Migration problems of Atlantic salmon (Salmo salar L.) in flow regulated rivers

Migration patterns of adult Atlantic salmon (Salmo salar) and smolts of salmon and brown trout (Salmo trutta) were studied in the flow controlled areas of two northern Swedish rivers. Fish behaviour and migration success at passages of various hydropower facilities were evaluated in different flow regimes. In addition, the impacts of the power-stations on the salmon populations were modelled. On average, 30% (annual mean 0-47%) of the upstream migrating salmon that were captured at the mouth of Umeälven and marked with radio-, PIT- or Carlin-tags in 1995-2005 (n = 2651), reached the fish-ladder 32 km upstream. The migration took, on average, 44 days from the river mouth to the fish-ladder. Salmon were hindered or delayed at the power-station outlet, waterfalls and the fish-ladder area. At the turbine outlet area, salmon generally responded to increased bypass flows by upstream migration. In total, a 70 % average loss of potential spawners to the catchment area was estimated. Predictions based on population modelling showed that if 75 % of the females passed the regulated section successfully and reached spawning areas in the tributary Vindelälven, the population could increase by about 500 % over a ten-year period. Radio-tagged smolts (n = 206) of Atlantic salmon and brown trout released upstream of the power-stations at Umeälven and Piteälven in 2002-2004 migrated downstream in the main flows at a speed of about 2 body length s-1, eventually leading them to the turbine intakes. Migrating smolts were observed surface oriented at depths of 1-3 m. Flow modelling estimated relatively low fish guidance efficiencies for the spillways at natural flows. About 13 % of the smolts at Piteälven were hindered as they approached the power-station, and mortality of smolts at turbine passage was positively related to body size. By using the data for radio-tagged smolts and data from Carlin-tagged smolts (n = 7450) in 1998-1999, the overall average mortality for smolts at the power-station was estimated to 17%. Population modelling predicted a potential increase in the escapement return from 5-30 % to 70-120 % in ten years if the smolts had no losses as they passed the power-station