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Professor Neil Gemmell has been studying DNA drawn from Loch Ness.

A DNA study carried out at Loch Ness rules out suggestions Nessie could be a plesiosaur or a shark.

Theories of a catfish or sturgeon have also been refuted by University of Otago geneticist Professor Neil Gemmel and his team who studied 250 samples of water taken from various sites in the loch.

"Most species are so small you can barely see them but there are a few that are larger and of course the question we're all asking is – is there anything big enough to explain the sorts of observations people have made over the years that have led to this myth or this legend of a monster or creature in Loch Ness?" Professor Gemmell said at a press conference held in Drumnadrochit this morning.

He said their studies provided no evidence of a plesiosaur present in the loch.

They also turned up no DNA evidence to support the idea of shark, catfish or sturgeon in the waters.

The remaining theory that Professor Gemmell cannot refute based on the environmental DNA data obtained is that what people are seeing is a very large eel.

"There is a very significant amount of eel DNA," he said.

"Eels are very plentiful in Loch Ness, with eel DNA found at pretty much every location sampled– there are a lot of them.

"So - are they giant eels?

"Well, our data doesn’t reveal their size, but the sheer quantity of the material says that we can't discount the possibility that there may be giant eels in Loch Ness.

"Therefore we can’t discount the possibility that what people see and believe is the Loch Ness Monster might be a giant eel."

Professor Gemmell added that "further investigation is needed to confirm or refute the theory, so based on our data, giant eels remain a plausible idea."

He noted that in 1933 researchers had also proposed that a giant eel might in fact be the explanation for some of the sightings made then but the idea waned as notions of extinct reptiles became more prominent.

"Divers have claimed that they've seen eels that are as thick as their legs in the loch.

"Whether they're exaggerating or not I don't know, but there is a possibility that there are very large eels present in the loch.

"Whether they are as big as around 4m as some of these sightings suggest, well, as a geneticist I think about mutations and natural variation a lot, and while an eel that big would be well outside the normal range, it seems not impossible that something could grow to such unusual size."

Despite his findings Professor Gemmell believes people will always believe in the Loch Ness monster.

"What I’m most satisfied with is that we came here to study environmental DNA (eDNA) and our analysis has captured everything we thought is in the loch," he said.

"We now have an excellent database which, if compared to any future testing, could enable us to identify trends and changes in the loch environment.

"That is essentially the benefit of eDNA – it is an extremely powerful and robust tool to document the living things (both large and microscopic) present in a given place.

"It’s going to be extremely useful in the future as the technology becomes quicker and more accessible, more data is created."

He said one of the more intriguing findings was the strong input of DNA from land-based species in the loch.

"We found substantial levels of DNA from humans and a variety of species directly associated with us such as dogs, sheep and cattle," he said. "However we also detected wild species local to the area such as deer, badgers, foxes, rabbits, voles and multiple bird species.

"These findings indicate eDNA surveys of major waterways may be useful for rapidly surveying biological diversity at a regional level."

A two-hour special on Professor Gemmell's study is set to air in the UK on the Discovery Channel on September 15.