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 fed pelleted food and exposed to the temperature cycles of that latitude. The weight distributions of the hatchery brown trout differed from those of a wild population by being skewed to the right and by having much wider ranges at comparable mean weights. A temperature of 3.5 degrees 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 than at corresponding temperatures in autumn. At particular temperatures, the age-1 brown trout had twice the growth rates previously found for 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 demonstrated by a logarithmic equation. Equations are also presented for the growth of age-0 fish and the known maximum growth of brown trout.