Young salmon in the River Usk are being held back by weirs and low water levels, a new study by Natural Resources Wales (NRW) has revealed.

The River Usk Smolt Telemetry Project, launched in 2021, has tracked juvenile Atlantic salmon (known as smolts) using acoustic tags as they migrate to sea each spring. The results show the fish face major delays at man-made barriers – especially in dry years.

River levels have a huge effect on both passage speed and survival. Tracking data has shown that in a dry year, only 24 per cent of tagged smolts successfully reach the sea, rising to 67 per cent in a wet year.

Results also showed that not only were the numbers of fish reaching the sea lower in a dry year, they were in fact taking longer to leave the river – five weeks longer in some instances. Some fish were spending in excess of a month trapped above Brecon weir.

Oliver Brown, Aquaculture Officer leading the project for NRW said: “The project has evolved over the last couple of years and is giving us a good indication of what’s happening to these fish at a critical point in their life cycle.”

“From the tracking data, which gives us fish passage, speed and survival, we can see that the highest loss per km during seaward migration is in the impounded section of water above Brecon weir.”

The findings support ongoing improvements on the River Usk this year, including the installation of a new smolt pass at the weir to help young salmon migrate downstream, as part of the Four Rivers for LIFE project.

The River Usk is a SAC (Special Area of Conservation) which means it is of international importance for its wildlife and plants such as Atlantic salmon, lamprey, shad, bullhead, otter and water crowfoot.