Effect of bull trout and brook trout interactions on foraging habitat, feeding behavior, and growth

Observations of free-ranging sympatric bull trout Salvelinus confluentus and nonnative brook trout S.fontinalis in two eastern Oregon headwater streams provided little evidence of habitat partitioning. Both species held focal feeding points in similar microhabitats and fed primarily from the water column rather than from the surface or benthos. In an instream experiment, 20 enclosures were assigned one of three treatments: two bull trout, four bull trout, or a mix of two bull trout and two brook trout. In the enclosures, macroinvertebrate drift was restricted and trout densities were elevated, creating an environment of reduced food and habitat resources. Under these conditions, there was no indication of a niche shift by bull trout; feeding behavior and habitat use by bull trout did not differ depending on the presence or absence of brook trout. Brook trout in the mixed-species treatment were the most aggressive, maintained dominance in 75% of the enclosures, and exhibited significantly higher growth than sympatric bull trout. However, the effects of intra-and interspecific interactions on bull trout growth were equivalent. Given the absence of resource partitioning and a niche shift by bull trout in the presence of brook trout (despite obvious interference interactions), we suggest that the displacement of bull trout by brook trout is likely when resources are scarce.