Growth variation in brown trout Salmo trutta across small spatial scales : effects of density dependence and habitat quality?

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.