Year of Award

2010

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Environmental Studies

Department or School/College

Environmental Studies Program

Committee Co-chair

Vicki J. Watson, Clint C. Muhlfeld

Commitee Members

Len Broberg

Keywords

bull trout, Glacier National Park, multiple regression, nonnative species, North Fork of the Flathead River, westslope cutthroat trout

Abstract

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. I 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 or older fish/100m2), and an information theoretic approach (AICc) 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 water temperatures (8.5-16oC), stream sizes and elevations, but were most abundant in small, complex streams 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 wetted width) streams with relatively cold mean August water temperatures (8-10oC) and in stream reaches not affected by lake trout. The low densities and limited distribution of bull trout observed in this study reflect the imperiled status of adfluvial populations 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.

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© Copyright 2010 Vincent Stephen D'Angelo