Investigating a major assumption of predictive instream habitat models: is water velocity preference of juvenile Atlantic salmon independent of discharge?

The mean column velocity preference of juvenile Atlantic salmon Salmo salar (L sub(F)30-55 mm) was investigated by observing their spatial pattern of habitat use in a laboratory flume while varying discharge (Q) over a 18-fold range (Q=2.6-46.8 l s super(-1)). Based on 341 fish observations at three discharges (Q=2.6, 15.0 and 46.8 l s super(-1)), three separate velocity preference curves were developed using standard procedures. The mean column velocities measured at 0.6 depth for the fish positions at the set low, medium and high discharges had medians of 7, 10 and 24 cm s super(-1), respectively, and varied significantly between the discharges. Across the range of flows, the fish utilized mean column velocities between 0 and 56 cm s super(-1), but the three velocity preference curves differed. Differences between juvenile Atlantic salmon use of habitat, defined according to mean column velocities at different discharges, were greatest at the lower end of the available range of velocities (