2Norwegian Institute for Nature Research. Tungasletta 2, N-7485 Trondheim,Norway.
3Institute of Freshwater Fisheries3, 110 Reykjavik, Iceland
Eight sea trout (Salmo trutta) and eleven Arctic charr (Salvelinus alpinus) (370-610 mm in length) were tagged externally or internally with data storage tags while entering sea water in the Alta fjord in North Norway in June 2002. The tags measured both pressure (depth) and water temperature with ten minutes intervals during the whole period from tagging until recapture. Six trout (75%) were recaptured after 1-28 days at sea, while ten (91%) charr were recaptured after 0.5 – 33 days at sea. In average, the trout preferred about one meter deeper and 1-2 °C warmer water than the charr, indicating that the two species may select different feeding areas at times during their migration. The charr spent 56% of their time between 0-1 m depth, while the trout spent 48% of their time between 1-2 m depth. Both species spent >90% of their time no deeper than 3 m depth. However, the trout dived more frequently to deeper water (max. 27 m) than the charr (max. 15 m), and these dives were most frequently seen in the late sea migration. The charr showed a diurnal diving pattern, staying in average about 0.5 m deeper during day than night, although there were 24 h with daylight during the whole period. No such clear pattern was observed for trout. The differences in migration behaviour between the two species may be related to different preference for prey selection and optimal temperature for growth.