Westslope Cutthroat Trout

Oncorhynchus clarkii lewisi

Historically, the westslope cutthroat trout (WCT) was the most widely distributed and abundant of the cutthroat subspecies. In the United States, WCT are native to much of the Columbia River drainage, including river systems on both sides of the Continental Divide in Montana and Idaho, portions of Washington state east of the Cascades Divide and a small portion of the upper John Day River drainage in Oregon. Robert Behnke hypothesizes that the presence of WCT in the middle-Columbia drainages may be the result of late-Pleistocene flooding of glacial Lake Missoula.

Historic Range Relief Map

Unfortunately, habitat degradation from livestock grazing, roads, mining and timber harvest has reduced the distribution of WCT in all states. The status of genetically pure WCT is problematic as this subspecies readily hybridizes with other cutthroat trout subspecies and rainbow trout. Previous assessments indicated that genetically pure WCT may occupy only 2-4% of their historic range. WCT are not protected by the ESA but are listed as sensitive or vulnerable by state and federal agencies. They also may be displaced by non-native brook trout and brown trout.

Including populations of varying genetic purity our CSI analysis documents WCT in 1,502 of 2674 subwatersheds (56%) within their historic range in the United States. Despite this seemingly broad range, many populations are threatened by habitat losses and introductions of non-native trouts. For example, most subwatersheds in Montana’s Blackfoot River Basin scored less than 10 out of 25 for Population Integrity indicating genetic concerns, vulnerability of small populations and loss of life history diversity.

Key CSI Findings

• 56% of subwatersheds within historic range are occupied by WCT
• Genetic purity of many populations is compromised by non-native trout introductions
• Expansion of isolated populations and restoration of life history diversity are highest restoration priorities
• 57% of presently occupied subwatersheds scored 81-98 out of 100 indicating large strongholds in central Idaho wilderness and roadless areas and areas near Glacier National Park

CSI analysis for management priorities indicates highest restoration priority in areas such as Montana’s Bitterroot and Blackfoot rivers, and Idaho’s Lemhi, Pahsimeroi, and Clearwater drainages. High reintroduction priorities are indicated for Washington’s Methow, northern Idaho’s Pack and Priest rivers, and Montana’s Smith, Madison, Big Hole and other systems.

Highest priorities for habitat restoration consist of expanding and reconnecting isolated habitats and restoring migratory life histories. Instream flow improvements are needed in many systems. However, site-specific knowledge is critical to planning effective restoration because of continuing problems with hybridization with introduced trouts. While fragmented populations are at high risk of extinction, removal of barriers and reconnection of tributaries may render populations more vulnerable to hybridization.

WCT also are highly susceptible to angling pressure, and active management and harvest regulations are needed for long-term maintenance of populations that are valued as sport fisheries.

Despite all the problems and pitfalls facing WCT, pockets of genetically pure fish – in both lakes and rivers – persist across the historic range. The presence of high quality habitat in large wilderness areas provides reasons to be optimistic about the future of this subspecies.

Prepared by Jack E. Williams, TU, 12/1/2006

Conservation Strategies Map

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