OCCURRENCE OF BULL TROUT IN NATURALLY FRAGMENTED HABITAT PATCHES OF VARIED SIZE

Bull trout Salvelinus confluentus and other salmonids in the Pacific N orthwest are believed at risk of local and regional extinctions becaus e of ongoing habitat loss and fragmentation. Biologists have focused o n defining and protecting critical stream channel characteristics, but there is little information regarding the scale or spatial geometry o f habitat that may be necessary for the species' long-term persistence . We investigated the influence of habitat patch size on the occurrenc e of bull trout by determining the presence or absence of fish in natu rally fragmented watersheds of the Boise River basin in Idaho. We defi ned patches of potential habitat for bull trout as watersheds above 1, 600 m elevation, a criterion based on the presumed restriction of loca l populations by stream temperature. We used logistic regression to in vestigate the possible influence of patch size as well as stream width and gradient on the occurrence of bull trout at reach, stream, and pa tch scales of analysis. Bath stream width and patch size were signific ant in the models, but individual effects could not be clearly resolve d because of collinearity. The predicted probability of occurrence bas ed on patch size alone was less than 0.10 for patches smaller than abo ut 1,000 ha and more than 0.50 for patches larger than about 2,500 ha. Our results support the hypothesis that area of available habitat inf luences the distribution of disjunct populations of bull trout. An are a effect is consistent with the predictions of island biogeography and metapopulation theory, and our work suggests that larger-scale spatia l processes may be important to the persistence of species like bull t rout.