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Abstract

In this thesis, a 23-years stocking program of brown trout (Salmo trutta) smolts, annually released into the rivers Ume- and Vindelälven is evaluated. The statistics of homing adults caught in a fish ladder as well as individually tagged trout caught in the fishery were evaluated. Stocked anadromous trout migrated only short distances since > 95 % of the recaptured tagged fish was caught <200 km from the home river. A 50 % loss of fish was seen when they were released upriver (and above a dam) compared to released more downstreams. Relative size of survival for smolts (recapture rate) was more important than their absolute size. Few trouts migrated to smaller forest rivers in the vicinity of the larger Umeälven. Upstream migrating trouts counted in a fish-ladder showed that 42.7 % of all passing fish from 1974 to 1997 were of wild origin and that the annual average was 29 individuals per year. It is concluded that the wild anadromous trout in these rivers are near extinction despite the effort with stocking programs. Growth and survival of stocked 1-year old brown trout were analysed in a field study where the effect of acclimatisation by keeping the fish in enclosures before stocking, increased the number and size of the fish recaptured within the stocked area, two month after the release. The first period after stocking is of vital importance for the juvenile fish stocked into a natural habitat. Growth of stocked 1-year old brown trout released into artificial stream tanks was strongly effected by individual characteristics, density, prior residents and food level. Competitive ability, defined as a behavioural profile is an important growth determinants. The two most dominant behavioural profiles differed in profitability depending on density and size of the fish. Large size combined with aggressive behaviour were beneficial at low competitive levels (densities), while large size and non-aggressive behaviour were beneficial at high densities. Prior residence was beneficial at intermediate and high-competitive levels but had no impact at low densities of fish. The mean growth rate of all fish decreased with increasing density. Both food abundance and behaviour profile of juveniles affected growth. Food abundance had no effect on the number of territorial individuals. Instead, the proportions of the alternative behavioural strategies, i.e. a floating behaviour (individuals that occasionally displayed a territorial behaviour) and a non-territorial (shoaling) behaviour changed with food abundance. In the lowest food regime, more individuals displayed a non-territorial than a floating behaviour.