Work has started on installing new fish passes at Blachford Lake to allow passage of salmon and sea trout past two weirs under a wide range of flows.
The fish passes are expected to be completed in September ready for the next migration season.
Blachford Lake provides the main barrier to salmon migration on the River Yealm, preventing access to the fish spawning grounds on Dartmoor.
This joint initiative is supported by other partner agencies including Action for Wildlife (AFW), Westcountry Rivers Trust (WRT) and the Yealm River Association (YRA).
A decline in salmon numbers nationally, as well as in Dartmoor's rivers has caused concern for some years. Work to improve spawning and freshwater habitats have been made by the Environment Agency and its partner organisations within numerous river catchments.
'We have all done a lot of work to improve the river Yealm's wildlife habitats but the fish are isolated in parts of the river because there is this major barrier at Blachford. We want to change this by installing a fish pass to allow the fish to move freely,' said Lesley Newport.
The work, which is being carried out by the Environment Agency's workforce, comprises the removal of the existing seven metre long stone upper weir and replacement with a pool and traverse fish pass, with minimal alterations to the lower weir to improve fish passage.
Like many salmon rivers, the River Yealm has a 'Salmon Action Plan' that pinpoints the main threats to local salmon stocks and helps the Environment Agency and its partners decide how best to boost fish numbers.
The plan includes a conservation limit. This is a target for the number of salmon eggs needed to ensure enough young salmon survive in the river to sustain a healthy population. At present, the River Yealm is failing to reach its salmon conservation limit.
Improving the future for salmon is not a straightforward business. Migratory fish species travel between the sea and freshwater in order to complete their lifecycles. Some, like salmon and sea trout are returning to their native streams to breed, while others, such as eels, come to our rivers to grow before returning to the sea to spawn.
'Thankfully these fish are benefiting from cleaner water in our rivers, but they still face major problems. Man made barriers to migration, such as weirs and dams can block fish migrating to their spawning grounds and back out to sea, thereby severely restricting fish numbers,' added Lesley Newport
At least 15km of spawning grounds will be opened up to salmon spawning by the new fish passes. Once built, the Environment Agency will monitor the biodiversity improvements and fish populations, using cameras in the fish pass and electric fishing surveys.
Russell Weston, of the Yealm River Association, said:'We are delighted that the partners have been able to find the funding for this vital improvement and look forward to it being completed.'