Factors Influencing the Distribution of Bull Trout and Westslope Cutthroat Trout West of the Continental Divide in Glacier National Park

The reported decline of native bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarkii lewisi populations west of the Continental Divide in Glacier National Park (GNP) prompted research to identify critical habitats and investigate factors influencing their distribution and relative abundance.  We evaluated the association of six abiotic factors (stream width, elevation, gradient, large woody debris density, pool density, mean August stream temperature) and a biotic factor (the presence of nonnative lake trout, Salvelinus namaycush) with the occurrence and density of bull trout and westslope cutthroat trout in 79 stream reaches in five sub-drainages of the North Fork Flathead River in GNP.  Logistic and linear regression models were used to quantify the influence of these independent variables on species occurrence (presence/absence) and density (>age-1 fish/100m²), and an information theoretic approach (AIC_c) was used to determine the most plausible combinations of variables in each case. The occurrence of westslope cutthroat trout was negatively associated with the presence of lake trout and positively associated with large woody debris and water temperature. Westslope cutthroat were detected throughout a wide range of mean summer (August) water temperatures (8.5 – 16^oC), stream widths and elevations, but were most abundant in narrow, complex reaches that were not connected to lakes supporting lake trout.  Bull trout occurrence was positively related to stream width and negatively related to channel gradient and water temperature.  Bull trout were most abundant in narrow (< 10 m) stream reaches with relatively cold August water temperatures (8 – 10^oC) and in stream reaches not affected by lake trout. The low densities and limited distribution of bull trout likely reflect their imperiled status in GNP, owing to the invasion and establishment of nonnative lake trout from Flathead Lake.  These data may be used to monitor critical habitats and populations, inform conservation and recovery programs, and guide suppression efforts to reduce the deleterious impacts of nonnative invasive fishes.