Fish and Aquatic Conservation

illustration of a Westslope cutthroat trout

Westslope cutthroat trout

Oncorhyncus clarki lewisi, (Richardson, 1836)

Cool Facts

Westslope cutthroat trout are one of 14 subspecies of cutthroat trout, Oncorhyncus clarki, found in Western North America. The westslope cutthroat trout is the official state fish for the state of Montana. Currently the only remaining genetically pure native westslope cutthroat trout are found in about 600 miles of Missouri River tributaries. Conversely, Westslope cutthroat trout have disappeared in 94% of their native range on the east side of the Continental Divide.

SIZE: The average size of Westslope cutthroat trout ranges between 6 in (15 cm) and 16 in (41 cm).

RANGE: Westslope cutthroat trout occur on both sides of the Continental Divide from Yellowstone National Park into British Columbia and Alberta Canada. There are also several unconnected populations of Westslope cutthroat trout in Oregon, Washington and British Columbia.

HABITAT: Westslope cutthroat trout are common in headwaters, lakes and streams. Westslope cutthroat trout tend to thrive in streams with more pool habitat and cover, than streams with very few pools and little or no cover.

DIET: Westslope cutthroat trout feed primarily on aquatic invertebrates.

Natural History

Westslope cutthroat trout reproduce in the spring, when the water temperature reaches about 10 degrees Celsius (50 degrees Fahrenheit). Female cutthroat trout bury their eggs in a nest or redd after they have been fertilized by the male cutthroat trout. The eggs of these fish normally hatch within a couple of weeks up to a few months. Newborn Westslope cutthroat fry (having just emerged from their eggs) frequently migrate back to lakes to rear after one to two years in their native stream.

Westslope cutthroat trout have three possible life history strategies. These strategies include adfluvial (migrates to lakes), fluvial (migrates to rivers) or resident (stays in streams). Migratory cutthroat can travel several hundred miles between their adult habitat and their spawning habitat.


Westslope cutthroat trout have suffered a reduction in their historical range because of habitat loss and fragmentation, isolation of existing populations and their ability to hybridize with rainbow trout/steelhead and other sub-species of cutthroat trout.

The U.S. Fish and Wildlife Service (FWS) was petitioned to list the Westslope cutthroat trout under the protection of the Endangered Species Act. In 2000, the FWS determined that listing the Westslope cutthroat trout was not warranted because of its wide distribution and the available habitat on public lands. Conservation efforts by state and federal agencies are currently underway to restore Westslope cutthroat trout.

Management of this species involves protecting the population strongholds and making tough decisions on restoration priorities for the depressed populations of Westslope cutthroat trout. Recovering depressed populations will involve habitat restoration and removing non-native species.