LOS- Holk is right. Why would that picture be such shit quality AND in black and white :/ And fifty meters off-shore? The entire surface area of the loch is only 56.4 km^2.
Also, it seems that you guys have yet to take into account that PLESSIES CANNOT BEND THEIR HEADS THAT WAY. Bio, the plesiosaurs evolved more so that they could reach their necks downwards rather than upwards. And even then, if their vertebrae could support such articulation, there's still this thing (you know what gravity is, right?) that would have tipped their bodies so that the majority of the neck was underwater anyway, making it completely impractical for them to have the thing in the first place, which would, inevitably, lead to them evolving so they had shorter necks. Through simple logic, your theory about them evolving so their necks went up that way is blown out of the water.
Think about it this way- you can bend your head forward pretty far, and you can curl up into a ball pretty easily. However, you cannot bend backwards as well as you can forwards. It's been the same way through our evolution for years. In fact, it's pretty much the same way for all vertebrates. If our backbones were able to bend all willy-nilly, what would be the point of having them? It isn't logical that the plesiosaur would have evolved so that its neck could bend up.
Because of their large lungs, the plessies would have been pretty bouyant. They probably did the thing alligators/crocodiles/whatever do today to be able to keep themselves submerged more easily. Still, they would have been float-y. And once again,PLESSIES WERE REPTILES. You know, cold-blooded. Why would a cold-blooded animal go down deeper into water where it is colder? (That's also another reason they wouldn't be in the Loch Ness- toocoldtoocoldtoocold.) So because of their buoyancy, and because they needed to stay close to the surface of the water where it was warmer, which direction do you think the plessies would have had to bend their necks so they could eat supper? Oh right! DOWNWARDS.
Also, Bio, you said plessies may have evolved to where they had some form of echolocation type of thing? Not even likely. Unlike whales or dolphins, plessies don't have the phonic lips used to MAKE clicks or squeals like the whales/dolphins do, and they never would- look at their teeth.
It's true though, that my theory about plessies fighting over space is purely speculation. I was just guessing- it's still certain that the Loch Ness is simply not large enough to support a thriving population of the things, pygmy or otherwise. There aren't THAT many fish in the loch, and the ones that are in there aren't huge, but are of sizes that wouldn't be so easy for a teeny plessie to attack.
Personally, I don't believe any type of plessie is alive today. It's true that there is a very, very, tremendously miniscule, minute chance that the things could have survived (one in a gazillionfillion), but there is really no way there could be a Nessie-plessie.