Mountain Whitefish (Prosopium williamsoni)

A member of the Salmonidae family, the mountain whitefish provides an important winter fishery in certain areas, especially where steelhead are absent.


Possessing an adipose fin and an axillary process, the mountain whitefish is long, slender, and nearly cylindrical, although not quite as cylindrical as the round whitefish. It is nevertheless among the species referred to as “round whitefish” and can be distinguished from the lake whitefish, which is more laterally compressed than is the mountain whitefish.

Silvery overall, it is dark brownish to olive or greenish to blue-gray above, with scales that often have dark borders and ventral and pectoral fins that may have an amber shade in adults. The small mouth is slightly subterminal, and the snout extends clearly beyond it. The caudal fin is forked, and there are 74 to 90 scales down the lateral line and 19 to 26 gill rakers.


The mountain whitefish can grow to 22.5 inches and 5 pounds. The all-tackle world record is a 1988 5.5-pound fish from Saskatchewan. The mountain whitefish can live for 18 years.

Spawning behavior

Spawning takes place from October through December in shallow, gravelly streams or occasionally in lakes at water temperatures of 42°F or less. Parents do not guard the eggs, which incubate over the winter to hatch in the spring.

Food and feeding habits

Mountain whitefish feed primarily on benthic organisms like aquatic insect larvae, mollusks, fish, and fish eggs (including their own), as well as on plankton and surface insects when primary food sources are unavailable.

Other Names

Rocky mountain whitefish, Williamson’s whitefish, grayling; French: ménomini des montagnes.


The mountain whitefish is endemic to the lakes and the streams of the northwestern United States and southwestern Canada. It occurs inland into Alberta and Wyoming, overlapping the range of the lake whitefish and slightly overlapping that of the round whitefish.


Generally inhabiting rivers and fast, clear, or silty areas of larger streams, as well as lakes, mountain whitefish usually occur in stream riffles during the summer and in large pools in the winter. They prefer temperatures of 46° to 52°F and are found in the deep water of some lakes, although in northern lakes they usually hold no deeper than 30 feet.