3.3 | S. trutta fry habitat associations

A quadratic relationship for D 50 appeared in the top four models of the S. trutta single pass count analysis and had the highest cumulative weight of any variable in the model set (cumulative AIC c weight = 0.81). The number of S. trutta fry per site was maximized at a D 50 of 151 mm and was ≥ 10 per site (658 fry per km) between a D 50 of 96 and 206 mm (Figure 2). Temperature (cumulative AIC c weight = 0.35) appeared in the top model, but the next highest model in which it was included had a ΔAIC c of 4.85. Depth and velocity (cumulative AIC c weights of 0.11), linear relationships of which were included in models with ΔAIC c of 2.29 and 2.39, respectively, appeared to have lesser effects on S. trutta fry counts (Figure 2). Upon further examination of fry sites with a D 50 between 96 and 206 mm containing ≥ 10 S. trutta fry per site (n = 13), counts were highest when depth averaged 0.18 (±0.03) m and velocity averaged 0.20 (±0.09) m s 1. Sites meeting these average depth and velocity criteria contained 2.2 and 1.5 times more S. trutta fry than stocked O. mykiss fry, respectively.

S. trutta abundance was similarly predicted by a quadratic relationship for D 50, which appeared in all but the second model of the set, and had a cumulative AIC c weight of 0.90. Abundance was highest in the site with a D 50 of 120 mm, within the optimum range obtained from the S. trutta fry count data. Presence of wood (cumulative AIC c weight = 0.99) was the only other variable to have an effect, appearing in the first two models. S. trutta abundance was lowest in one of the four sites that contained wood (Figure 3).