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

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 inter-cohort habitat exclusion confines fry to marginal habitats. This possibility was tested using a semi-natural 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 inter-cohort 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.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author