Effects of variable discharge on the velocity use and shoaling behaviour of Phoxinus phoxinus

The swimming and shoaling behaviour of Phoxinus phoxinus was investigated in four identical channels connected to the River Frome, U.K. Minnows (mean length 29.5 mm) were stocked at an initial density of 100 fish per channel (22.2 fish m^?2). The position of all visible fish in the channels was recorded at basal and elevated discharges, and use of velocity habitats inferred. Although a range of velocities (including zero) was present at both discharges, the fish selected higher mean velocities at elevated discharges than at basal discharges. Habitat suitability was significantly different between periods, emphasizing the effect of habitat availability and past experience upon habitat use. This appeared to result from the fish holding position at points with variable flow conditions, rather than moving to points with more suitable conditions. Shoal size tended to be reduced during and after elevated discharge, although variability between replicates was high. Small groups of fish tended to break off during the high flow events.