DOMINANCE IN DEMAND-FEEDING BEHAVIOR IN ARCTIC CHARR AND RAINBOW-TROUT - THE EFFECT OF STOCKING DENSITY

The effects of stocking density on the demand feeding behaviour of Arc tic charr Salvelinus alpinus and rainbow trout Oncorhynchus mykiss wer e evaluated using demand feeders in combination with a monitoring syst em based on automatic passive integrated transponders. The proportion of total bites accounted for by top-ranked charr and trout decreased f rom 87 to 15% and From 66 to 15%, respectively, when moving from the l owest to the highest densities, indicating that the ability of the hig hest-ranked fish to monopolize the demand feeders was reduced at high densities. However, when the individuals were grouped into quartiles, based on their individual rank, both charr and trout ranked within the upper quartile accounted for the majority of bites independent of sto cking density (87 and 72% of the total, respectively). Instead of ther e being a single dominant individual, as was the case under low densit y conditions, a small group of individuals dominated the actuation of the trigger at high densities. The two species responded differently t o crowding, with the total daily bite activity of charr increasing lin early with increasing density, whereas the corresponding relationship was quadratic for rainbow trout, with a peak value al 24 to 36 kg m(-3 ). (C) 1996 The Fisheries Society of the British Isles