The energy budget of the brown trout population of øvre Heimdalsvatn

Energy pathways of a natural brown trout Salmo trutta population were constructed. The main subjects taken into account were: the biomass, calorific values and production of the population, natural mortality and annual yield, energy intake, calorific values and composition of the food, efficiency of food absorption, spawning and migration of young trout. The specific energy value of trout increased with increasing weight of the fish. An energy decrease of almost 50% was demonstrated for the spawning trout of both sexes, and the calorific values prior to spawning were not reached before the next summer. Energy intake was estimated to 6.87, 10.31, 8.42 and 9.15 kcal m −2 lake surface for the years 1969, 1970, 1971 and 1972 respectively. Although a great number of food items were found in the trout stomachs, the four groups Gammarus lacustris, Lepidurus arcticus , Trichoptera and Chironomidae accounted for 2/3 of the total consumption. The efficiency of food absorption varied between 42 and 53%. The mean age of trout entering the lake from the nursery streams was ca. 3 yr. The annual amount of this immigration was estimated to be 0.28 and 0.24 kcal m −2 lake surface for 1969 and 1970 respectively. Annual production of trout in the take was estimated to be 1.46 and 2.66 kcal m −2 respectively for the same years. For both these years balance was found between biomass at the beginning of the year plus immigration of young and total production, minus catch by man, natural mortality and spawning on the one hand and biomass at the end of the year on the other hand.