I know I have seen it somewhere on the site, but I can't seem to find it again. No doubt others may find it difficult to locate.

It is a very telling document from the Angling Trust on Small Scale Hydro. I was unable to read the document from the original provided link, but it is also on this page, there are a couple of links below the picture of the mutilated perch. Should be mandatory reading.

Most hydropower schemes are a waste of taxpayers' money and damaging to the environment, says new report - The Angling Trust

The picture shows no less than 7 mutilated perch, suggesting that far more than "the occasional fish" gets mangled by these screws.
The screw at Settle seems to be reducing the salmon run, and the suggestion is that noise scares the fish away, preventing them from reaching spawning areas. An alternative possibility is that they have been turned into soup by the turbine blades.

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No trees were damaged by this e-mail, but a hell of a lot of electrons were severely disturbed by it.