EFFECTS OF WATER VELOCITY, GROUP-SIZE, AND PREY AVAILABILITY ON THE STREAM-DRIFT CAPTURE EFFICIENCY OF BLACKNOSE DACE, RHINICTHYS-ATRATULUS

In laboratory flow-tank experiments, I determined the effects of water velocity, group size, and prey arrival rate on the proportion of drif ting prey a group of blacknose dace, Rhinicthys atratulus, captured. W ater velocity, group size, and the interaction between the two account ed for significant proportions of the variance found in the stream-dri ft capture efficiency of the fish. Neither prey arrival rate nor any o f the interactions which included prey arrival rate explained a signif icant proportion of the variance in the capture data. I present a regr ession relating water velocity and group size to the proportion of dri ft items fish capture that should be valuable for future studies of fi sh habitat selection. Further analysis of the regression found an opti mal water velocity for drift-feeding blacknose dace at between 24 and 27 cm.s-1. Because of the significant interaction between water veloci ty and group size, the advantage of foraging in sites with optimal wat er velocity decreased as the number of intraspecific competitors incre ased.