Live plesiosaurs: weighing the evidence

Video evidence

Video evidence is a step up from photographic evidence, but is thin on the ground. Proponents of living plesiosaurs provide 'the Dinsdale film', taken in 1960, as the most compelling video evidence for a monster in Loch Ness. For example, Bauer (2002a, p.227) states that this footage represents the "strongest objective evidence for Nessies". The film contains a "large hump [which] moves in a curving path, submerging after a time" (Bauer 2002b p. 457). I have not seen the footage so I cannot comment in detail, suffice to say, a moving hump is not robust evidence for a living plesiosaur.
Sightings

There are plenty of reported sightings of sea monsters (see Huevelmans for a thorough account), but anecdote is the least reliable form of evidence. There are two main problems with sightings;firstly they are untestable and therefore do not provide scientific evidence. Even if the witness is honest, this brings us onto the second problem with sightings: eye witness accounts are notoriously inaccurate. This was demonstrated during the filming of the BBC documentary, 'Search for the Loch Ness monster'. A simple experiment was performed in which a simple stick was raised out of the water of the Loch for a few seconds in view of a group of unsuspecting tourists. They were asked to draw what they saw. A surprisingly large number reconstructed a 'head' on the end of the 'neck'. It seems that we sometimes see what we would like to see. The human brain is fallible, open to misinterpretation, and even the most sceptical observer can be misled.
Sonar

The best known sonar evidence is of too low resolution to be conclusive. It may simply represent physical artefacts in the water, such as layers of differing temperature (Bauer 1988).