Assessing Competition Between Brook and Bull Trout in an Artificial Stream

Bull trout are the most widely distributed native Salvelinus species in mountainous regions of western inland North America. Threats to bull trout populations are many, and one frequently cited example is displacement from introduced exotic brook trout. In this laboratory experiment, we conducted trials of competition between these two species to determine if various abiotic and biotic features of bull trout populations and streams affects the competitive interaction in cold (8 °C) water; typical mean daily temperatures of headwater mountain streams where the two species commonly overlap. We measured aggression, foraging success and ability to defend profitable feeding territory for each species at both night and day periods. The latter was based on the hypothesis that diel activity patterns may be different between the two species. We also examined whether bull trout from populations with migratory or resident life histories competed differently against brook trout, and if fish density affected the interaction. Bull trout generally foraged more successfully than brook trout at low densities, but not at high densities, where brook trout were the more aggressive competitor. Time of day did not affect interspecific competition, and bull trout of migratory life histories competed more successfully against the exotic than those of resident life histories. These results suggest that migrant populations of bull trout may have more biotic resistance to invasion than resident ones, and that the two species respond differently to fish density. This may help explain some of the patchy patterns of successful brook trout invasion in cold headwater mountain streams.