Seasonal variation in the thermal performance of juvenile Atlantic salmon (Salmo salar)

1. Experimental data on the maximum growth and food consumption of winter-acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70degreesN were compared with predictions from published models of growth and food consumption of summer-acclimatised fish from the same populations. 2. All winter-acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1-6 degreesC). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 degreesC. 3. Growth and food consumption varied significantly among populations. Winter-acclimatised fish from a Northern population had a higher mass-specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer-acclimatised salmon, where fish from the Northern population had the lowest growth efficiency. 4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.