Narrowing down the possibilities
What could the Stronsay Beast have been?
Well, the Stronsay beast did not have bones as such. It had 'gristly bones'. This means it was cartilaginous. This rules out the following creatures and all others with bones.
Smaller oarfish washed ashore in Orkney which I examined at the Orkney Marine-Life Aquarium
The picture above shows an oarfish. This oarfish washed up in Orkney in 2006 - I examined it after it was stored at the Orkney Marine-Life Aquarium. Others have washed up since and they can grow to great lengths.
What about whales and seals? They are found around Orkney and there are certainly whales large enough to match or exceed the length of the Stronsay Beast. The Walrus shown above visited the islands in 2013 and is a rare sighting. The Stronsay beast was cartilaginous and mammals have bone skeletons, so that rules out whales and seals. The second largest animal in the world, the Finn whale, was washed ashore in Orkney in 2007. The photo below shows the unfortunate creature.
The same reason rules out dinosaurs or reptiles - we're not looking for a reptile or living dinosaur in this case.
So, if it is none of the above, then what's left? Cartilaginous fish are the sharks, rays and some small groups such as chimera fish. We can rule out all except the sharks on the basis of size . We need something big. There are only two known shark species which could even be considered because of the size of the Stronsay Beast. The biggest shark in the world is the Whale Shark. The biggest whale shark was 25 metres. The Stronsay beast was 16.8 metres.
Whale shark with diver)
In the case of the Stronsay Monster, the witnesses did not mention any of the characteristic markings of this shark (the spots on the skin). The mouth is too wide for the Stronsay Beast and they are not meant to be found anywhere near Orkney's coast (see distribution in red below), although a local diver reported seeing a live one in 2012 and visitors from tropical waters are not unknown e.g. sunfish and leatherback turtles. What about a basking shark then? See the basking shark discussion on the next page