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S The three parameters (L ,K, and to> used in the von Bertalanffy growth curve (equation 7) were estimated for 'fish from the Colorado, Green, White, and Yampa rivers. Information from back-calculation tables from previous studies was used to estimate von Bertalanffy growth parameters of fish from the Green River and length at age was compared with lengths from this study. RESULTS AND DISCUSSION Records of tagged or recaptured Colorado squawfish during the 12-year period from 1978-1989 totaled 2176. Most records were from the Colorado (422), Green (1086), White (146), and Yampa (465) rivers (Table 1). Fish were captured from March through November, but number varied among months due to greater sampling effort or vulnerability in some months. Of the tagged fish, 279 were recaptured at least once, 42 recaptured twice, and three recaptured three times. Most fish sexed during spawning were males. Of 165 fish sexed, 79% (130) were males. Identifications of sex based on secondary sexual characteristics were potentially inaccurate. One fish (tag 3136- R) was identified as a male at one capture and as a female at another (Appendix A, Table 11). Seethaler (1978) sexed 147 fish and identified a similar proportion (83%) as males. Length freQuency Total length of tagged Colorado squawfish from all rivers averaged 536 mm with a range of 8 1-896 mm. The modal length group was 500-550 mm for fish from the four major rivers. Mean lengths of fish by river were 541 mm in the Colorado, 519 mm in the Green, 498 mm in the White, and 577 mm in the Yampa (Figure 2). All tagged fish from the Yampa River were over 350 mm TL. The common occurrence of fish less than 450 mm in the Green River and their virtual absence in the Yampa River suggests possible longitudinal separation of different life stages. Appearance of fish in the Yampa River after they reach 450 to 500 mm suggests they recruit into this river after reaching sexual maturity (> 42S mm TL), possibly after migrating to the spawning area on the lower Yampa River. Tyus (1990) noted a net downstream movement of smaller fish and recruitment to adult stocks by an upstream movement of larger fish. Samples from the Colorado and Green rivers included both large and small fish, but distributional differences may have occurred historically before impoundments blocked redistribution into upstream reaches. Yearly length frequency was examined for fish collected from the Colorado, Green, and Yampa rivers during 1979-19S8 and from the White River during 1981-1985. Other years were not included because the number of fish tagged each year was small (n<10; Table 1). Yearly length-frequency distributions for fish from each river were similar to their cumulative length-frequency distribution. Length-frequency distributions were consistent through the 10 year period (Figures 3-9). Fish between 500 and 600 mm TL were the most common sizes collected each year, suggesting yearly recruitment into the sampled adult population. The consistent length-frequency distributions for each river suggest a gear bias, but a variety of gear was used and there were some sub-adult fish colIected. The low number of fish less than 450 mm TL that were tagged could be the result of sampling bias. Either gear ineffectively sampled small fish or these fish were not present in sampled habitats or river reaches. Weight-Iemah relationship Least-squares regressions of weight-length plots were calculated for each monthly sample with 10 or more fish. Slopes of weight-length regressions for mature fish (> 428 mm TL) within each river were not significantly different, but intercepts were significantly different (P < 0.05) between months (Table 2). Based on equal slopes, regression coefficients were pooled for all fish from each river. Pooling placed equal weight on each month and prevented months with a large