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BIOLOGY OF YOUNG COLO DO SQUA WFISH 85 waters in which the fish were absent and present, respectively. Backwater sizes in the lower Green River were smaller, and no size selection was not- ed (Table 4). We detected no relationship between the presence or absence of young and backwater substrate (Table 4). There were no strong corre- lations (r 2 < 0.1) among the temperature, depth, or size of backwaters. Growth and Survival Age-O Colorado squawfish averaged about 40 mm in total length in October (range, 29.~7.3 mm). Average total lengths were similar in the upper and lower Green River (upper: mean = 39.7 mm, range = 29-46.3 mm; lower: mean = 39.9 mm, range = 30-47.3 mm; Table 5), although the fish were larger in the upper area in four of the nine years (t-tests, P < 0.01; Table 5). The relative abundance (log"fish/IOO m2) of age-O Colorado squawfish captured in October was inversely correlated with late-summer flows in the upper (r = -0.80, P < 0.01 for strata D, E, F) and the lower (r = -0.86, P < 0.01 for strata A, B, C) Green River (Figure 3). Total length was also inversely correlated with late-summer flows, particularly in the upper Green River (r = - 0.81, P < 0.01 for upper; r= -0.57,P= 0.11 forlower; Figure 3). Spring catches of age-O Colorado squawfish (range, 0-29 fish/IOO m2) were greater than au- tumn catches for four of eight comparisons. They ranged from 0.2 to 2.1 times higher than autumn catches in the lower Green River and from 0.0 to 9.2 times higher in the upper river. Age-I Colo- rado squawfish caught in spring were larger (mean, 45.2 mm, N = 1,193) than the same cohort cap- tured the previous autumn (mean, 41.5 mm, N= 1,212). When fish were partitioned into upper and low r Green River groups, the mean total length of sh in March and April (upper: mean = 45.4 m , N = 191; lower: mean = 45.0 mm, N = 1,0 2) were greater than those offish sampled in ! . the ame areas the prevIOUS October (upper, mean .8 mm, N = 137; lower, mean = 43.2 mm, N = 1,075) in five of six comparisons (t-tests, P < 0.05). Discussion Distribution and Hatching Larval Colorado squawfish dispersed down- stream soon after hatching and occupied ephem- eral shoreline embayments (backwaters) by mid- summer. Our drift-net and seine samples in 1987 illustrated this downstream transport from the Yampa River spawning site. We did not capture larvae in the lower Yampa and upper Green rivers between June 29 and July 3, 1987, but they were present from the Yampa River spawning site downstream to RK 362 on the Green River from July 13 to 24. Drift-net catch at the mouth of the Yampa River showed that peak larval abundance occurred July 17, when the mean back-calculated age of the larvae was II d (average TL, 9.0 mm). Ifwe assume that the fish hatched at the midpoint of the spawning area (RK 29), and that the fry emerged about 5 d after hatching (Hamman 1981), the larvae drifted 29 km in approximately 6 d. By mid-August, age-O postlarvae were not captured in the Yampa River, but they were taken imme- diately below, in the Green River. In October, postlarval Colorado squawfish were rare in the Green River for about 80 km below the Yampa River confluence. Rapid downstream transport was documented by Nesler et al. (1988), who noted that young Colorado squawfish were transported downstream and out of the Yampa TABLE 5.-Mean total lengths ofage-O Colorado squawfish in the lower and upper Green River in October 1979- 1985 and 1987-1988. N = number of fish. Asterisks denote significance at P ~ 0.0 I **; NS = not significant (P > 0.05). Lower Green River Upper Green River Total length (mm) Total length (mm) Year Mean SD N Mean SD N I-test 1979 44.5 7.02 639 47.3 7.07 267 ** 1980 36.9 7.02 810 43.1 8.47 424 ** 1981 46.3 8.34 47 45.2 9.33 267 NS 1982 33.2 4.52 127 35.7 5.03 398 ** 1983 29.0 5.33 27 32.8 6.50 4 NS 1984 43.0 11.31 2 30.0 4.76 23 NS 1985 41.4 7.54 67 35.5 6.06 59 ** 1987 43.6 6.35 155 43.1 7.17 74 NS 1988 41.3 6.97 1,329 45.0 8.65 288 **