Thermal performance of juvenile Atlantic Salmon, Salmo salar L.

1. Experimental data for maximum growth and food consumption of Atlantic Salmon (Salmo salar L.) parr from five Norwegian rivers situated between 59 and 70 degreesN were analysed and modelled. The growth and feeding models were also applied to groups of Atlantic Salmon growing and feeding at rates below the maximum. The data were fitted to the Ratkowsky model, originally developed for bacterial growth. 2. The rates of growth and food consumption varied significantly among populations but the variation appeared unrelated to thermal conditions in the river of population origins. No correlation was found between the thermal conditions and limits for growth, thermal growth optima or maximum growth, and hypotheses of population-specific thermal adaptation were not supported. Estimated optimum temperatures for growth were between 16 and 20 degreesC. 3. Model parameter estimates differed among growth-groups in that maximum growth and the performance breadth decreased from fast to slow growing individuals. The optimum temperature for growth did not change with growth rate. 4. The model for food consumption (expressed in energy terms) peaked at 19-21 degreesC, which is only slightly higher than the optimal temperature for growth. Growth appeared directly related to food consumption. Consumption was initiated approximate to2 degreesC below the lower temperature for growth and terminated approximate to1.5 degreesC above the upper critical temperature for growth. Model parameter estimates for consumption differed among growth-groups in a manner similar to the growth models. 5. By combining the growth and consumption models, growth efficiencies were estimated. The maximum efficiencies were high, 42-58%, and higher in rivers offering hostile than benign feeding and growth opportunities.