she saw a black creature come about three feet out of the water near the north end of the loch. It happened so quickly, she was unable to get a camera out to take pictures.
Two days earlier, a couple on visiting Urquhart Castle - above Nessie’s rumoured lair - reported seeing a large dolphin sized shape emerging from the waters.
Mr Feltham, who said he became obsessed with the legend of Nessie after a family holiday to the Highlands when he was seven, lives in a converted mobile library which is parked in a pub car park near the loch at Dores.
“At least my search for Nessie has given me the best view of the world,” said Mr Feltham, who makes money by creating models of the Loch Ness Monster and selling them to tourists.
Mr Feltham has become such a feature of the loch that he is even a member of the local community council.
“Film crews and journalists from all over the world turn up on a regular basis, and I answer all their questions, but they are invariably focused on one subject: is there a monster, or isn’t there? Which is perfectly understandable, but it frustrates me that I never have the chance to get an equally important point across: that if you have a dream, no matter how harebrained others think it is, then it is worth trying to make it come true. I’m living proof that it might just work,” he says on his website.
“Have I ever regretted my decision? Never, not for one second.”