COMPETITIVE INTERACTIONS FOR FORAGING MICROHABITAT AMONG INTRODUCED BROOK CHARR, SALVELINUS-FONTINALIS, AND NATIVE BULL CHARR, SALVELINUS-CONFLUENTUS, AND WEST SLOPE CUTTHROAT TROUT, ONCORHYNCHUS-CLARKI-LEWISI, IN A MONTANA STREAM

Competitive interactions for foraging microhabitat among introduced br ook charr, Salvelinus fontinalis, and native bull charr, S confluentus , and westslope cutthroat trout, Oncorhynchus clarki lewisi, were stud ied by species removal experiments in a tributary of the Flathead Lake and River system, northwestern Montana, focusing on brook charr influ ences on bull charr. When the three species were in sympatry, they int eracted with each other, forming a size-structured, mixed-species domi nance hierarchy in two stream pools. The influences of interference in teractions 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 remo val of two species. Cutthroat trout removal resulted in increased fora ging 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 p artly compensated for by subsequent bull charr immigration, implying t hat competitive interactions with brook charr are an important factor in the mechanisms responsible for the regulation of bull charr densiti es, at least on a local scale.