Westslope and yellowstone cutthroat trout (Oncorhynchus clarki lewisi and O.c. bouvieri, respectively) are declining in both population and spatial distribution throughout their native range, driven in part by invasion of non-native fishes including rainbow trout (O. mykiss) and brown trout (Salmo trutta). The magnitude of cutthroat decline and pattern of rainbow/brown trout invasion is not spatially uniform, even within watersheds that share similar flow regimes.  Therefore we hypothesize that, in addition to flow regime, other physical characteristics contribute to the invasibility of cutthroat trout habitat within a given tributary. Over the next three years, we will test this hypothesis by: 1) selecting study catchments by mapping and analyzing the present distribution of the four salmonids (mentioned above) across the native range of both westslope and yellowstone cutthroat trout; 2) assessing existing patterns of invasion success within study catchments by utilizing stable isotope analyses of otoliths and fin rays to determine the key natal streams for each species and; 3) correlating patterns of catchment characteristics (e.g., landuse) with patterns of invasion success within study catchments. Information on relationships between catchment characteristics and invasion success will provide new approaches for furthering conservation efforts of native resident trout populations.