The Peace/Williston Fish and Wildlife Compensation Program (PWFWCP)

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Mountain Whitefish
Prosopium williamsoni

What do they look like?

A small-scaled silvery whitefish with a large adipose fin (about 1.5 times the eye diameter). In riverine and adfluvial populations, the body (length up to 300 mm) is round (cylindrical) in cross section and, when viewed from above, the snout usually is pinched and pointed. In the field, these populations are easily confused with lake whitefish but are separated by the shape of the dorsal fin (when depressed, the anterior dorsal rays in mountain whitefish do not extend beyond the posterior dorsal rays; while, in lake whitefish the anterior dorsal rays extend well beyond the posterior dorsal rays). Two forms of mountain whitefish coexist in many B.C. rivers. The 'normal' form that, in profile, has the typical mountain whitefish head shape, and a 'pinocchio' form that has a 'turned-up' nose and an elongate snout.

Where do they live?

Although mountain whitefish in Central and Northern British Columbia are often the most abundant species in the upper parts of many rivers, little is known about their habitat use. In B.C., adult mountain whitefish occur in lakes and rivers. In fluvial populations, seasonal habitat shifts are common. These habitat changes often involve complex, multiple migrations of over 100 km. There is a post-spawning fall migration to over-wintering sites, a spring feeding migration, a summer feeding migration, a fall pre-spawning migration, and a spawning migration.

In rivers, adults typically occur in loose aggregations in runs or pools. Usually adults are close to, but not on, the bottom in water 1 m to 2 m deep over coarse gravel or cobble substrates. Mountain whitefish are remarkably active and rarely maintain position. Instead, they bob and weave in the current, constantly changing position within the group. Juveniles in rivers appear to avoid riffles and backwaters and are associated with glides and runs. They also show a preference for water 1 m to 2 m deep and large substrates (25-40 cm). In lakes, adult mountain whitefish usually occur at depths of less than 20 m but there are seasonal changes in adult habitat that are associated with temperature changes. In lakes, juveniles remain in shallow (<2 m) inshore habitats throughout the spring and summer. They usually occur over sand or coarse gravel substrates. Newly emerged mountain whitefish occur in shallow water (<50 cm) over fine gravel or sand substrates.

What is their life like?

In B.C., this species displays three basic life-history patterns: a lacustrine life history that is completed entirely within lakes, a riverine life history associated entirely with flowing water, and an adfluvial life history that involves movements between lakes and rivers. Mountain whitefish usually spawn in flowing water, although lake-spawning populations occur in some B.C. lakes. Most lake populations migrate into streams to spawn. Fluvial populations in large rivers often migrate into smaller streams to breed but mainstem spawning does occur in the Peace river. In flowing water, spawning usually occurs at the lower end of riffles or near the head end of pools. In lakes, mountain whitefish appear to preferentially spawn at sites where there is upwelling water.

Mountain whitefish spawn in either the fall or early winter (October or November), usually when water temperatures drop below about 10 °C. There is no site preparation and the eggs are released over the substrate and incubate over winter. In B.C., fecundity ranges from about 1,000 to 15,000 eggs. The fry emerge in the spring or early summer. Newly emerged fry are small (16-20 mm). By late summer, the young-of the-year reach a length of about 60-100 mm. In lakes and reservoirs, newly emerged fry remain inshore. Growth is rapid and most individuals attain sexual maturity by the age of six. The maximum age recorded for this species is 23 years but in B.C. relatively few individuals live beyond 12 years.

What do they eat?

In lakes, newly emerged fry feed primarily on plankton, while in streams they feed on the smallest life-stages of aquatic insects. In rivers, adults and subadults forage on the nymphs of aquatic insects and occasional terrestrial insects. In lakes, the main prey are plankton, snails, surface insects and, occasionally, young fish.

What is their distribution? Mountain whitefish are exclusively North American. Here, they are widely distributed along both slopes of the Rocky Mountains from northern Utah north to the middle Mackenzie River. They also occur in California where they are native to the east slope of the Sierra Nevada. In B.C., mountain whitefish are primarily an interior species and only reach the coast where major rivers broach the coastal mountains. Thus, mountain whitefish are absent from coastal islands and most short coastal rivers. The B.C. distribution of mountain whitefish extends from the U.S. border north to the Stikine and Liard rivers.

View Peace Williston distribution map.