A channel is being built in a Dorset river to allow salmon to reach spawning grounds more easily this winter.
Salmon catches in the south west are at an all time low
The Environment Agency said that the weir at Louds Mill on the River Frome in Dorset was a "serious obstacle" to salmon trying to swim upstream.
As a result the number of salmon has dropped to an all time low. The new fish pass will open at least 15km (9.3 miles) of spawning ground by November.
Salmon catches in the south west have dropped by 78% since a peak in 1994.
Saving the salmon
The agency said that the gauging weir, used to measure water flow downstream of Dorchester, is also a "complete barrier" to smaller fish.
The Dorset Frome, which is a salmon river, has an action plan that includes a conservation limit and sets targets for the number of eggs needed to ensure the species does not collapse.
"At present, the Dorset Frome is failing to reach its salmon conservation limit," the agency said.
According to agency statistics, the total salmon catch in the south west fell to its lowest point of 2,297 fish in 2005, 78% below the 1994 peak of 10,650.
Catch of sea trout increased by 3% between 2004 and 2005, from 8,147 fish to 8,362, but remains 39% below 1994 when 13,681 fish were caught.
The fishing season for salmon on the Frome is 1 March to 31 August and for sea trout it is 15 April to 31 October.
The Environment Agency said this will be a landmark installation because the type of structure they will be using has never been using in a gauging weir before in the UK.
The Frome will become a demonstration site for the new technology.