The aim of the study was to use the nutrient and energy budgets from a practical farming situation for rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar to show how ration size and feed utilization determined the amount of nutrients in the effluent water. Separate budgets for nitrogen, phosphorus, and energy were determined showing the fractions of the effluent nutrients that came from feed loss, feces, and other excreta. The interrelationships between these fractions were compared between a group of fish whose ration size was well adapted to its biomass and whole feed loss was low and a group of fish fed in excess whose feed loss was high. This comparison indicated the uncertainty in composition of the effluent water in situations where the amount of feed loss is not determined. A Triangle filter was installed for filtering the effluent water. The flow diagrams pinpointed how optimalization of ration level and feed composition contribute to reduce pollution
