Effect of habitat types on survival, spatial distribution and production of an allopatric cohort of Atlantic salmon, Salmo salar L., under conditions of low competition

An allopatric cohort of Atlantic salmon, Salmo salar L., introduced to a small previously fishless stream was studied from parr to the smolt stages. In May 3900 0+ parr (mean total length 30mm) were planted at three different densities in habitats with slow, intermediate and fast water velocities. During the first year, high mortality occurred during the first 7 weeks after planting in May, and in September–October. Survival from May 1985 to April 1986, before the smolt emigration, was24.8%. The smolt yield 1 year after planting was 15.5%. It is suggested that the high survival was caused by low competition. Most of the redistribution of the fish took place during the first months. Type of planting habitat affected the timing of redistribution. The parr left slow-flowing, deep habitats with fine substrate soon after planting, while redistribution was slowest in the fastest flowing habitats with coarse substrate. The observed avoidance of slow, deep habitat types in the absence of interspecific competition, suggests that this may be a fixed behavioural response, and not due to competition. Long movements, up to 800 m, were recorded only within the first 7 weeks after planting. The effect of planting densities on population density was most pronounced immediately after planting in the fast and also intermediate habitats. Planting density effects declined and were not detectable after 1 year. The effect of habitat type on fish numbers and biomass was pronounced irrespective of planting densities. Growth was fastest in the intermediate habitat, and at the lower planting densities. Production was 7.2 g m^?2 the first summer-autumn. Due to smolt emigration, few fish remained in the stream the second summer-autumn, and the production was 1.0 g m^?2.