Validity of Age Determinations from Scales of Brown Trout, Rainbow Trout, and Brook Trout

A total of 1,055, scale samples from tagged, recaptured brown trout, rainbow trout, and brook trout and 129 scale samples from planted, marked rainbow trout were taken from Prickley Pear Creek, Montana, between June 1949 and October 1951. In addition, scale samples were obtained from 69 rainbow trout planted as fry in Savage Lake, Montana, and from 42 known-age rainbow trout of the brood stock at the U. S. Fish and Wildlife Service Fish Hatchery, Ennis, Montana. Studies of these scales substantiated the general validity of the scale method of aging wild trout and trout planted as fry or young-of-the-year fingerlings. Recaptured brown trout whose growth rate and annulus formation between growing seasons could be determined totaled 233; of these, 46 failed to form the expected annuli. Of 70 recaptured rainbow trout, 8 failed to form annuli, and of 3 brook trout, 1 failed to form an annulus. Most fish that failed to form annuli were more than 3 years old and had made little or no increase in body length. Scales from rainbow trout brood stock of known age had “annuli” in varying numbers and positions on fish of the same age. These check marks did not conform to the known age of the trout and were probably caused by hatchery feeding practices and by handling during spawning operations. Annuli beyond the third were, in general, in close proximity to one another and were difficult to detect. In some older fish, erosion or resorption obliterated scale characters.