Synthetic note on spatial niche selection and competition in young Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in lotic environments

Habitat selection data from spatio-temporally heterogeneous streams must be interpreted in the light of site and scale studied. The fundamental spatial microniche in summer in both young Atlantic salmon, Salmo salar, and brown trout, S, trutta, is characterized by strong preference for low snout water velocities, because of the principal drift-feeding strategy of energy-intake maximizing sit-and-wait. The strategy is described by a simple model. Fundamental niche is size-structured, with larger parr being more dynamically robust in salmon, whereas trout show increasingly strong preferences for pools as they grow. Realized niche in trout is deep-slow relative to salmon's swift(-shallow) niche, which is a result of greater tolerance for higher water velocities in salmon combined with competitive niche displacment in the presence of the more aggressive trout. Interspecific interference competition cause a narrower spatial niche with respect to both water velocity and depth in salmon, but not in trout.