Mountain Whitefish

Identification

The mountain whitefish is a salmonid built for the bottom. The body is cylindrical rather than laterally compressed, silvery on the sides with an olive-green to dusky back, and the tail is forked. The head is small relative to the body and the snout overhangs a small, subterminal mouth with no teeth — a mouth designed to be pressed against gravel, not to seize a swimming fish. Like all salmonids it carries an adipose fin, and in this species the adipose is conspicuously large, its base more than one and a half times the diameter of the eye.

The scales are the fastest field mark. They are large and obvious — 74 to 90 along the lateral line — where a trout of the same size has scales you have to look for. Colorado Parks and Wildlife notes the practical consequence of the second field mark: the mouth is weak and tears easily, so a whitefish hooked and handled like a trout often does not come to hand intact. Counts are dorsal 12–13 rays, anal 11–13, pelvic 10–12, pectoral 14–18, with 19 to 26 short gill rakers on the first arch. Breeding males develop tubercles on the head and sides. Juveniles are pencil-thin and silvery with seven to eleven dark oval parr marks (California Department of Fish and Wildlife species account).

Historic confusion with suckers, and the loose application of the word “whitefish” to several large minnows, did the species lasting reputational damage. A 1971 University of Idaho literature review put it flatly: the mountain whitefish “does not appeal to Utah fishermen” and “most anglers still do not appreciate” it, and traced the attitude directly to that superficial resemblance (Daily 1971). It is not a sucker. It is a salmonid, and in most of the rivers it occupies it is the native one.

Range and distribution

The native range runs from the Mackenzie River drainage in the Northwest Territories south through western Canada and the northwestern United States, spanning Pacific, Hudson Bay and upper Missouri basins, and reaching its southern limits in the Truckee River drainage of Nevada and the Sevier River drainage of Utah (USGS). It does not occur in Alaska. In Alaska’s interior the ecological equivalent is round whitefish, Prosopium cylindraceum, a congener with a similar body plan and the same bottom-feeding habit.

In Montana the species is widely distributed and found “in abundance in most clear, cold rivers in the western drainages and eastern mountain front.” In Idaho it is native and regularly occurring statewide (IDFG). In Washington it occupies large rivers and a long list of eastern and central lakes and reservoirs. Colorado is the exception that clarifies the rule: the species is native only to the Yampa and White river drainages there, and the fish in the Colorado River and Cache la Poudre drainages were introduced in the 1940s and have been recorded by USGS as established since 1955.

The southern populations are the fragmented ones. In California the species persists in the Truckee, Carson and Walker systems on the east slope of the Sierra Nevada and in Lake Tahoe and Fallen Leaf Lake, but it disappeared from Sagehen Creek after Stampede Reservoir was built and from the South Fork of Prosser Creek by 1983. Those populations are now disconnected from one another, a marked decline from their nineteenth-century distribution (Moyle et al., State of the Salmonids).

Habitat and water temperature

Mountain whitefish are fish of medium to large cold mountain streams, and secondarily of lakes and reservoirs. Within a river they occupy runs and deeper riffles, holding in loose shoals of five to twenty fish close to the bottom. Critically for anyone reading a temperature gauge, they are concentrated in low-gradient reaches — the same reaches that warm first and stay warm longest.

The thermal numbers are specific enough to be useful. USGS describes the species as inhabiting clear, cool waters below 20°C (68°F). Northern populations show seasonal temperature preferences of 10–18°C (50–64°F), and the species is largely found in waters whose summer temperatures stay under 21°C (70°F). Maximum growth of fry occurs between 13 and 14°C (55–57°F). Upper lethal thermal tolerance has been measured at 23.6°C (74.5°F) — a number worth sitting with, because it is essentially identical to the 23.5°C figure for bull trout, a species universally understood as a cold-water indicator (Moyle et al., citing Brinkman et al. 2013 and Dunham et al. 2003).

Spawning temperatures are colder still: 1–11°C (34–52°F), usually 2–6°C (36–43°F), with optimal embryo development below 8°C. Montana FWP places peak spawning at 35–44°F. The 1971 Idaho review recorded a population that did not spawn until after 14 October, when water temperatures fell to 42°F, and reported eggs taking roughly five months to hatch at 35°F but about one month at 48°F. Incubation is therefore a winter-long process whose duration is set almost entirely by temperature.

The combination of a 74.5°F ceiling and an obligate low-gradient habitat is why Brinkman and colleagues concluded that mountain whitefish may be disproportionately affected by warming compared with other salmonids: the trout can move up; the whitefish, largely, cannot.

Life history

Spawning runs from October into early December. Fish spawn at dusk or after dark in groups of more than twenty, over coarse gravel in riffles deeper than about 75 cm, scattering eggs across gravel and rock with no redd construction and no parental care. Some populations move into the lower reaches of tributary streams to spawn; lake populations may spawn along shorelines instead. Fecundity ranges from 770 to more than 24,000 eggs per female, averaging around 5,000. Embryos hatch in six to ten weeks in early spring — Montana FWP gives March.

Maturity comes in the second to fourth year. Growth is highly variable and driven by productivity rather than age: fish in alpine lakes reach roughly 11 cm at age one and 20 cm at age five, while river fish run 25 to 30 percent larger at any given age after the first year. Eight-year-old fish from one unproductive lake averaged 9.0 inches; eight-year-olds from a productive one averaged 16.8 inches. Large individuals of 25 to 50 cm standard length are typically five to ten years old. Longevity is contested in the older literature — one author reported fish of at least 18 years, another believed the natural lifespan rarely exceeds nine — and that disagreement has not been cleanly resolved.

Diet

Mountain whitefish are benthic feeders. Juveniles take small chironomid midge, blackfly and mayfly larvae; adults take mayfly, caddisfly and stonefly larvae through the summer. Lake Tahoe fish add snails, crayfish and amphipods. They take drifting and terrestrial insects that fall to the water, and they will eat fish eggs, but they rarely eat fish. Most feeding happens at dusk or after dark.

Two details matter for river ecology. First, Montana FWP records that mountain whitefish feed actively in winter, when most trout in the same river have throttled down — and the Idaho review found that from January to March they ate aquatic insect larvae almost exclusively. Second, because they feed from the substrate rather than the drift, their body condition tracks benthic invertebrate production directly. A river whose stoneflies and caddis have thinned out shows it in the whitefish before it shows in anything else.

Angling

Mountain whitefish are caught incidentally by trout anglers across the West and deliberately by relatively few. The nymph fished deep and dead-drifted, which is the standard western trout presentation, is also the presentation best matched to a bottom-oriented insectivore, which is why so many are hooked by people who did not intend to hook them. The small, toothless, subterminal mouth means takes are often felt as a check in the drift rather than a strike, and the weak mouth tissue means hooks pull free readily.

Winter is when the species is most distinctly available. Whitefish continue feeding on benthic larvae through January to March in water where trout have largely stopped, and in many western rivers the cold-months fishery is functionally a whitefish fishery whether or not anyone calls it that. The 1971 Idaho review noted that the species “has increased in popularity in recent years”; more than half a century later that increase remains modest relative to how many fish are actually present.

They are excellent eating and are traditionally smoked. Colorado Parks and Wildlife notes that smoked whitefish are considered a delicacy in many parts of the country — an assessment that sits oddly against the fish’s reputation among the anglers who catch the most of them.

Conservation

Rangewide the species is secure: NatureServe ranks it G5. The state-level picture is less settled. Montana ranks it S4 with the explicit note that it “is widely distributed but is declining in some areas and faces significant threats from warming water temperatures and drought.” Idaho ranks it S3 and lists it as a Species of Greatest Conservation Need under the State Wildlife Action Plan. In California, Moyle and colleagues assigned it a status of Moderate Concern and rated it highly vulnerable to extinction within the state over the next hundred years — a striking assessment for a fish widely assumed to be everywhere.

The clearest demonstration of what that vulnerability looks like came in August 2016 on the Yellowstone River. An outbreak of proliferative kidney disease, caused by the myxozoan parasite Tetracapsuloides bryosalmonae, killed mountain whitefish and trout in numbers that led Montana to close almost 200 miles of the Yellowstone below Yellowstone National Park and its tributaries. USGS attributes the scale of the mortality to low stream flows and high temperatures. The subsequent peer-reviewed investigation estimated total PKD mortalities in the tens of thousands and an economic loss of about US$500,000, and found the parasite was neither novel nor a uniquely virulent strain — the environmental conditions were what changed (Hutchins et al. 2021, Ecosphere).

That is the case for treating mountain whitefish as an indicator. It is the native salmonid across most of its range, it is abundant enough to constitute a major portion of fish biomass in many cold-water streams, it is pinned to low-gradient reaches it cannot escape, its upper lethal temperature is no higher than bull trout’s, and it dies first. A river whose whitefish are thinning is a river with a problem, whatever its trout counts say. The other named threats are the familiar ones: dams fragmenting populations, agricultural diversions removing cold water, and predation by non-native trout, particularly lake trout.