Size-related habitat use in juvenile Atlantic salmon: the importance of intercohort competition

Abstract

In stream-living salmonids, an underlying mechanism for the critical period after emergence has generally been assumed to be size-dependent swimming capacity constraining fry (age-0) to low-velocity habitats with reduced food availability and intense competition. A further plausible mechanism is that intercohort habitat exclusion confines fry to marginal habitats. This possibility was tested using a seminatural stream with 16 test arenas, each comprising one high-velocity, deep habitat and one low-velocity, shallow habitat. We observed groups of newly emerged Atlantic salmon (Salmo salar) fry, either alone or in sympatry with one or two age-1 salmon. Salmon fry used high-velocity areas (42.2 ± 0.4 cm·s−1) most extensively in the absence of intercohort competition, where they obtained more food than in low-velocity areas (3.3 ± 0.3 cm·s−1), even though foraging efficiency was lower (though not significantly so). In sympatry with older cohorts, fry increased their use of the low-velocity habitat, with a reduced foraging activity, suggesting that strong older cohorts in natural populations may have the potential to influence the strength of the recruiting cohort by negative density dependence due to interference competition for habitat.