h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

What do the critters that live in them think of all this up-ing and down-ing?

Both are natural I think, though the upper lake has various man made additions, like a dam and a big plughole drilled into it. There's a legend that Marchlyn Mawr has "Arthurian treasue" lurking in it, which perhaps gives the fish something to look at while they're beeing bounced up and down every day

I visited the place last year - it is awesome! I think that a small river was built to allow migrating fish to miss the lake, and I vaguely remember about one of the lower lakes supporting a colony of a type of fish normally found in much colder climes. Any bikers reading this - the ride from Llanberis south is the best several miles I've ever done - although the road curves a lot, visibilty through the bends is generally good - on the left hand bends glance down (briefly!) over the very low wall and be prepared to have kittens - its a hell of a way down!

BuskingBob is right about the fish. A significant number of them were transplanted to another lake.
My brother was Chief Biologist at the original project. As far as I remember, another small lake was traded for the upper lake, Marchlyn Mawr, and Marchlyn Mawr was enlarged by what you might call a dam, but it's actually an entire surrounding hillside. I saw the grasses sown over it, in the Welsh fog, as single blades of grass, before they covered it.

The fish is a species of char known locally as the torgoch, and internationally as Salvelinus alpinus, a trout-like fish in the same genus as the Speckled Brook Trout, different from the brown and rainbow trout, both of which used to be classified in the genus Salmo, although the rainbow is now reckoned an Oncorhyncus, like the Pacific salmon.

The addition to the lake's bowl increased its capacity, and a water-impenetrable coating was added, including the plughole. I'm pretty sure all the fish were transported elsewhere, although the lower pond is actually part of the river.