Evaluation of Depth and Velocity Preferences of Juvenile Coho Salmon in Washington Streams

Abstract

We determined depth and velocity preference curves for juvenile coho salmon Oncorhynchus kisutch for input into the physical habitat simulation system of the instream flow incremental methodology. Preferences were determined by scaling the maximum value of the ratio of observed use to expected use to 1.0. We developed composite preference curves by compiling weighted preferences for depth and velocity from four separate studies. The values of depth and velocity preference determined in each of the four studies were weighted by the sample size of the study, summed for each depth or velocity interval, and then normalized such that the sums peaked at 1. Juvenile coho salmon showed maximum preference for depths of 76–100 cm and velocities of 3–6 cm/s. To test the composite preference curves, we used an independent data set, marking the locations of juvenile coho salmon and comparing the products of preferences for depth and velocity (Pref (dv) = Pref (d) · Pref (v), where Pref (d) is depth preference and Pref (v) is velocity preference) where coho salmon were observed with those at measurement points on a grid. In June and September 1996, most coho salmon were found most frequently at locations with high combined preferences.