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25 Only 18 of 278 recaptured fish had readable scales taken at both capture and r:ecapture. Time interval between capture and recapture ranged from less than I year to 6 years; . Known time interval was compared to observed annuli between capture and recapture. Recapture interval was correctly predicted for seven fish. Ages of these fish ranged from 7 to 13 years at first capture. Intervals of eleven fish were incorrectly determined. Ten of these were incorrect by plus or minus 2 years of the known time interval. One fish, 15 years old, was underestimated by 6 years. A possible source for discrepancies between expected and observed age difference is whether the first true annulus was present on the scale. Another source of variation could be time of year when annulus formation actually took place. It is difficult to identify when annulus formation Occurs on older fish collected between May and July. Incorrectly ageing a scale by only 1 year at both capture and recapture could account for a total difference of up to 2 years from the known time interval. Age composition was determined for fish from the Colorado, Green, White, and Yampa rivers. Colorado River samples were probably influenced by the small, inadequate sample of 14 fish. The majority of older fish collected from the Green River were age VII, age VIII in the White River, and age IX in the Yampa River (Figure 13). The Yampa River contained a broader range of older fish than any other river. Scale samples of fish from the Colorado, Green, White, and Yampa rivers were treated both separately and combined for back-calculated lengths (Tables 3-7). Length at first annulus was unknown due to the lack of a first annulus on any scale. The shortest fish length calculated from an annulus was 57 mm. Only 25 fish were calculated to have lengths under 75 mm at the first observable annulus. Some of these may have been the true first annulus, but for most fish, the first annulus was apparently not observed. Back-calculated fish length was plotted against age for each river (Figure 14). Growth rates of fish younger than 7 years were similar among the four rivers. After age VII or VIII, fish reached sexual maturity based on lengths greater than 428 mm (Table 8) and growth rates differed among rivers. Musker (1981) used a values from equation (6) that were derived from fish from each river to back-calculate lengths. Due to small samples, these values were not accurate estimates of the body length to scale radius relationship. These estimates of a caused inaccurate calculations of lengths at each annulus. Carlander (1981) warned of the problem of calculating a from inadequate samples and suggested using a standard a for each species. Using a (45.151) calculated from 336 Colorado squawfish (Figure 6), lengths at each annulus were recalculated from annuli measurements reported by Musker (Appendix B). These adjusted calculations were used in further comparisons. Back-calculated lengths (this study) were shorter than lengths at age reported by other studies (Table 8). Fish tended to be 1 year older for a given size when ~ompared with lengths at age from previous studies (Figure 15). The oldest fish aged by Musker (1981) was an 879-mm-TL, 13-year-old Yampa River fish. An ll-year-old female, 610 mm long was the oldest fish aged by Vanicek and Kramer (1969). Seethaler (1978) also aged his oldest fish at lJ years. The two oldest fish I aged were from the Yampa River and were 18 years old and over 800 mm long. Oldest fish from other rivers were 16 years (Green and White rivers) and 12 years (Colorado River). A verage annual growth increments were determined from back-calculation table summaries (Tables 3-7; Figure 16). Growth increment between years was greatest (over 60 mm) between 3-4 or 4-5 years. Yearly growth decreased to about 30 mm per year after age XII, but tended to increase in later years (approximately Age XVI). Relative growth calculated from back-calculation tables decreased with age to between 5-10%, and fish over 12 years grew only