It could be argued that a weirpool is a noisy environment underwater, BUT the noise of water falling over the weir is "white" noise, and as such has no particular frequency. An Archimedes screw is sure to generate a low frequency throb. Probably felt rather than heard by the fish. So it is entirely feasible that the noise scares away migrating fish. Another possibility is that the screw is mashing the salmon. Salmon are attracted to any rush of water as they migrate, and it seems highly conceivable that some, perhaps many are swimming straight into the turbine blades.
The New Mills design is probably worse still: there is a piddling little fish pass very near to the screw outlet. I cannot see why salmon would be attracted to the pass rather than the screw outfall, EVEN IF the noise does not deter the fish from going anywhere near.
Maybe all these hydro schemes should be shuit down during both game and coarse fish migration and breeding seasons. As the hydros run at the expense of the subsidising taxpayer, it would be no financial loss to the public if they were shut down. A monetary gain actually.
Will the noise at Otterspool drive ALL the fish away from the weirpool? Why should noise just affect migratory fish? Maybe resident fish might come to ignore it, but not pleasant for them.
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