Habitat shifts in rainbow trout: competitive influences of brown trout.

We compared habitat use by rainbow trout sympatric (three streams) and allopatric (two streams) with brown trout to determine whether competition occurred between these two species in the southern Appalachian Mountains. We measured water depth, water velocity, substrate, distance to overhead vegetation, sunlight, and surface turbulence both where we collected trout and for the streams in general. This enabled us to separate the effects of habitat availability from possible competitive effects. The results provided strong evidence for asymmetrical interspecific competition. Habitat use varied significantly between allopatric and sympatric rainbow trout in 68% of the comparisons made. Portions of some differences refelected differences in habitats available in the several streams. However, for all habitat variables measured except sunlight, rainbow trout used their preferred habitats less in sympatry with brown trout than in allopatry if brown trout also preferred the same habitats. Multivariate analysis indicated that water velocity and its correlates (substrate particle size and surface turbulence) were the most critical habitat variables in the interaction between the species, cover in the form of shade and close overhead vegetation was second most important, and water depth was least important.