Foraging, growth, and loss rate of young-of-the-year Atlantic salmon (Salmo salar) in relation to habitat use in Catamaran Brook, New Brunswick

The ideal despotic distribution predicts that individuals occupying preferred habitats will have higher fitness than those in less preferred habitats, whereas the ideal free distribution predicts that average fitness will be equal in all habitats. To test between these two alternatives, we studied habitat use in relation to foraging, growth, and loss rates of 216 individually tagged young-of-the-year Atlantic salmon (Salmo salar). Fish were observed by snorkelling between 2 July and 4 September 1999 in Catamaran Brook, New Brunswick. In a multiple logistic regression, the variables that best discriminated between the habitats used and not used by fish were mean flow velocity and water depth; the fish preferred habitats of intermediate flow velocity (6–48 cm·s–1) and depth (20–39 cm). Fish in preferred habitats experienced higher levels of food abundance and had higher foraging rates but did not differ in body size or growth rate compared with those in less preferred habitats, perhaps because of higher energetic costs. In addition, loss rate did not differ significantly between preferred and less preferred habitats. Our data suggest that salmonid populations at low density may be better described by an ideal free distribution rather than by an ideal despotic one.