Genetic status and distribution of native westslope cutthroat trout in Glacier National Park

Science Center Objects

After 14,000 years of surviving extreme environmental events, such as floods, fires and glaciations, Glacier’s greatest native trout is at high risk of disappearing from several streams and lakes east and west of the Continental Divide. The decline of westslope cutthroat trout (Oncorhynchus clarkii lewisi; WCT) in Glacier National Park (GNP) has been attributed to the establishment of nonnative fish species, which consistently displace and hybridize with native cutthroat trout populations in systems where they have been introduced. In response to these declines, GNP wants to develop a management plan to protect and restore WCT in GNP; however, little information exists regarding the distribution and genetic composition of WCT populations throughout the park. This research project will aid in this effort by assessing the genetic status, population structure, and evolutionary history of WCT throughout GNP using stream electrofishing and non-lethal genetic testing. This information may be used to inform management actions in GNP and elsewhere in the western United States.


Genetic status of westslope cutthroat trout populations in waters surrounding Glacier National Park.

Figure 1. Genetic status of westslope cutthroat trout populations in streams and lakes of the Upper Columbia, Missouri and South Saskatchewan river basins, Glacier National Park, as of 2012.   Public domain

Westslope cutthroat trout have been evolutionarily isolated from other cutthroat taxa for 1-2 million years and are all genetically related in this region. Westslope cutthroat trout are highly divergent from the other major cutthroat trout subspecies (coastal O. c. clarki, Yellowstone O. c. bouvieri, and Lahontan O. c. henshawi cutthroat trout) and their geographic range is the largest of the 13 interior subspecies, encompassing the Columbia, Fraser, Missouri, and Hudson Bay drainages of the United States and Canada. Westslope cutthroat trout historically occupied a wide variety of aquatic habitats, from small headwater streams to large rivers and lakes. However, native populations have dramatically declined due to habitat degradation, fragmentation, over-exploitation, and hybridization and competition with nonnative trout. Consequently, non-hybridized populations of WCT currently inhabit less than 10% of their historic range in the United States and 20% in Canada. Many of the remaining non-hybridized populations are restricted to headwater areas above barriers to fish migration.

The WCT is considered a “Species of Concern” in Montana according to the Natural Heritage Program and American Fisheries Society, and classified as a “Species of Management Concern” by the National Park Service. This unique subspecies of inland native trout has been petitioned for listing under the Endangered Species Act (ESA). The status review in the petition to list the WCT as “threatened” was conducted based on genetics information, and populations with less than 20% non-native WTC geneticmakeup were included as part of the unit considered for listing. The decision to use the 20% threshold continues to be controversial because many populations that have not been genetically tested were assumed to be non-hybridized. Moreover, protection of fish with even low levels of hybridization may facilitate continued expansion of hybridization. Thus, assessing the hybridization status of WCT is needed because this information may influence the decision to list by determining the number of fish classified as part of the native WCT population. Consequently, there is a need to better understand the genetic status of WCT, especially in protected areas in the western United States.

Although GNP in northwestern Montana is considered a range-wide stronghold for WCT, the long term persistence of non-hybridized populations is threatened by hybridization with nonnative rainbow trout (RBT). Recent work in the Flathead system has shown a rapid increase in hybridization spreading in an upstream direction, threatening pure WCT populations in GNP. However, little information exists regarding the genetic status and population structure of WCT in GNP. Westslope cutthroat trout have long been considered an integral component of biodiversity, culture and economy in GNP. They are part of a historic fishery that is a fundamental part of the biodiversity of the Park. As such, protecting native fish resources is a high priority for conservation and management programs in GNP. Invading nonnative species such as rainbow and lake trout, could overwhelm and replace native WCT due to hybridization, competition (and predation) respectively, as they have done in other park streams and lakes. Delay makes it more difficult to develop management plans for control and management of invasive species to protect core WCT populations, so existing pure populations that have evolved and adapted to GNP's waters may be lost forever. Thus, there is a need to better understand the status and trends in the distribution of WCT in GNP. This information is needed to make informed conservation and management decisions, to provide recreational opportunities for visitors in the Park, and to maintain the evolutionary heritage of Glacier’s greatest native trout for future generations to enjoy and appreciate.


Map of westslope cutthroat trout conservation populations in Glacier National Park.

Figure 2. Westslope cutthroat trout conservation populations in the Missouri (A), South Saskatchewan (B), and Upper Columbia (C – Middle Fork Flathead; D – North Fork Flathead) river basins in Glacier National Park. The colored lines indicate genetic risk degree.  Public domain


The project will develop information necessary to understand the genetic status (historic and present) and distribution of WCT in GNP, and will develop specific management alternatives to reduce or eliminate these threats and protect remaining populations. The project will use new and innovative molecular DNA techniques successfully applied to meet these objectives by the Principal Investigator in the system. Activities include collecting tissue samples (30 sites west and 30 sites east of the Continental Divide) genetic analyses, and reporting and presenting results and management alternatives. The specific objectives are to:

  • Map the distribution of WCT and hybridization in GNP.
  • Assess spatial and temporal patterns of hybridization and relatedness among WCT populations.
  • Develop site-specific management alternatives for conservation and protection.