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36 about 6% of their body length each year (Figure 17). Relative growth of recaptured fish over 500 mm TL was only about 1-2% of body length (Figures 17). This may be evidence of the negative effect of tagging on fish growth. Growth of recaptured fish also exhibited a slight mc;:rease at larger sizes. The reason for this is unknown, although it occurred with both back-calculated and recaptured fish data. Walford plots were used to estimate asymptotic length (L ) based on back-calculated lengths (Figure 18). Asymptotic length approximated maximum length. It identified the largest average length that fish in the population were approaching with increased age. Negative slopes of these plots supported the adequacy of the von Bertalanffy growth equation. Growth coefficient (K) and asymptotic length were calculated for fish from rivers included in this and previous studies (Table 9). Because adequate samples were available, lengths at age for Colorado squawfish collected from the Green River were calculated based on von Bertalanffy growth curves using data I compiled and data from previous studies (Figure 19). Calculated growth rates were apparently equal among all studies except Seethaler's (1978) data which tended to approach a lower asymptotic length (752 mm). The von Bertalanffy curves again showed that I must have aged fish about one year older at a given size than either Vanicek and Kramer (1969) or Musker (1981). SUMMARY A total of 2176 Colorado sQuawfish were tagged between 1978-1989 in the Upper Colorado River Basin. Total length of tagged fish ranged from 81-896 mm. Modal size groups were 500-550 mm TL for fish from the Colorado, Green, White, and Yampa rivers. All fish from the Yampa River were over 350 mm TL, most were over 450 mm TL, indicating recruitment of fish after they attained sexual maturity. Weight-length relationships were very similar between Colorado River and Yampa River fish and between Green River and White River fish. Fish from the Colorado or Yampa rivers tended to be more rotund at a given" length than fish from the Green or White rivers. Fish reached maximum relative condition in June prior to spawning, and minimum condition in July or August, probably as a result of migration or " spawning. Scales from 326 fish collected between 1978-1990 were aged. The first annulus was not observed on scales, and it may not form on most fish due to their small size as they go through their first winter. Adjusting for this missing annulus, the scale method appears suitable for ageing Colorado squawfish during at least the first 10 years of life. Scale ages were validated by correctly ageing two 10 year old stocked fish that were recaptured. Ages over 10 years may be accurate but are subject to an unknown amount of error as shown by ages of recaptured fish. The Yampa River contained a broader range of older fish than any other river. The two oldest fish were from the Yampa River and were 18 years old. Ages of oldest fish from the Green, White, and Colorado rivers were 16, 16 and 12 years, respectively. Relative growth calculated (rom tag-recapture data was lower than relative growth calculated from scales. This suggests a . possible negative effect of tagging on growth. Back-calculated lengths and von Bertalanffy growth curves were similar for fish up to ages VII or VIII, at which time growth rates changed among rivers. ACKNOWLEDGEMENTS C. Carlson (Colorado State University, CSU), K. Fausch (CSU), T. Nesler (Colorado Division of Wildlife, DOW), and E. Wick (CSU) provided helpful discussions and technical advice. I thank all researchers who provided data. They include: C. McAda (USFWS), H. Tyus (USFWS), K. Karp (USFWS), K. Paulin (USFWS), S. Platania (University of New Mexico), D. Propst (New Mexico Department of Fish and Game), D. Osmundson (USFWS), S. Cranney (Utah Department of Natural Resources, UDNR), T. Chart (UDNR), T. Nesler, P. Martinez (CDOW), R. Valdez (BiojWest, Inc., BW), L. Crist (BW), P. Holden (BW), and E. Wick. P. Chapman (CSU) provided assistance in statistical analysis of weight-length relationships. T. Nesler, R. Muth, D. Weber (DOW), and D. Vanicek (California State University) reviewed earlier drafts of the manuscript.