Summer and fall microhabitat utilization of juvenile bull trout and cutthroat trout in a wilderness stream, Idaho

Microhabitat use and availability were evaluated and compared between diffe rent size classes of juvenile resident bull trout (Salvelinus confluentus) and cutthroat trout (Oncorhynchus clarki) in a small wilderness stream with in the South Fork Clearwater River basin, Idaho. The objective was to deter mine if utilization of measured habitat characteristics changed from summer to late fall. Sampling of fish was conducted with night snorkeling. During the summer, smaller juvenile bull trout (< 66 mm) total length (TL) were a ssociated with shallow stream margins over coarse substrates. In the fall, they moved to significantly deeper, lower velocity water, and closer to cov er (p <0.05), but maintained their association with coarse substrates. Duri ng the summer, larger juvenile bull trout and larger juvenile cutthroat tro ut (66-130 mm TL) occupied significantly deeper water than smaller juvenile bull trout (p <0.05). Generally, larger juvenile bull trout were found clo ser to the bottom and in lower velocity water than larger juvenile cutthroa t trout (p <0.05). In the fall, larger juvenile bull trout and larger juven ile cutthroat trout were associated with significantly deeper, lower veloci ty water located closer to cover than in summer (p <0.05). However, cutthro at trout occupied slightly deeper water over finer substrates than bull tro ut. Deep water with low velocities evidently provide important rearing area s for large bull trout and large cutthroat trout in the fall. Land manageme nt practices that maintain such environments will benefit these species.