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Chapter 1 Westslope Cutthroat Trout Oncorhynchus clarki lewsi Patrick Van Eimeren Fisheries Biologist USDA Forest Service Flathead National Forest 1935 3rd Avenue East Kalispell, Montana 59901 INTRODUCTION Westslope cutthroat trout Oncorhynchus clarki lewisi and Yellowstone cutthroat trout O. C. bouvieri most likely separated from a common ancestor prior to the last glacial period during Pleistocene times (Behnke 1992). It appears that cutthroat trout were the earliest of the trout and salmon to populate the headwaters of the Columbia River basin enabling them to colonize above major barrier falls (Behnke 1979). Headwater transfers established westslope cutthroat trout populations in the upper Missouri River and South Saskatchewan River basins. Schultz (1941) describes a direct connection between the Flathead and Marias river systems. Current distribution (Figure 1) includes disjunct populations in Oregon and Washington and popula- tions in the upper Columbia River above barrier falls on the Pend Oreille, Spokane, and Kootenai rivers, including British Columbia headwaters (Behnke 1992). Isolated and disjunct populations of westslope cutthroat trout in the John Day drainage of Oregon and Lake Chelan, Methow, Entiat, Yakima, and Wenatchee river drainages of Washington may have ensued from catastrophic floods as glacial Lake Missoula broke through its ice dam 12,000- 15,000 years ago (Behnke 1992). Westslope cutthroat trout are also established in the Salmon and Clearwater drainages apparently through headwater transfers from the Clark Fork River after the Yellowstone form was likely displaced by redband trout (0. mykiss) (Behnke 1992). East of the Continental Divide, westslope cutthroat trout occur in fragmented populations in the upper Missouri River including headwaters of the Gallatin and Madison rivers in Yellowstone National Park downstream to the confluence with the Musselshell River. Populations also occur in the South Saskatchewan drainage in Glacier National Park and southern Alberta (Behnke 1992) ASSESSMENT METHODS Standardized questionnaires were developed for determining the status of inland cutthroat trout. A sample of the questionnaire is included in Appendix A. Attempts were made to use the questionnaire, however due to the numerous populations of wests lope cutthroat trout across its range other methods to document status were also employed. In Idaho, the status and distribution of westslope cutthroat trout was recently compiled by Rieman and Apperson (1989). Discussions with biologists suggested that this data was current and little updated data existed, therefore the information included in this assessment for Idaho was derived from Rieman and Apperson (1989). 1