Water Temperature as the Primary Influence on Timing of Seaward Migrations of Atlantic Salmon (Salmo salar) Smolts

Using multiple regression models we tested if water temperature, water flow, cloudiness, and lunar cycle were significantly correlated with the timing of the yearly descent of Atlantic salmon (Salmo salar) smolts in the Norwegian Imsa River, 1976–83. Water temperature between 9 April and 16 May explained 91, 95, and 89% of the yearly variation in date of 25, 50, and 75% cumulative smolt descent, respectively. The regression models were not very sensitive to variations of which date we started and stopped recording the temperature. The start of the yearly smolt run was not triggered by a specific water temperature or a specific number of degree-days, but was controlled by a combination of temperature increase and temperature level in the river during spring. There was no significant correlation between smolt descent and any of the other environmental variables tested.