Another excellent post my friend, a real eye-opener to an area that I was (until now) unaware of. In relation to the circles on the lake I think this is caused by the ice melting, this makes the water under the ice denser which (alongside the lakes natural currents) causes a circular motion of melt water which is then displayed on the ice surface. Not to say that there isn't a methane element (it's certainly a convenient global warming fit) but I don't think it's as simple as that, indeed this process does occur throughout the world and in places with no methane.

That said, the missing ships could have some kind of gas element (possibly methane) i.e an underwater movement/gas release which causes a gigantic bubble on the surface of the water which in turn incapacitates the crew and sinks the boat. A smaller gas bubble may not sink the boat but the crew may become incapacitated by it, hence the missing time? Equally there could easily be some kind weather condition or natural phenomenum at play here.

I have no problem believing that the monster could be attributed to sturgeon, indeed within their isolation they could easily evolve into a far larger sub species. The alien aspect is also fascinating and indeed if you were looking to have some form of permanent presence on the earth then what better place to hide? Surely if you were to see underwater humanoid creatures you would know what you saw? it's certainly a strange story to make up. Was it Enlil that was known to be the god of water and land? Sorry if I'm beginning to rant :D but this is a fascinating post that sends my thought processes off into multiple directions! What a brilliant post to start 2018 with @tremendospercy excellent work!!