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Montana Field Guides

Westslope Cutthroat Trout - Oncorhynchus clarkii lewisi

Species of Concern
Native Species

Global Rank: G5T4
State Rank: S2
(see State Rank Reason below)

Agency Status


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State Rank Reason (see State Rank above)
The Westslope Cutthroat trout is currently ranked "S2" in Montana because it is at risk due to very limited and/or potentially declining population numbers, range and/or habitat, making it vulnerable to extirpation in the state.
General Description
The Westslope Cutthroat Trout is one of two subspecies of native cutthroat found in the state. Together, they have been designated Montana's state fish. Cutthroat trout are so named for the red slashes near the lower jaws. The Westslope Cutthroat Trout's historical range was all of Montana west of the Continental Divide as well as the upper Missouri River drainage. This fish has been seriously reduced in its range by two primary factors: hybridization with Rainbow and/or Yellowstone Cutthroat Trout, and habitat loss and degradation. Since the Westslope is recognized as a very important part of our native fish fauna it has been designated a Montana Fish of Special Concern in Montana. Pure Westslope Cutthroat Trout have been identified by genetic analysis and form the broodstock maintained by the Montana Department of Fish, Wildlife, and Parks at its Anaconda hatchery. The average size of these fish is 6 to 16 inches, depending on habitat, but they rarely exceed 18 inches in length.

Westslope Cutthroat Trout are common in both headwaters lake and stream environments. They feed primarily on aquatic insect life and zooplankton. Cutthroat spawn in the spring in running water, burying their eggs in a nest called a redd. The eggs hatch in a few weeks to a couple of months. The newborn fry frequently migrate back to lakes to rear after 1 to 2 years in their native stream. Westslope Cutthroat Trout is a trout with small, non-rounded spots, with few spots on the anterior body below the lateral line. Coloration varies, but generally is silver with yellowish hints, though bright yellow, orange, and especially red colors can be expressed to a much greater extent than on coastal or Yellowstone Cutthroat Trout (Behnke 1992). Hybridization between Westslope and Yellowstone Cutthroat Trout can produce a spectrum of spotting and coloration ranging between the typical patterns of each subspecies. Some populations that have been affected by hybridization show little or no phenotypic signs of hybridization (Behnke 1992). Hybridization with Rainbow Trout can be detected by the appearance of spots on the top of the head and on the anterior body below the lateral line, as well as by reduced scale counts, increased caecal counts, and loss of basibranchial teeth (Behnke 1992).

For a comprehensive review of the ecology, conservation status, threats, and management of this and other Montana fish species of concern, please see Montana Chapter of the American Fisheries Society Species of Concern Status Reviews.

Diagnostic Characteristics
Tiny teeth are usually present on the floor of the mouth behind the tongue. The lower sides are red during spawning season (Montana Fish, Wildlife, and Parks). In Montana both pure and moderately hybridized populations of Westslope Cutthroat Trout have a high incidence of basibranchial teeth, whereas pure Rainbow Trout lack these teeth. The presence of basibranchial teeth in some individuals of a Rainbow Trout population indicates hybridization with Westslope Cutthroat Trout (Leary et al. 1996).

It can be difficult to visually distinguish Westslope from other cutthroat trout subspecies, but the Westslope Cutthroat Trout tends to have more small spots by the tail and none by the pectoral fin and the fish is more of a silvery or greenish color. The only way to be certain about identification of this subspecies is by genetic testing (Montana AFS Species Status Account).

Species Range
Montana Range

All Ranges
(Click legend blocks to view individual ranges)

Western Hemisphere Range


Range Comments
The westslope cutthroat trout is found in the Kootenai watershed, the Clark Fork watershed, the headwaters of the Missouri river and the headwaters of the Saskatchewan River. This subspecies is also found in the Salmon River and Coeur d'Alene Basins of Idaho and Canada and has a few scattered populations in Wyoming, Washington and Oregon.

Observations in Montana Natural Heritage Program Database
Number of Observations: 19736

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Westslope cutthroat trout have three possible life forms, adfluvial (migrates to lakes), fluvial (migrates to rivers) or resident (stays in streams). All three life forms spawn in tributary streams in the springtime when water temperature is about 10 degrees C and flows are high with spring run-off.

Spawning and rearing streams tend to be cold and nutrient poor. Westslope Cutthroat Trout seek out gravel substrate in riffles and pool crests for spawning habitat. Cutthroat trout have long been regarded as sensitive to fine sediment (generally defined as 6.3 millimeters or less). Although studies have documented negative survival as fine sediment increases (Weaver and Fraley 1991), it is difficult to predict their response in the wild (McIntyre and Rieman 1995). This is due to the complexity of stream environments and the ability of fish to adapt somewhat to changes in micro-habitat (Everest et al. 1987, Montana AFS Species Status Account).

Westslope Cutthroat Trout also require cold water, although it has proven elusive to define exact temperature requirements or tolerances. Likewise, cutthroat trout tend to thrive in streams with more pool habitat and cover than uniform, simple habitat (Shepard et al. 1984). Juvenile cutthroat trout overwinter in the interstitial spaces of large stream substrate. Adult cutthroat trout need deep, slow moving pools that do not fill with anchor ice in order to survive the winter (Brown and Mackay 1995, Montana AFS Species Status Account).

Food Habits
Westslope cutthroat trout primarily are invertivores through most life-stages: eating insects and zooplankton and do not grow very large, usually just between 6 and 12 inches. Larger-aged fish (greater than 12 inches) may eat sculpin or other small aquatic vertebrates.

Westslope Cutthroat Trout have three possible life forms, adfluvial (migrates to lakes), fluvial (migrates to rivers) or resident (stays in streams). All three life forms spawn in tributary streams in the springtime when water temperature is about 10 degrees Celsius and flows are high (Liknes and Graham 1988). While resident fish spend their entire life in tributary streams, migratory life forms can travel several hundred kilometers as they move between adult and spawning habitat (Montana AFS Species Status Account).

In Montana, the maximum age of 475 fish from 29 headwater streams was 8 years-old based on otoliths (Downs 1995).

Reproductive Characteristics
In headwaters streams in Montana, age at sexual maturity ranged from 2 to 4 years-old for males and from 3 (26%) to 5 (93%) years-old in females (Downs 1995). Size at sexual maturity in these streams ranged from 110 to 160 millimeters in males and 150 to 180 in females (Downs 1995). Female fecundity ranged from 198 to 630 eggs and was generally correlated with length (Downs 1995).

Management of this species involves protecting the population strongholds and making tough decisions on restoration priorities for the depressed populations. The State of Montana has altered fishing regulations to reduce fishing mortality. Montana has also developed a Conservation Agreement signed by nine government agencies and conservation groups (Montana Department of Fish, Wildlife and Parks 1999). This agreement prioritizes protecting genetically pure populations first, then slightly introgressed populations. Recovering depressed populations will involve habitat restoration and removing non-native species. Research suggests that it is not a good idea to bolster populations with stocked fish from other watersheds due to considerable genetic variation between watersheds (Leary et al. 1998). It will be especially challenging to recover migratory life forms. Governmental agencies will need to work together to share expertise, pool financial resources and monitor progress toward restoration of this species (Montana AFS Species Status Account).

Threats or Limiting Factors
There are four primary reasons for the decline of this species.
1) Habitat loss is considered to be a widespread problem. Cutthroat trout have declined due to sedimentation and warming water temperatures in streams due to poor grazing practices, logging, mining, agriculture, residential development. Loss of hundreds of stream miles of spawning habitat due to dewatering of streams for irrigation and barriers created by dams and road culverts.
2) Non-native species have also taken a huge toll on westslope cutthroat trout. Brook trout, rainbow and brown trout outcompete juvenile cutthroat trout for food and dominate a stream.
3) A third reason for decline is the role of hybridization with other species. Westslope cutthroat trout readily hybridize with rainbow trout and other non-native cutthroat trout subspecies.
4)A fourth cause of decline has been overfishing and harvesting. Westslope cutthroat trout are highly susceptible to angling.

  • Literature Cited AboveLegend:   View Online Publication
    • Behnke, R. J. 1992. Native trout of western North America. American Fisheries Society Monograph 6.
    • Brown, R.S. and W.C. Mackay. 1995. Fall and winter movements of and habitat use by cutthroat trout in the Ram River, Alberta. Transactions of the American Fisheries Society 124(6): 873-885.
    • Downs, C. C. 1995. Age determination, growth, fecundity, age at sexual maturity, and longevity for isolated, headwater populations of westslope cutthroat trout. M.S. Thesis, Montana State University, Bozeman. 81 pp.
    • Everest, F.H., R.L. Beschta, J.C. Scrivener, K.V. Koski, J.R. Sedell and C.J. Cederholm. 1987. Fine sediment and salmonid production: a paradox. Pp 98-112 In: Streamside Management: Forestry and Fishery Interactions. University of Washington, Seattle, Washington. E.O. Salo and T.W. Cundy, tech Ed.
    • Leary, R.F., F.W. Allendorf and N. Kanda. 1998. Lack of genetic divergence between westslope cutthroat trout from the Columbia and Missouri River drainages. Wild Trout and Salmon Genetics Laboratory Report 97/1. Missoula, Montana.
    • Leary, R.F., W.R. Gould, and G.K. Sage. 1996. Success of basibranchial teeth in indicating pure populations of rainbow trout and failure to indicate pure populations of westslope cutthroat trout. North American Journal of Fisheries Management 16:210-213.
    • Liknes, G. A. and P. J. Graham. 1988. Westslope cutthroat trout in Montana: life history, status, and management. American Fisheries Society Symposium 4:53-60.
    • McIntyre, J. D., and B. E. Rieman. 1995. Westslope cutthroat trout. Pp 1-15 In: M. K. Young, technical editor. Conservation assessment for inland cutthroat trout. USDA Forest Service General Technical Report RM-GTR-256. iv + 61 pp.
    • Montana Chapter of the American Fisheries Society species status accounts.
    • Montana Department of Fish, Wildlife & Parks. 1999. Memorandum of understanding and conservation agreement for westslope cutthroat trout (Oncorhynchus clarki lewisi) in Montana. 28 pp.
    • Shepard, B.B., K.L. Pratt, and P.J. Graham. 1984. Life histories of westslope cutthroat and bull trout in the upper Flathead River Basin, MT. Environmental Protection Agency. 85 pp.
    • Weaver, T. and J. Fraley. 1991. Fisheries habitat and fish populations. Flathead Basin Forest Practices Water Quality and Fisheries Cooperative Program. Flathead Basin Commission. Kalispell, Montana.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • American Gem Corporations. 1996. Application for an Operating Permit and Proposed Plan of Operations: Gem Mountain Sapphire Mine, Granite County, Montana. August 1996
    • Bear, E.A. 2005. Effects of temperature on survival and growth of Westslope Cutthroat Trout and Rainbow Trout: implications for conservation and restoration. M.Sc. Thesis. Bozeman, MT: Montana State University. 68 p.
    • Gustafson, D.R. 2001. Westslope cutthroat hypothesis. Presentation to the American Fisheries Society Meeting, Bozeman, MT.
    • Leathe, S. 2005. Westslope cutthroat trout restoration in north-central Montana, Project performance report, July 1, 2003 through June 30, 2005
    • Leathe, S. 2006. Westslope cutthroat trout restoration in north-central Montana, Final project performance report
    • Liknes, G.A. 1984. The present status and distribution of the westslope cutthroat trout (Salmo clarki lewisi) east and west of the Continental Divide in Montana. Montana Deptartment of Fish, Wildlife, and Parks. 163 pp.
    • McIntyre, J.D. and B.E. Rieman. 1995. Westslope Cutthroat Trout IN Conservation Assessment for Inland Cutthroat Trout. General Technical Report RM-256. US Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado. M.K. Young, tech. Ed. Pages 1-15.
    • Montana Dept. of Fish, Wildlife and Parks. Region Four., 1996, Draft Environmental Analysis for Weed Management.
    • Moser, D., Tews, a., Enk, M. 2006. Northcentral Montana cooperative westslope cutthroat restoration project, 2005 Annual Report.
    • Western EcoTech, Helena, MT., 1999, Wetland delineation report for the Haskins Landing Proposed Wetland Mitigation Area. MWFE? June 2, 1999.
  • Web Search Engines for Articles on "Westslope Cutthroat Trout"
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Westslope Cutthroat Trout — Oncorhynchus clarkii lewisi.  Montana Field Guide.  .  Retrieved on , from