Yellowstone Cutthroat Trout

Yellowstone cutthroat trout occupy the interior heart of the group’s range and represent both its greatest surviving stronghold and its most public near-collapse. The subspecies holds the largest inland cutthroat population anywhere, in Yellowstone Lake, and that population was reduced precipitously within a decade by a single introduced predator.

Range and Drainages

The USGS Nonindigenous Aquatic Species database places the native range in the Snake River drainage north of Shoshone Falls, across Idaho, Nevada and Utah, and in the upper Yellowstone River drainage in Montana and Wyoming. Shoshone Falls is the structural fact behind that distribution: an impassable waterfall on the Snake that cut the upper basin off from downstream colonization and left the upper Snake with a fauna of its own.

On the Wyoming side, Wyoming Game and Fish maps Yellowstone cutthroat across a specific and widely separated set of drainages: the upper Yellowstone, Clarks Fork, Shoshone, Greybull and Wood River systems on the Absaroka front; the Popo Agie, East Fork Wind and upper Wind River in the Wind River range; and the Little Tongue and Little Bighorn in the Bighorns. That last pair sits east of the Bighorn Basin, well removed from the rest — a reminder that this subspecies’ range is a set of fragments rather than a block.

The subspecies has also been stocked far outside its native range. USGS records nonindigenous Yellowstone cutthroat in Colorado’s North and South Platte and Arkansas drainages and the San Luis Valley, in Rocky Mountain National Park, commonly within Glacier National Park, and in Montana’s South Fork Flathead River. That last case is notable: Yellowstone cutthroat stocked into westslope cutthroat range become a hybridization threat to a different native lineage.

Identification

Yellowstone cutthroat are the large-spotted interior cutthroat. Wyoming Game and Fish describes them as olive-green to brown dorsally with yellowish-brown to silver flanks, carrying spots that are noticeably larger and more sparsely distributed than those of the Snake River fine-spotted form with which they share the upper Snake basin. Spotting concentrates toward the caudal peduncle and tail. The red jaw slashes are typically strong. In the Snake River drainage the practical field distinction is spot size and density rather than color, and intermediate fish occur.

Yellowstone Lake and the Lake Trout Invasion

The best-documented event in this subspecies’ modern history took place in Yellowstone Lake, a 34,000-hectare water with 239 km of shoreline, a mean depth of 48 m and a maximum depth of 137 m. As Koel and colleagues (2020), Fishes 5(2):18, record it, invasive predatory lake trout Salvelinus namaycush were discovered in the lake in 1994 and “caused a precipitous decrease in abundance of native Yellowstone cutthroat trout.” Lake trout are deep-water piscivores that neither compete for the same spawning habitat nor feed the terrestrial predators — bears, otters, ospreys, eagles — that had depended on cutthroat running up shallow tributaries each spring. Replacing one with the other removed a whole trophic pathway.

The response has been the largest sustained invasive-fish suppression program in North America. An adaptive management strategy was implemented in 2010 and intensified after 2012, with a gillnetting effort benchmark of more than 46,800 100-metre net nights reached in 2012. Between 1995 and 2019, 3.35 million lake trout were killed. Lake trout biomass fell from 432,017 kg in 2012 to 196,675 kg in 2019, total abundance fell from 925,208 to 673,983 over the same period, and the adult population was reduced by 79 percent.

The cutthroat response is measurable. By 2019 Yellowstone cutthroat weights had doubled and large individuals over 400 mm had become more abundant. The program is not a cure — suppression must continue indefinitely, because lake trout cannot be eradicated from a lake of that size — but it is evidence that a cutthroat population can rebuild when predation pressure is mechanically removed.

Conservation Status

Yellowstone cutthroat are not listed under the Endangered Species Act. Montana Fish, Wildlife and Parks designates them a Species of Concern with a state rank of S2 — at risk — and notes federal monitoring without listing. Range-wide status is coordinated across Montana, Wyoming, Idaho, Utah, Nevada and the National Park Service, and the subspecies is a Species of Greatest Conservation Need in several of those states. Range-wide figures for the proportion of historical habitat still occupied were not verified for this article and are not reported here.

Hybridization and Competition

Introduced rainbow trout are the standing genetic threat, as they are across the group. Montana’s statewide plan identifies hybridization between introduced rainbow trout and native cutthroat as a management priority addressed through suppression, barriers and selective stocking. Rainbow trout occupy the lower and mid-elevation reaches of most Yellowstone cutthroat rivers, so introgression advances from downstream upward, and the same warming-driven upstream expansion documented by Muhlfeld and colleagues (2014) for westslope cutthroat applies mechanistically here.

Two further complications are specific to this subspecies. Brook trout displace Yellowstone cutthroat from small headwater streams by competition rather than hybridization, and USGS Northern Rocky Mountain Science Center research explicitly pairs native Yellowstone cutthroat against nonnative brook trout in evaluating climate influences on growth, survival and life-history expression. Separately, in the upper Snake, Yellowstone cutthroat interbreed with the Snake River fine-spotted form — an interaction whose significance depends entirely on whether the fine-spotted fish is a distinct taxon at all.

Water Temperature

No controlled thermal tolerance study specific to Yellowstone cutthroat was verified for this article. The best available reference point is the closely related westslope form, where Bear, McMahon and Zale (2007) measured an ultimate upper incipient lethal temperature of 19.6 C (67.3 F) against 24.3 C (73.7 F) for rainbow trout, with growth optima near 13-14 C for both and a management recommendation to hold maximum daily temperatures near 13-15 C (55-59 F).

Applied to Yellowstone cutthroat waters, those figures are an extrapolation rather than a measurement, and should be read as such. What they do establish is the direction and rough magnitude of the disadvantage: a cutthroat river approaching the upper 60s F is at or past its thermal ceiling while the rainbow trout in the same reach are not. The interior range of this subspecies — high, arid, with heavy summer irrigation demand on the Snake and its tributaries — makes late-summer temperature the practical constraint on where these fish persist.

Life History and Spawning

Yellowstone cutthroat spawn in spring into early summer, timing keyed to snowmelt, with the tributary runs of Yellowstone Lake historically among the most concentrated salmonid migrations in the interior West. Fish express resident, fluvial and adfluvial strategies; the adfluvial lake fish grow largest. Diet is dominated by drifting aquatic invertebrates and terrestrial insects, with larger lake fish taking amphipods and, in some systems, small fish. Migratory life histories depend on unobstructed tributary access, and irrigation diversions on the Snake and Yellowstone systems remove that access at scale.

Taxonomy

ITIS lists Oncorhynchus clarkii bouvieri as a valid subspecies (TSN 553416), and USFWS, USGS and the state agencies all use that trinomial. The American Fisheries Society’s 8th edition names list (2023) reorganized cutthroat trout into multiple species; the specific rank and binomial that list assigns to the Yellowstone lineage could not be verified for this article and is therefore not stated here. Complicating the picture further, Loxterman and Keeley (2012), BMC Evolutionary Biology 12:38, found that Bonneville and Yellowstone cutthroat trout did not form separate monophyletic groups in mitochondrial DNA, instead mixing across divergent clades — evidence that the boundary between the Yellowstone lineage and its neighbors is not as clean as the subspecies list implies.