The present status and distribution of the westslope cutthroat trout (Salmo clarki lewisi) east and west of the Continental Divide in Montana

The distribution and abundance of westslope cutthroat trout ( Salmo clarki lewisi) has drastically declined over its historic range in the last 100 years.Although previous studies in Montana have identified strongholds, the status of westlope cutthroat statewide continued to be an uncertainty.The purpose of this report was to quantitatively determine the present and historic range as well as identify strongholds of genetically pure westslope cutthroat in Montana.The historic range of westslope cutthroat in Montana includes all waters west of the continental divide, that portion of the Missouri River drainage upstream from Fort Benton and the headwaters of the Marias, Judith, Musselshell and Milk rivers.Three life history forms are found within this range: 1) adfluvial stocks, 2) fluvial populations and 3) nonmigratory resident fish.Behavioral rather than morphological differences appear to separate these stocks.Migratory adults travel long distances during high streamflows and spawn when water temperatures are near 10 C (50 F).Most migratory adults quickly leave the spawning grounds.Sexual maturity is attained at age III or older.Alternate year spawning is predominant in some drainages.Westslope cutthroat are opportunistic in their food habits, but are not highly predaceous;they tend to specialize as an invertebrate feeder.The original range of westslope cutthroat in Montana was estimated to be 25,547 km, which was considered a conservative estimate.A total of 655 streams, comprising 6,993 stream km, are currently inhabited by westslope cutthroat trout in Montana; this represents 27.4% of the estimated historic range.However, these figures include waters that contain contaminating species and known introgressed populations.Genetically pure populations were documented in 25 streams, making up 290.3 stream km, which represents 1.1% of the historic range and 4.1% of the current range of stream km.Streams most likely to contain additional pure populations, which are those that received a genetic value rating of 2 or 3, comprised 24.7% of the current stream km.These, combined with the already known pure populations, comprise 2,017.1 stream km or 28.8% of the current and 7.9% of the historic range.These figures represent an optimistic estimation of the current status of genetically pure westslope populations in Montana streams.The upper Flathead River basin above Flathead Lake represents the largest stronghold of westslope cutthroat in Montana today; 161 streams and approximately 1,903.5 stream km, which is 85% of the historic range in the drainage, was considered westslope cutthroat habitat.Fifty-eight percent of the known pure populations of westslope are found in the drainage.Within the drainage, the S. Fork Flathead River is the largest and most secure stronghold.The Clark Fork River drainage below the mouth of the Bitterroot River could be considered the second largest stronghold in the state, since 1,108.6stream km were found to be westslope waters.The Smith River drainage is the largest stronghold of native cutthroat east of the divide in terms of total stream km (315.7 stream km) and in terms of the area where contaminants are absent and haven't been planted (86.3 stream km).viii Statewide, 259 lakes contain or are believed to contain westslope cutthroat populations.Six percent are known to contain genetically pure populations.Fifteen of the 16 pure populations are found within Glacier National Park; the other population is located on the Flathead Indian Reservation.Only 4 lakes or reservoirs east of the continental divide were found to contain westslope populations.General factors affecting population abundance as well as existing conservation measures are discussed.Recommendations for future actions are included.IX I than in other areas of the lake.Spawners quickly moved back down into th main river after spawning was completed, arriving by mid-summer.The longest and swiftest upstream movement documented was 212 km in 87 days.II Juveniles emigrate downstream primarily at ages II and III, but the number of age I out-migrants can also be substantial.Most juveniles move downstream in the spring and early summer.Some juvenile cutthroats overwintered in the North or Middle Fork while migrating but most moved downstream to the main river.They remained in the lower river overwinter or longer before entering Flathead Lake.Emigration has also been documented in the fall (May and Huston 1974 and 1975; Bjornn and Mallet 1964), which may be a result of inadequate overwintering habitat.TrailCreek, a tributary of the North Fork Flathead River, had a high number of juveniles emigrate in the fall.It also appeared that juveniles migrated up into Trail Creek in the spring. ReproductionBoth migratory adults that move into the tributaries of the upper Flathead basin during high streamflows and resident adults spawn during Ma ; and June.Spawning is dependent on water temperature and occurs from Marcl to July (Roscoe 1974) when water temperatures are near 10 C (50 F) (Scott and Crossman 1973).Most migratory adults spend little time in the tributaries; most left by the beginning of July.The migrants tend to us« only the lower reaches of the tributaries (Johnson 1963).Cutthroat may select smaller tributaries for spawning to decrease mortality associated with scouring of the streambed which can occur in larger streams during high water conditions (Johnson 1963).Cutthroat trout in the Flathead basin attain sexual maturity at age 1^ 8 9 10