Westslope Cutthroat Trout - Oncorhynchus clarkii lewisi
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'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, 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 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 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 (FWP). Westslope cutthroat is a trout with small, nonrounded 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 (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).
Tiny teeth are usually present on the floor of the mouth behind the tongue. The lower sides are red during spawning season (FWP). 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 (AFS website 2003).
Summary of Observations Submitted for Montana
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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 milimeters 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 microhabitat (Everest et al. 1987) (AFS website 2003).
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, Pratt and Graham 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) (AFS website 2003).
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 (AFS website 2003).
In Montana, the maximum age of 475 fish from 29 headwater streams was 8 years-old based on otoliths (Downs 1995).
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 milimeters in males and 150 to 180 in females (Downs 1995). Female fecundity ranged from 198 to 630 eggs and was generally correlated with lenght (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, Allendorf and Kanda 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 (AFS website 2003).
- Additional ReferencesLegend: View WorldCat Record View Online Publication
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- American Fisheries Society (AFS), Montana Chapter Website.
- American Gem Corporations, USA, Helena, MT., 1996, Application for an Operating Permit and Proposed Plan of Operations: Gem Mountain Sapphire Mine, Granite County, Montana. August 1996
- 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 MT. MT. Dept. 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.
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