Overview

Distribution

National Distribution

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

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Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) West of the Continental Divide, this subspecies is believed to be native to several major drainages of the Columbia River basin, including the upper Kootenai River drainage from its headwaters in British Columbia, through northwest Montana, and into northern Idaho; the Clark Fork River drainage of Montana and Idaho downstream to the falls on the Pend Oreille River near the Washington-British Columbia border; the Spokane River above Spokane Falls and into Idaho's Coeur d'Alene and St. Joe River drainages; and the Salmon and Clearwater River drainages of Idaho's Snake River basin (USFWS 2003). The native distribution also includes disjunct areas draining the east slope of the Cascade Mountains in Washington (Methow River and Lake Chelan drainages, and perhaps the Wenatchee and Entiat river drainages), the John Day River drainage in northeastern Oregon, and the headwaters of the Kootenai River and several other disjunct regions in British Columbia (USFWS 2003). East of the Continental Divide, the native distribution is believed to include the headwaters of the South Saskatchewan River drainage (United States and Canada); the entire Missouri River drainage upstream from Fort Benton, Montana, and extending into northwest Wyoming; and the headwaters of the Judith, Milk, and Marias rivers, which join the Missouri River downstream from Fort Benton (USFWS 2003).

Behnke (1992) regarded the mountain cutthroat trout of British Columbia (nominal subspecies alpestris) as disjunct populations of O. clarkii leweisi.

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Physical Description

Diagnostic Description

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; presence of basibranchial teeth in some individuals of a rainbow trout population indicates hybridization with westslope cutthroat trout (Leary et al. 1996).

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Ecology

Habitat

Montana Valley and Foothill Grasslands Habitat

This taxon can be found in the Montana valley and foothill grasslands ecoregions, along with some other North American ecoregions. This ecoregion occupies high valleys and foothill regions in the central Rocky Mountains of Montana in the USA and Alberta, Canada. The ecoregion the uppermost flatland reaches of the Missouri River drainage involving part of the Yellowstone River basin, and extends into the Clark Fork-Bitterroot drainage of the Columbia River system. The ecoregion, consisting of three chief disjunctive units, also extends marginally into a small portion of northern Wyoming. Having moderate vertebrate species richness, 321 different vertebrate taxa have been recorded here.

The dominant vegetation type of this ecoregion consists chiefly of wheatgrass (Agropyron spp.) and fescue (Festuca spp.). Certain valleys, notably the upper Madison, Ruby, and Red Rock drainages of southwestern Montana, are distinguished by extensive sagebrush (Artemisia spp.) communities as well. This is a reflection of semi-arid conditions caused by pronounced rain shadow effects and high elevation. Thus, near the Continental Divide in southwestern Montana, the ecoregion closely resembles the nearby Snake/Columbia shrub steppe.

A number of mammalian species are found in the ecoregion, including: American Pika (Ochotona princeps), a herbivore preferring talus habitat; Bighorn Sheep (Ovis canadensis), Black-tailed Prairie Dog (Cynomys ludovicianus), who live in underground towns that may occupy vast areas; Brown Bear (Ursos arctos); Hoary Marmot (Marmota caligata), a species who selects treeless meadows and talus as habitat; and the Northern River Otter (Lontra canadensis), a species that can tolerate fresh or brackish water and builds its den in the disused burrows of other animals.

There are six distinct anuran species that can be found in the Montana valleys and foothills grasslands, including: Canadian Toad (Anaxyrus hemiophrys); Western Toad (Anaxyrus boreas); Northern Leopard Frog (Lithobates pipiens); Plains Spadefoot Toad (Spea bombifrons); Columbia Spotted Frog (Rana luteiventris), an anuran that typically breeds in shallow quiet ponds; and the Boreal Chorus Frog (Pseudacris maculata).

Exactly two amphibian taxa occurr in the ecoregion: Long-toed Salamander (Ambystoma macrodactylum), a species who prefers lentic waters and spends most of its life hidden under bark or soil; Tiger Salamander (Ambystoma tigrinum).

Reptilian species within the ecoregion are: Milk Snake (Lampropeltis triangulum), an adaptable taxon that can be found on rocky slopes, prairie and near streambeds; Painted Turtle (Chrysemys picta); Western Plains Garter Snake (Thamnophis radix), a taxon that can hibernate in the burrows of rodents or crayfish or even hibernate underwater; Yellow-bellied Racer (Coluber constrictor); Spiny Softshell Turtle (Apalone spinifera); Western Terrestrial Garter Snake (Thamnophis elegans); Rubber Boa (Charina bottae); Western Skink (Plestiodon skiltonianus); and the Western Rattlesnake (Crotalis viridis).

The ecoregion supports endemic and relict fisheries: Westslope Cutthroat Trout (Oncorhynchus clarki lewisi), Yellowstone Cutthroat Trout (Oncorhynchus clarkii bouvieri), and fluvial Arctic Grayling (Thymallus arcticus), a relict species from past glaciation.

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Habitat Type: Freshwater

Comments: Small mountain streams, main rivers, and large natural lakes; requires cool, clean, well-oxygenated water; in rivers, adults prefer large pools and slow velocity areas (stream reaches with numerous pools and some form of cover generally have the highest fish densities); often occurs near shore in lakes (Spahr et al. 1991). Juveniles of migratory populations may spend 1-4 years in their natal streams, then move (usually in spring or early summer, and/or in fall in some systems) to a main river or lake where they remain until they spawn (Spahr et al. 1991, McIntyre and Rieman 1995). Many fry disperse downstream after emergence (McIntyre and Rieman 1995). Juveniles tend to overwinter in interstitial spaces in the substrate. Larger individuals congregate in pools in winter.

Spawns in small tributary streams on clean gravel substrate; mean water depth is 17-20 cm and mean water velocity is 0.3-0.4 m/sec; tends to spawn in natal stream (see McIntyre and Rieman 1995). Adfluvial populations live in large lakes in the upper Columbia drainage and spawn in lake tributaries. Fluvial populations live and grow in rivers and spawn in tributaries. Resident populations complete the entire life history in tributaries. All three life-history forms may occur in a single basin (McIntyre and Rieman 1995). Migrants may spawn in the lower reaches of the same streams used by resident fishes. Maturing adfluvial fishes move into the vicinity of tributaries in fall and winter and remain there until they begin to migrate upstream in spring. Of migratory spawners, some remain in tributaries during summer months but most return to the main river or lake soon after spawning (Behnke 1992).

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Migration

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: Yes. At least some populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

Resident populations spend entire life in small stream; migratory cutthroat may travel about 25-50 miles between breeding and nonbreeding habitats (Spahr et al. 1991). Trout from large lakes such as Coeur d'Alene, Priest, Pend Oreille, and Flathead lakes may migrate 150 km or more between spawning (upstream) and nonspawning (lake) habitats (Behnke 1992). Extent of migrations may vary with the availability of high quality pools suitable for overwintering. See also reproduction comments.

In the Blackfoot River drainage (Montana), 16 of 22 radio-tagged individuals migrated during the spawning period; migrations to tributaries occurred during the rising limb of the hydrograph; migratory fishes moved both upriver and downriver to reach spawning tributaries; in one year the mean distance traveled to access tributaries was 31 km (range 3-72 km); once in tributaries, individuals generally remained within a 200-m reach; neither of two repeat migrants spawned within 3 km of the previous year's spawning location, though both traveled in the same tributaries; after leaving tributaries fishes moved both upriver and downriver to overwintering areas and did not move more than 100 m thereafter; individuals did not exhibit fidelity to their prespawning main-stem locations; in general, fishes demonstarted the large spatial extent over which fluvial westslope cutthroat trout use aquatic resources (Schmetterling 2001).

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Trophic Strategy

Comments: Feeds mainly on aquatic and terrestrial insects and zooplankton; diet includes relatively few fishes (Spahr et al. 1991, McIntyre and Rieman 1995).

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Population Biology

Number of Occurrences

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: > 300

Comments: This subspecies is represented by numerous robust populations, including several hundred "conservation" populations (USFWS 1999, 2000, 2003; Shepard et al. 2005; May 2009).

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Global Abundance

100,000 to >1,000,000 individuals

Comments: Total adult population size is unknown but very large.

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General Ecology

Metapopulation theory may apply to this species (see McIntyre and Rieman 1995).

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Life History and Behavior

Reproduction

Spawns March-July, depending on elevation, at water temperatures near 10 C; usually first spawns at age 4 or 5; alternate-year spawning has been reported in the Flathead River basin in Montana and elsewhere; repeat spawners may comprise up to about 24% of the adult population (Spahr et al. 1991, McIntyre and Rieman 1995).

In the Blackfoot River drainage, Montana, fishes spawned as flows subsided after the peak discharge; 38% of individuals died after spawning (Schmetterling 2001).

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Conservation

Conservation Status

National NatureServe Conservation Status

Canada

Rounded National Status Rank: N3 - Vulnerable

United States

Rounded National Status Rank: N4 - Apparently Secure

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NatureServe Conservation Status

Rounded Global Status Rank: T4 - Apparently Secure

Reasons: Range much reduced but still widespread in British Columbia, Alberta, Washington, Oregon, Idaho, Montana, and Wyoming; currently occupies approximately 54,000 stream-kilometers; many protected and appropriately managed populations; major threat is genetic introgression from introduced exotic fishes.

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Global Short Term Trend: Decline of 10-30%

Comments: Trend over the past 10 years or three generations is uncertain, but area of occupancy and abundance probably have been slowly declining.

Shepard et al. (2005) concluded that while the distribution of westslope cutthroat trout has declined dramatically from historical levels, the subspecies is not currently at imminent risk of extinction because (1) it is still widely distributed, especially in areas protected by stringent land use restrictions; (2) many populations are isolated by physical barriers from invasion by nonnative fish and disease; and (3) active conservation of many populations is occurring.

Global Long Term Trend: Decline of 30-50%

Comments: Westslope cutthroat trout historically occupied 90,800 stream-kilometers and currently occupy 54,600 kilometers (however, these are probably underestimates) (Shepard et al. 2005).

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Threats

Degree of Threat: Medium

Comments: Hybridization with nonnative rainbow trout or their hybrid progeny and descendants, both of which have established self-sustaining populations in many areas in the range westslope cutthroat trout, remains the greatest threat to westslope cutthroat trout (USFWS 2003). The available empirical evidence and speculations of many fishery scientists indicate that introgression of rainbow trout genes will continue to move upstream into many stream reaches presently inhabited by westslope cutthroat trout, although there may be limits to that upstream spread set by environmental factors and the superior fitness of extant westslope cutthroat trout populations in their native habitats. The eventual extent that such hybridization moves upstream may be stream-specific and impossible to predict (USFWS 2003). However, numerous nonintrogressed westslope cutthroat trout populations are distributed in secure habitats throughout the subspecies' historical range. USFWS (2003) considered slightly introgressed westslope cutthroat trout populations, with low amounts of genetic introgression detectable only by molecular genetic methods, to be a potentially important and valued component of the overall westslope cutthroat trout subspecies. USFWS (2003) concluded that westslope cutthroat trout are not threatened by introgressive hybridization. Genetic analyses found no evidence of genetic introgression in 768 samples (58 percent of samples tested) (the numbers of individuals tested per sample were variable and sample sites were not randomly selected) (Shepard et al. 2005).

Impacts of introduced kokanee, lake trout, and brook trout have eliminated populations in some areas (e.g., kokanee may outcompete cutthroat for zooplankton, lake trout is an effective predator on cutthroat). Some westslope cutthroat trout populations have persisted despite the presence of large kokanee populations (see McIntyre and Rieman 1995). Lake whitefish and non-native mysid shrimp also evidently have caused cutthroat declines through competitive interactions. However, USFWS (2000, 2003) concluded that extant headwater populations of westslope cutthroat trout are relatively secure from colonization by non-native fishes (and from adverse effects of human activities).

Stocked, hatchery-reared steelhead that do not migrate to the ocean (residual steelhead) sometimes migrate over 12 km upstream from their release point and may move into areas occupied by westslope cutthroat trout (McMichael and Pearsons 2001). Locally, residual steelhead could pose a threat through ecological interactions.

This subspecies has been negatively affected by loss/degradation of habitat from logging, road construction, mining, and grazing (Spahr et al. 1991), which may result in sedimentation and increased water temperature . Habitat loss has been a primary cause of depressed populations in Idaho (McIntyre and Rieman 1995). These fishes are sensitive to pollution and generally to siltation of streams (some populations may persist despite abundant sediment). Dams, irrigation diversions, and other migration barriers have negatively affected habitat and probably have interfered with metapopulation dynamics (McIntyre and Rieman 1995). Populations have become increasingly fragmented. However, many populations exist in streams that are not affected by these factors (USFWS 2003).

Westslope cutthroat trout are sensitive to fishing pressure (McIntyre and Rieman 1995); restricted or catch-and-release fishing has been needed to maintain wild populations (Spahr et al. 1991). Climate warming would eliminate some habitat.

Overall, USFWS (2000, 2003) concluded that the magnitude and imminence of existing threats are small.

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Management

Global Protection: Very many (>40) occurrences appropriately protected and managed

Comments: Most of the habitat for extant populations is on lands administered by federal agencies, particularly the U.S. Forest Service. Most of the strongholds for the subspecies are within roadless or wilderness areas or national parks, all of which afforded considerable protection. Numerous federal and state regulatory mechanisms protect the subspecies (USFWS 1999, 2003). USFWS (2000, 2003) noted the existence of hundreds of protection and restoration projects throughout the range.

Approximately 42 percent of the stream length occupied by this subspecies in the United States is protected by stringent land use restrictions in national parks (2 percent), wilderness areas (19 percent), and roadless areas (21 percent) (Shepard et al. 2005). A total of 563 populations (39,355 km) are being managed as "conservation populations," and while most (457, or 81 percent) conservation populations were relatively small, isolated populations, large and interconnected metapopulations occupied much more stream length (34,820 km, or 88 percent) (Shepard et al. 2005).

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Relevance to Humans and Ecosystems

Risks

Stewardship Overview: Effective conservation probably will require the maintenance or restoration of well-connected mosaics of habitat (see McIntyre and Rieman 1995).

Restoration should focus on rehabilitating degraded watersheds and removing/excluding undesirable non-native fishes (Van Eimeren 1996).

Schmetterling (2001) recommended for the Blackfoot river drainage in western Montana riparian timber management that continues long-term input of large woody debris to tributaries, continued closure of the Blackfoot River to angling harvest, and the use of culvert designs that will pass spawning fishes under most flow conditions.

Relatively little is known about the appropriate amount or distribution of habitat necessary to ensure long-term persistence of populations. Larger scale patterns in habitat and fish distribution, dispersal rates and mechanisms, and disturbance regimes need to be considered (McIntyre and Rieman 1995). More effective measures of habitat suitability are needed (McIntyre and Rieman 1995). More information is needed on the roles of the different life-history forms in the long-term persistence of populations (McIntyre and Rieman 1995).

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Names and Taxonomy

Taxonomy

Comments: According to Allendorf and Leary (1988), coastal, Lahontan, and westslope subspecies of O. clarkii are electrophoretically divergent from other subspecies, closer to rainbow trout. However, mtDNA comparisons agree with other systematic and zoogeographical evidence that all subspecies of cutthroat trout are more closely related to each other than any of them is to rainbow trout (Gyllensten and Wilson 1987). Much of genetic variation within westslope cutthroat (subspecies lewisi) results from alleles found in only one or two local populations (Allendorf and Leary 1988).

Extensive introductions of Yellowstone cutthroat trout have been made in the range of westslope cutthroat trout, and "hybridization" has resulted. In Glacier National Park, hybridization occurred in previously barren lakes into which both subspecies were introduced but did not occur where Yellowstone cutthroats were introduced into areas with native westslope cutthroat populations (Yellowstone cutthroats did not survive) (Behnke 1992). Widespread hybridization with introduced rainbow trout has occurred (but not where cutthroat trout and rainbow trout evolved in sympatry) (McIntyre and Rieman 1995).

Forbes and Allendorf (1991) found that mitochondrial genotypes had no detectable effects on meristic traits in interbreeding trouts of the subspecies lewisi (westslope) and bouvieri (Yellowstone), which exhibit substantial genetic divergence.

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