Species Dominance by Brook Trout and Rainbow Trout in a Simulated Stream Environment

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Abstract
An instream viewing facility was used to determine whether brook trout Salvelinus fontinalis or rainbow trout Salmo gairdneri show hierarchical dominance in slow‐ and fast‐flow stream habitats. Nine trials with interspecific pairs of wild immature fish matched for length (mean fork length, 13.7 cm) were conducted in each flow regime. In the slow‐flow habitat, brook trout dominated rainbow trout in five of the six trials in which dominance was established. No species advantage was observed in the fast‐flow habitat; each species achieved dominance four times. Dominant fish, regardless of species or flow, initiated most of the aggressive behaviour and gained weight whereas subordinates lost weight. We suggest that the demonstrated dominance of brook trout in slow flows may be related to the speciesˈ preference for lentic habitats in natural stream environments and that this phenomenon may affect species segregation. Received September 26, 1983 Accepted August 25, 1984