Variability in Timing and Characteristics of Atlantic Salmon Smolt in Icelandic Rivers

Since the late 1980s, studies of smolt emigration patterns in Atlantic salmon Salmo salar have been conducted in three Icelandic rivers: Ellidaar in the southwest, Nupsa (tributary of the Midfjardara River) in the northwest, and Vesturdalsa in the northeast. Migration time in the southwest was from mid‐May to mid‐June, but in the two northern rivers it was from mid‐June to early August. The smolts in all three rivers migrated mainly at low light intensity: at night early in the summer or early in the day late in the summer. There was significant correlation between the onset of the smolt emigration and the water temperature. Increased flow, which in the south was associated with warm southerly winds and rain, stimulated the smolt run in the Ellidaar River but depressed the run in the north, where cold northerly wind brought predominantly snow. In cold summers in the Vesturdalsa River, smolt emigration was delayed until August, when the smolts reached a larger size. Fewer smolts emigrated in the colder years. The smolt run in the northern rivers coincided with a narrow range in sea surface temperatures in spite of wide range in emigration time. The return ratio of the smolts varied among years and was generally higher in the Ellidaar River (average, 7.9%) than in the Nupsa (2.4%) and Vesturdalsa rivers (2.7%). We suggest that northern Iceland smolts have adapted to unstable environmental conditions and thereby use environmental cues (e.g., river water temperatures) to guide them to the most favorable conditions in the sea.