• discharge;
  • water velocity;
  • salmonid;
  • microhabitat

Abstract –  The energetic costs associated with foraging and social interaction for Atlantic salmon (Salmo salar) parr was estimated by measuring gross differences in performance (growth and lipid content) during two experimental trials conducted in an artificial river channel, under similar food input at two discharges. The discharges provided velocities within the range commonly experienced by salmon in the wild. Fish growth (second trial) and lipid content (first trial) were the highest at the lower discharge. Fish were less active and used a smaller area of the experimental arena under high flow. These behavioural adjustments are consistent with the reduction of energy costs at high discharge. However, there was no effect of discharge on aggressive behaviour or food intake. Therefore, despite evidence of energy economy in response to high discharge and velocity, the energetic costs of foraging were sufficiently high to cause substantial reduction in performance. In common with the findings of previous studies, dominance status was associated with individual variation in performance and habitat use.