Age, Growth, and Movement of Mountain Whitefish, Prosopium williamsoni (Girard), in the North Fork Clearwater River, Idaho

The growth and age structure of mountain whitefish, Prosopium willlamsoni, was compared from three sample locations in the North Fork Clearwater River, and a tagging study initiated to assess movement and migratory behavior. No difference was detected in growth or age structure of whitefish in two streams of the upper drainage, and juveniles representing age groups I and II were generally absent from upper river populations. Age groups I and II whitefish from the lower North Fork Clearwater had significantly greater growth than similar fish from the upper river. The sample from the lower river contained whitefish as old as age IV, whereas fish to age XIV were found in the upper reaches. Young-of-the-year whitefish which spent their first growing season in the upper river moved downstream in September, and apparently remained in the lower river until reaching sexual maturity. Mature whitefish moved upstream (82 km) in late spring and early summer, remained in the upper reaches until spawning in November, then, with the completion of spawning, moved downstream (88 km) to overwinter in the deeper pools of the lower river. Repeat homing by spawning adults was demonstrated by the return of tagged fish (37%) to a small tributary in the following year.