Spatial and temporal migration of wild Atlantic salmon smolts determined from a video camera array in the sub-Arctic River Tana

Spatial and temporal migratory behaviour of wild Atlantic salmon smolt (Salmo salar L.) was investigated at the River Utsjoki, a tributary of the sub-Arctic River Tana, using submerged video cameras that covered the entire water column across the river. The matrix of cameras provided information on the exact time, position in water column, and swimming direction of individual smolts. The smolt migration started in the beginning of June and ended in the middle of July, i.e. during the period of midnight sun. In contrast to most other studies, smolts migrated all day, with a peak from 07:00 to 10:00 h. Hours of sunshine and change in water level explained 55% of day-to-day variation in numbers of migrating smolts. Most smolts migrated actively (head first) in lower part of the water column with only 4% observed in the upper 30 cm surface water layer.