Sammendrag
Whether time of seaward migration of young Atlantic salmon Salmo salar influences their subsequent
survival and growth was investigated in the River Imsa, south-western Norway. Salmo salar were
tagged when moving downstream through a trap near the outlet between 1976 and 2010 and recaptured
on their adult return.Most descended as smolts in April and May, but some descended during the other
months of the year. Annual variation in timing of the smolt migration was significantly correlated with
variation in water temperature during spring. Mean total body length of the descending S. salar varied
with month of seaward migration. The sea survival of S. salar emigrating from the River Imsa between
January and May was 2⋅8 times higher than for those descending between June and December. The
sea survival of the various cohorts decreased with increasing river temperature in April to May, prior
to the smolt migration, and decreasing day number when the smolts moved to sea. The size of smolts
descending the river between April and May did not affect the survival at sea as much as it affected
the survival of migrants descending in any other month of the year. The majority of the downstream
migrating S. salar were 2 years old, but proportionally, more 1 year olds moved downstream in the
autumn than in the rest of the year.Mean duration between downstream migration of the young and the
return migration of the grilse was shortest (12⋅7 months) for those descending in July and August and
longest for those descending in October (21 months). Mean monthly specific growth rate was highest
for those migrating downstream between May and July and lowest for those emigrating in September.
Based on the present results, it was hypothesized that S. salar emigrating between April and August
migrated directly out into the ocean, while those that emigrated between October and March stayed in
the estuary until the subsequent spring.
© 2014 The Fisheries Society of the British Isles
post-smolts; recapture rate; sea growth; seasonal seaward migration; water temperature.
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