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Table 7. Average total length (range in parentheses) of razorback suckers of different estimated ages from the Colorado, Yampa, and Green rivers, 1974-75. Colorado River Yampa and Green rivers Female Male Female M ale No. of Length No. of Length No. of Length No. of Length fish (mm) fish (mm) fish (mm) fish (mm) - - 1 485 - - 1 499 6 571 3 522 4 510 8 483 (538-595) (498-561) (472-531) (442-514) 17 561 5 510 5 508 8 489 (499-609) (479-541) 4461-539 (468-515- 8 583 - - 1 515 - - (551-618) 1 592 - - - - - - aNumber of growing seasons completed. area varied considerably in size and shape, and the relation between total length of fish and scale radius (average for two scales from the key area) was not sig- nificant for fish from either river. Consequently we did not back-calculate the lengths of razorback suckers at different ages. However, mean lengths and weights at capture were determined .for each group (Table 7-. Inasmuch as fish were collected in early spring before growth began, or in late fall when growth for the year had ceased, we arbitrarily credited a "virtual" annulus at the edge of each scale; consequently "age" as used here refers to the number of growing seasons com- pleted. Although ages were assigned to the fish on the basis of agreement between two readers, the assignment of age cannot be considered completely accurate because the annuli were difficult to identify and because the method has not been validated for razorback suckers. Since no small fish were collected, verification of the first few annuli by the length-frequency method was not possible (Tesch 1971). Because the razorback sucker is rare, no fish were sacrificed for otoliths, opercles, or vertebrae, nor were fin rays removed for age determination. Other data suggest that the ages we assigned to the fish may be incorrect or that the presence of the tags markedly retarded growth. One fish recaptured 1.5 years after its original capture had not grown in length, and an additional annulus could not be de- tected. Asecond fish, 508 mm long at tagging, had grown only 8 mm when it was recaptured by P. B. Holden (personal communication) 3.5 years after release. Furthermore, two razorback suckers from California were estimated to be much older than those from the upper basin, on the basis of analysis of oto- liths. Amale (662 mm long) was estimated to be 22 years old and another fish (length unknown) 17 years old (J. A. St. Amant, personal communication. Although there may be errors in the assignment of ages to razorback suckers in the present study, these assessments are the only ones available to provide an estimate of the growth of razorback suckers in the upper Colorado River basin. There was no definite distinction in lengths at dif- ferent ages, but overlap of lengths among age groups is common among large, mature fish. On the basis of our estimates, male razorback suckers from the Colo- rado River were significantly larger than males of the same age from the Yampa River (P <_ 0.10-; the same relation held for females from the two areas (P <_ 0.01-. Among fish from the Yampa River that had completed 5 growing seasons, females were significantly larger than males. Females 5 and 6 years old from the Colo- rado River were significantly larger than males of the same ages from the same river (P <_ 0.05 and P <_ 0.1, respectively). In 1975 fish 4 and 6 years old (n = 20 and n = 35, respectively) were dominant in both rivers. The faster growth rate of fish in the Colorado River could be partly due to higher water temperatures. The water temperature of the Green River decreases in summer due to discharges- from Flaming Gorge Dam IU. S. Bureau of Reclamation 1975-. Water from the Yampa River warms the Green River, but water tem- peratures in the summer are still lower than they were before the river was impounded. Growth of Colorado squawfish and roundtail chubs (Vanicek and Kramer 1969-, and rainbow trout, Salmo gairdneri (U.S. Bureau of Reclamation 1975-, was reduced in the Green River by decreased water temperatures below the dam. McAda (1977) reported a faster growth rate in flannelmouth suckers from the Colorado River than in fish from the Green and Yampa rivers. However, the intakes to the penstocks of Flaming Gorge Dam were modified in :June 1978 to provide warmer water that would improve the growth of trout (Kramer 1979-.