THE ANNUAL GROWTH-RATE CYCLE OF BROWN TROUT

Growth of age-0 and age-1 brown trout (Salmo trutta) was studied at a Norwegian hatchery situated at latitude 63 degrees N. The fish were fe d pelleted food and exposed to the temperature cycles of that latitude . The weight distributions of the hatchery brown trout differed from t hose of a wild population by being skewed to the right and by having m uch wider ranges at comparable mean weights. A temperature of 3.5 degr ees C was found to be the minimum for growth. The optimum temperature was 15 degrees C or higher. The maximum size-independent growth rates in weight of age-0 and age-1 brown trout were 4.24% per day and 5.80% per day, respectively. At given temperatures, the age-0 fish initially grew more slowly than age-1 fish but reached the growth level of the age-1 brown trout after 5 months. Growth rates were higher in spring t han at corresponding temperatures in autumn. At particular temperature s, the age-1 brown trout had twice the growth rates previously found f or brown trout fed to satiation, but less than half the rates recorded for some Norwegian lake brown trout populations. The growth rates of the age-1 brown trout, when the initial autumn temperature decrease is ignored, were strongly related to temperature below the optimum, as d emonstrated by a logarithmic equation. Equations are also presented fo r the growth of age-0 fish and the known maximum growth of brown trout .