Landscape controls on boulder-rich, winter habitat availability and their effects on Atlantic salmon (Salmo salar) parr abundance in two fifth-order mountain streams

Abstract

We test the effect at river reach and segment scales of landscape controls on the distribution of Atlantic salmon (Salmo salar) parr densities, as well as associated variations in boulder (diameter ≥ 256 mm) abundance and potential overwintering habitat. This study encompasses data from 45 km of fifth-order mainstem channels along two neighbouring river catchments in the Gaspé region, Québec. At both scales, winter habitat availability was correlated with boulder availability. At the river segment scale (1–5 km), parr densities significantly correlated (P < 0.05) with boulder availability along the Bonaventure River, which presented significant intersegment variations in boulder abundances. In contrast, segment-scale boulder and parr abundances were uniformly low along the Petite Cascapédia River. At the reach scale (600 m), positive but less strongly significant boulder – parr abundance correlations were observed in both the Bonaventure and Petite Cascapédia rivers. Spatial variations in boulder abundances in these systems reflected variations in the degree of channel to valley walls coupling and imposed channel formative shear stresses. In similarly boulder-poor segments with comparable fry abundances, parr abundances were significantly greater along the Bonaventure than the Petite Cascapédia River, possibly because of the presence in the former system of nearby boulder-rich refugia segments.