Factors That May Be Influencing Nonnative Brook Trout Invasion and Their Displacement of Native Westslope Cutthroat Trout in Three Adjacent Southwestern Montana Streams

Abstract

I examined trends in community structure and abundance in populations of native westslope cutthroat trout (WCT) Oncorhynchus clarki lewisi and nonnative brook trout Salvelinus fontinalis and fish habitat characteristics in three adjacent southwestern Montana streams during 1988, 1993–1995, and 1997–1999. Delano and upper Jerry creeks had relatively undisturbed riparian forests, whereas Libby Creek's riparian forest was harvested along much (0.9 km) of the stream during 1971. Libby Creek had higher summer water temperatures (average daily temperatures were 1–2°C higher) than Delano Creek. Libby Creek also contained more woody debris, a higher percentage of fine sediments within the streambed, and a higher proportion of pool habitats than Delano and upper Jerry creeks. Brook trout dominated the trout community in Libby Creek (78– 91%), whereas WCT dominated in Delano and upper Jerry creeks (77–100%). Similarly, brook trout densities were significantly higher in Libby Creek, and WCT densities were significantly greater in Delano and upper Jerry creeks. These data suggest that brook trout invasion and the displacement of WCT in these streams may have been influenced by water temperature, debris and pool frequencies, erosion and deposition of fine sediments on the streambed, or some combination of these factors. Past research has linked changes in these physical parameters to riparian timber harvest activities. More research is needed to better quantify potential influences of abiotic factors on brook trout invasion and the subsequent displacement of WCT. Until such research is completed, resource managers who wish to protect and conserve stream populations of native WCT should consider the effects of land management and fish restoration activities on the aforementioned stream habitat conditions, especially in systems that contain or could potentially be invaded by nonnative brook trout.