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84 TYUS AND HAINES TABLE 3.-Catch of age-O Colorado squawfish in different habitats, September-October 1979-1981; C = fish seined/lOa m2, E = number of seine hauls. Backwater Shoreline Eddy Main channel Side channel Year C E C E C E C E C E 1979 22.6 106 0.0 2 0.0 8 0.6 II 0.0 9 1980 22.6 187 4.7 49 0.0 II 0.0 I 3.0 II 1981 11.2 72 0.1 80 0.2 48 Mean 18.8 122 1.6 44 0.1 22 0.3 6 I.5 10 gradient reaches, including stratum C, D, and G, yielded lower average catches (mean, 4.1/100 m2; Table I). the lower Green River (strata A and B) and in 59% in the upper river (strata E and F). In autumn, Colorado squawfish occupied back- waters that were warm, deep (t-tests, P < 0.05), and turbid (G-tests, P < 0.01, Table 4). Back- waters with Colorado squaw fish averaged about 17'C, about 2' warmer than the average of those without the species (Table 4). Colorado squawfish were usually captured in deeper backwaters (mean depth, 38 cm), but the fish was more abundant in larger backwaters only in the upper Green River (t-test, P = 0.05), where average sizes were 269 m2 and 1,158 m2 for back- Habitat Use Catch ofpostlarval Colorado squawfish in low- er-gradient reaches during 1979-1981 was great- est in backwaters (84%) but postlarval fish were also captured in shorelines, side channels, runs, and eddies (Table 3). In all years combined (I 979- 1985, 1987, 1988), the catch of age-O Colorado squawfish averaged 13.3/ I 00 m2 in backwaters. The young were absent in 37% of backwaters in TABLE 4.-Relationships between some variables in backwaters and presence of Colorado squawfish in the Green River, September and October 1979-1985 and 1987-1988. N = number of backwaters. The study area in lower Green River ranged from river kilometer (RK) 35 to RK 211; the study area in upper Green River ranged from RK 340 to RK 515. The symbols I and G denote the I-test and G-test of differences between means for species- absent and species-present; asterisks denote significance at P :5 0.05* or P :5 0.0 I **; NS = not significant. Species absent Species present Variable Mean SD N Mean SD N Test Lower Gree" River Temperature ("C) 17.7 5.7 82 19.4 4.4 118 t* Maximum depth (cm) 22.4 22.2 87 33.9 26.5 147 /.. Size (m2) 309.9 644.8 32 493.7 622.6 49 /. NS Turbidity (%) (? Clear 12.5 2 7.0 4 Moderate 68.8 II 26.3 15 Turbid 18.7 3 66.7 38 Substrate % G.NS Silt 89.7 78 84.6 126 Sand 10.3 9 14.1 21 Gravel 0.0 0 0.7 I Cobble 0.0 0 0.7 I Upper Green River Temperature ("C) 12.2 4.5 166 14.6 4.6 100 /.. Maximum depth (cm) 33.1 24.3 190 41.8 25.3 128 /.. Size (m2) 269.0 354.5 55 1,158.0 1,638.4 30 t*. Turbidity (%) (? Clear 39.6 19 0.0 0 Moderate 27.1 13 33.3 7 Turbid 33.3 16 66.7 14 Substrate % G. NS Silt 68.9 128 66.7 86 Sand 30.1 56 33.3 43 Gravel 0.0 0 0.0 0 Cobble 1.0 2 0.0 0 waters in which the fish WI respectively. Backwater si;- River were smaller, and no ed (rable 4). We detected n the presence or absence of substrate (Table 4). There lations (r 2 < 0.1) among t, or size of backwaters. Growth and Survival Age-O Colorado squawfi mm in total length in Oct< mm). Average total lengt~ upper and lower Green Riv mm, range = 29-46.3 mrr mm, range = 30-47.3 mm; fish were larger in the up~ nine years (t-tests, P < 0.0 The relative abundanCe age-O Colorado squawfish c: inversely correlated with la upper (r = -0.80, P < O. and the lower (r = -0.86, B, C) Green River (Figurf also inversely correlated w particularly in the upper G: P < 0.01 for upper; r = -0. Figure 3). Spring catches of age-C (range, 0-29 fish/lOa m2) tumn catches for four of eir ranged from 0.2 to 2.1 tim catches in the lower Green 9.2 times higher in the up rado squawfish caught in sp 45.2 mm, N = 1,193) thar tured the previous autumn 1,2 I 2). When fish were pan TABLE 5.-Mean totallengtt 1985 and 1987-1988. N = nu 0.05). Lo Total lengl Year Mean 1979 44.5 1980 36.9 1981 46.3 1982 33.2 1983 29.0 1984 43.0 1985 41.4 1987 43.6 1988 41.3