Migration of Atlantic Salmon Smolts in River Orkla of Central Norway in Relation to Management of a Hydroelectric Station

The migration pattern of Atlantic salmon (Salmo salar) smolts in River Orkla, central Norway, was studied in 1980 and 1981 before construction of a hydroelectric plant was finished in 1983. Smolts were caught in traps operated from movable racks fastened to a bridge and lowered by winches. This sampling device worked satisfactorily at a water discharge less than 200 m 3/s and a water velocity less than 2.5 m/s. The smolt run started in early May and lasted until about June 10. The migration was initiated by the first peak in water discharge (>100 m 3/s), when the water temperature was 2-3°C. Significantly more smolts migrated when the discharge was increasing, followed by a drop in water temperature, than under the opposite conditions (P < 0.05). The smolts exhibited a predominantly nocturnal migration pattern as 87% were caught between 2200 and 0200 hours. Generally, the salmon smolts followed the main current during the run. River Orkla was to be diverted through a tunnel to turbines for produc...