Habitat Use by Adult Rainbow Trout under Moderate Artificial Fluctuations in Flow

Abstract Adult rainbow trout Oncorhynchus mykiss were observed in a 20-m reach of river to determine habitat use (at four flow levels) and preference (at two flow levels) under daily fluctuations in discharge from a hydropower peaking operation. Maximum increase in discharge was threefold (from 1.6 to 5.1 m3/s), which is small compared with that of some hydropower peaking operations. Available habitat (based on velocity and depth) was different under low and high flows. At the low discharge level, nearly twice as much of the lowest velocity-class (0.00–0.15 m/s) and eight times more of the shallowest depth-class (0.0–0.2 m) were available. Distributions of adult habitat use revealed that fish focal point and water column velocities increased with increasing discharge. At the highest discharge levels, more fish were found in the deepest water, and the fish assumed positions closer to the streambed, than at the three lower discharges. Fish were usually associated with boulders at all discharges. Habitat pre...