Low water levels and weirs delay salmon migration in River Usk, NRW study finds.
New research from Natural Resources Wales (NRW) has highlighted the extent to which juvenile Atlantic salmon (smolts) are being delayed or lost during their seaward migration in the River Usk, due to man-made barriers and low river flows.
The findings are the latest from the River Usk Smolt Telemetry Project, launched in 2021 to track tagged fish using acoustic telemetry. The study shows that in dry years, only 24 percent of smolts successfully reach the sea, compared to 67 percent in wetter conditions.
Delays in passage were also significant. In some cases, tagged fish remained in the river for more than a month longer than in wet years, with many held back above Brecon weir. NRW identified this impounded section of the river as having the highest loss per kilometre of tracked smolts, according to the Brecon and Radnor Express.
“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,” said Oliver Brown, Aquaculture Officer at NRW. “From the tracking data, we can see that the highest loss per km during seaward migration is in the impounded section of water above Brecon weir.”
The telemetry results are informing ongoing habitat interventions. This year, a new smolt pass is being installed at Brecon weir as part of the EU-backed Four Rivers for LIFE initiative, aimed at improving fish passage and biodiversity in designated Special Areas of Conservation (SACs).
The River Usk is a SAC under the EU Habitats Directive, recognised for species including Atlantic salmon, lamprey, shad, and bullhead. The river’s salmon populations have long been under pressure due to hydrological alteration, habitat loss and climate-related factors.
The report underlines the need for improved flow regimes and infrastructure upgrades to support viable wild salmon populations, as part of broader conservation and water management efforts in Wales.