The Plesiosaur - this is the most popular candidate. The plesiosaur was not a dinosaur, it was actually a prehistoric aquatic reptile which lived in the warm seas which surrounded Scotland 70 million years ago.
Giant Squid and Octopi - The class of invertebrates, animals
without a backbone, have never been seriously suggested as candidates for the
Loch Ness Monster, but it would seem sensible to consider them
anyway as they are real monsters of the oceans.
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Owing to Loch Ness being fresh water and the North Sea being salt water, plus the fact that the only way into the loch was from the sea after the last ice age, we can conclude that no large invertebrate could have made that transition.
Sorry, the kraken is an unlikely candidate to live in Loch Ness.
All of the problems with invertebrates apply to amphibians with the added complication that there are no large amphibians in the world today.
It is not possible to completely discount a large unknown amphibian if a way can be found to overcome the entry problem. Could a giant amphibian have sufficiently small spawn to stick to the legs of herons and other birds? If so where did they come from? Why are they not found elsewhere?
However unlikely it may be, a giant amphibian cannot be completely ruled out.
It has often been put forward that the Loch Ness Monster could be some sort of long-necked seal, sea cow or whale, but no such animal is known to exist anywhere in the world and it is now considered that all known large mammals have been catalogued.
There are two other serious problems. Mammals are all warm-blooded, consume vast quantities of food and surface regularly to breath. These factors alone completely rule out mammals as candidates.
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Leaving aside the very, very remote possibility of an unknown giant amphibian, if reptiles, mammals and invertebrates are ruled out, the fish category becomes the only sensible possibility.
There are three feasible fish candidates. The first of these is the giant eel. For centuries there have been reports of sea serpents in the oceans and there is no practical reason why a huge species of eel should not exist and enter the loch from time to time. The biggest problem with this is that they have never been caught in rivers, lakes and lochs anywhere else in the world. The second is the Wells catfish which has been known to grow to nine feet (3m), but this would appear to be too small as a surface object.
A far more likely candidate, and one which is found in other lakes, is the sturgeon, a migratory fish which comes into fresh water to spawn. While the Atlantic sturgeon is not known to reach immense proportions, like many fish, they can continue to grow into old age. The biggest sturgeon ever recorded was from a river in Russia. It was 8m (27 feet) long and from the bony scales along the side of its body, its age was estimated at well over 200 years.
The fish shown above was washed up in Lake Washington and was 3.5m (11 feet) long.
There is one interesting sighting of a monster in the River Ness back in the nineteen-thirties. The lady concerned described a huge animal which resembled a crocodile with tusks swimming up the river towards Loch Ness. This description could well be applied to the bone large-scaled back of a sturgeon when viewed from above. Interestingly the sturgeon also has long barbels under its mouth which could be interpreted as tusks if the fish were wallowing. There are no other records of sturgeon in the river or loch although they have been found in the Moray Firth into which the River Ness empties. Another, but unsubstantiated report, is that one was caught in the River Garry. Any sturgeon found here must have come through Loch Ness.
The sturgeon may well be the most likely candidate for the Loch Ness Monster, but they are certainly not a very romantic solution. Who would come all this way to see the Loch Ness Fish?
We shall see.