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Ronan's report


Sunday January 29th 2012

I've just finished reading To Sea & Back by Richard Shelton. It's a really nice book, blending autobiography (the author was head of the Freshwater Fisheries Lab. at Pitlochry, and is the Research Director of the Atlantic Salmon Trust), with the natural history of the Salmon.

I don't know how many articles, books, and TV programmes I've seen over the years on Atlantic salmon. Lots. So I wasn't expecting to learn much from Richard Shelton on the subject.

Wrong! Yep, I know I've quoted Socrates on this page before (The more I learn, the more I learn how little I know) but I'm a slow learner OK?

One of the more interesting new facts for me is that Atlantic salmon and brown trout do, on occasion, hybridise. And not all the progeny of this intermingling are infertile.

It gets even more interesting when you read that the frequency of interbreeding between wild salmon and wild brownies in the River Tweed is higher than in any other UK river; the Tweed is known for its large and fast growing sea trout; and that Tweed sea trout are known to travel further off shore (like salmon) than other UK sea trout.

To cap it all a huge sea trout taken from the Tweed (28lb) was shown to have some gene sequences usually only found in salmon.

To quote Shelton "it is just possible that the residual presence, in at least some Tweed sea trout, of DNA derived from past matings with salmon has been favoured by natural selection".

These kind of things can make a mockery of our attempts to classify things. When does a brown trout become a sea trout? On a slightly longer timescale, when will sea trout stop being sea trout? Evolution is generally a relatively slow process, but it grinds on relentlessly.

Richard Dawkins (what is it with all these clever Richards?) made the point well when he talked about rabbits. In the Natural History Museum in London there is what is known as the type specimen for a rabbit (and lots of other species too). A type specimen is the scientific community's ultimate definition of the species. A scientist can open up a drawer, or take a big jar off a shelf, point to a lump of fur and exclaim "that, is a rabbit".

However as Dawkins pointed out, the sharing of genes in the process of reproduction means that the immediate ancestors of our type specimen rabbit, and all of its progeny, will be slightly different. No two rabbits are the same. If you go back generation-by-generation each one will differ slightly until, if you go back far enough you end up with something very un-rabbit-like.

So the notions of rabbityness, and seatroutyness if you like, are fuzzy. Evolution makes things very seatrouty for a while and then in time the gene genies get to work and things become less sea-trouty and a bit more something-else-y.

Salmon and brown trout share the same ancestor, possibly a smelt-like fish. Which reminds me of the joke:

I say, I say, I say, this smelt has no nose.

Well, how does it smell then?

Of cucumber...

Will


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