The most important point is when the other stated that there was no creature larger than salmon in loch. This shows that there is truly no real "monster." The other begins to prove this by saying that if there was a monster it would have died and in order for the monster sighting to be true, it would have had to been the offspring of another. This means that there would have been multiple creatures in the loch which would had led to multiple sightings(or a lot more than there has been, since the long-necked dinosaurs are air breathers. Not to mention the tourist drew what they wanted to see and not what they really saw. The tourists are biased in that means. This can lead to the proving that the plesiosaur sighting or other monster-like sighting are truly myths because again no creature is bigger than a salmon in the loch.
