Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis, and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, in a Montana stream

Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis, and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, were studied by species removal experiments in a tributary of the Flathead Lake and River system, northwestern Montana, focusing on brook charr influences on bull charr. When the three species were in sympatry, they interacted with each other, forming a size-structured, mixed-species dominance hierarchy in two stream pools. The influences of interference interactions were examined by measuring changes in five characteristics of foraging microhabitat and behavior, focal point height and velocity, cover use, and foraging rate and distance, after the successive removal of two species. Cutthroat trout removal resulted in increased foraging rates and distances, and decreased cover use for brook charr, but no changes for bull charr. After removal of brook charr from the two-species system, bull charr also increased foraging rates and distances and occupied more exposed positions. Moreover, total fish densities, which had initially decreased owing to the removal experiments, were partly compensated for by subsequent bull charr immigration, implying that competitive interactions with brook charr are an important factor in the mechanisms responsible for the regulation of bull charr densities, at least on a local scale.