The influence of temperature and habitat on the distribution of chiselmouth, Acrocheilus alutaceus, in British Columbia

We intensively sampled fish in rivers and streams within a single major dra inage basin (the Blackwater River) and across major drainages in British Co lumbia to assess the factors influencing distribution of chiselmouth, Acroc heilus alutaceus, and to develop models for predicting chislemouth presence . Chiselmouth were typically absent from sites with maximum temperatures be low 20 degreesC or 2100 annual degree days, both within a single drainage a nd between larger drainages. Indices of stream size (bankfull channel width and basin area) were the most significant predictors of chiselmouth presen ce within the Blackwater drainage (p = 0.016 and p = 0.032, respectively), and inclusion of thermal variables only marginally increased classification success. In contrast, bankfull channel width and basin area were poor pred ictors of chiselmouth presence in mainstem habitat within larger drainage b asins throughout British Columbia. Inclusion of thermal variables (particul arly degree days > 12 degreesC) doubled correct classification rates of chi selmouth presence across larger drainage basins. These habitat associations suggest that water temperature is the primary constraint on presence of ch iselmouth populations in larger drainages across a landscape, while selecti on of different habitat types (mainstem habitat over smaller tributaries) d etermines distribution within any given basin.