Normally I like to report things I've seen myself, but this time it's an account of something at second hand. It raises an interesting question, though, and I'd love to hear from somebody who can illuminate it.
One of our regular visitors and very good friends came into the shop yesterday to say she had seen an otter at the top of the lake. A good close-up view, just where the river enters the lake, next to the woodland clearing. Nice - but the interesting part was that the river was visibly full of fish, crowding together and all pushing to get upstream. Hence, we suppose, the attraction for the otter. Not knowing much about fish, I guess this is spawning behaviour, where the trout in the lake aim to move up into the headwaters to lay their eggs.
Of course, we're talking about an artificial lake with a big tall dam at the other end, and no fish ladders, so the fish population is all introduced for sporting purposes. Interesting, therefore, this spawning behaviour among introduced fish. It must be an instinct to swim against the current in whatever water they find themselves in, rather than some kind of inbuilt GPS that sends them back to their birthplace.
But we also regularly see fish below the dam, leaping in a vain effort to get up the weir and back into the Stilling Pool below the dam. Are these ones that have in the past come over the top in the overflow? Are they trying to get back to their birthplace to spawn? If so, instinctive GPS must surely be operating, since they would have had a choice of rivers to swim back up. But since we don't know where they originated, we can't say anything reliable about them. Unless you know better?
Never mind - an otter is good.
Graham
PS It occurred to me just after posting this that the introduced fish in the lake may well have been there for many generations, and may easily be responding to instinct to return to the birthplace. So my thought about swimming against the current may well be tosh. Hey ho, sigh.... Think before you post.