Abstract
When two or more species coexist, there is often a differentiation in habitat use, temporally and/or spatially, either due to differences in habitat preference or in competitive abilities. Trout and salmon, for example, often coexist in the same river but whereas trout generally occupy pools, salmon are more often found in riffles. This segregation might be due to differences in habitat preference but could also occur as a consequence of salmon being out‐competed by trout from the pools. Food in riffle sections has been found to be relatively unpredictable in time and space whereas pools are believed to be more spatially predictable. It should therefore be easier for fish to monopolize food resources in pools than riffles and hence the importance of dominance might be most significant for pool‐dwelling fish. In agreement with this theory, dominance have been shown to increase growth rate in brown trout whereas no such relationship has been found in salmon. This experiment was therefore designed to detect whether the relationship between growth rate and dominance in salmon might vary depending on whether they co‐exist with trout or are free to select pool habitat with no trout present. The experiment was carried out in an indoor stream landscaped with both pools and riffles and equipped with a feeding system designed to mimic natural spatial and temporal variation in food supply. This set up allowed us to perform controlled behavioural observations under near‐natural conditions. The results from this study will be presented and discussed.