Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar (n = 30) and sea trout Salmo trutta (n = 19) were simultaneously released from a sea pen outside the mouth of the River L ae rdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 kin and did not differ between the species (mean +/- S.D. 15(.)9 +/- 4(.)3 and 14(.)9 +/- 5(.)2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8-12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up- and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13-31 days, respectively). Mean +/- S.D. length of the river stretch used during the search phase was larger for sea trout (3(.)3 +/- 2(.)5 km) than for Atlantic salmon (1(.)2 +/- 0(.)9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non-significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r(2) = 0.43) and holding phase (r(2) 0.24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r(2) = 0.67). (c) 2005 The Fisheries Society of the British Isles.