Abstract
The aim of this study was to investigate to what extent population density and small scale
habitat variability affect somatic growth of resident, stream-living brown trout Salmo
trutta. There is an ongoing debate whether density-dependent regulation of stream-living
salmonids operates via mortality and emigration or growth. A capture-mark recapture
study of brown trout was performed in Bellbekken, a small stream in east Norway. The
1.5 km stream was divided into 25 contiguous Sites. Trout movement was limited, and
76 % of the recaptures were caught at the same Site as the previous capture. Microhabitat
quality and trout density were estimated and used as predictors in the statistical
modeling of instantaneous growth rate. Growth was estimated from length increments of
recaptured fish, and from back-calculated lengths, determined from scale readings. A
general linear model (GLM) showed that instantaneous growth rate was negatively
related to length and age, and that immature fish grew better than mature fish. After
adjusting for differences in length, age and the maturity state, ~40 % of the variability in
growth was not accounted for. Some of this variability was related to Site, but most of it
remained unexplained. Fish density and estimates of habitat quality accounted for
roughly half of the variation explained by Site. The remaining variation could be related
to other habitat variables which were not estimated, or alternatively that the habitat
variables in this study were not measured at the most relevant spatial scale.