| Summary: | This thesis investigates the underlying mechanisms and the density-regulatory effects of age-class interactions, using juvenile Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) as study species. Field experiments were performed in streams along the western coast of Sweden, in which densities of older age-classes were reduced and the response on young-of-the-year habitat use and performance (growth, movement and survival) was observed (Papers I and II). Observational data from 159 trout populations was extracted from the Swedish Electro-fishing Register to test the generality of age-class competition (Paper III) and observations in controlled artificial stream environments were used to establish the underlying mechanisms with regard to habitat use and behavioural interactions (Papers IV and V). The combined findings of these studies show that age-classes of stream-living salmonids compete for limited resources in the stream habitat. This competition favours old individuals, although the behavioural observations of Paper V suggest that their competitive benefit may decrease at increasing densities of young-of-the-year fish. Density-reductions of older cohorts in field increased the growth of young-of-the-year trout, an effect that was observed at the later part of the growth season (Papers I and II). The observational data-set (Paper III), provided further evidence of the prevalence of inter-cohort competition, reflected as a negative association between density of older cohorts and young-of-the-year body-size, in the same magnitude as on an intra-cohort level. In accordance with previous studies, juvenile salmon and trout were segregated in the stream habitat, with young-of-the-year individuals using shallow, low-velocity, habitats close to the spawning area while older cohorts were positioned in deep, high-velocity, areas (Papers II and IV). However, when experimentally reducing the density of older cohorts in field and lab (Papers II and IV), this spatial pattern was shown to be an effect of ...
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