The sound warns the fish of danger and prevents them from swimming into intake pipes feeding a reservoir.

It is part of an �800,000 scheme to restore a beck near Penrith to its former glory.

It will also give the salmon coming up the River Lowther a better chance of breeding.

A barrage, which takes water from Heltondale beck to Haweswater reservoir, has been rebuilt so it always leaves enough water in the stream for the fish.

The project is backed by water company United Utilities and the Environment Agency.

Both have been trying to reduce the damage to fish from the vast network of aqueducts which feed the Haweswater reservoir.

A spokesman for United Utilities said the project will make a significant different to salmon stocks in Cumbria.