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952 OSMUNDSON ET AL. TABLE 4.-Diets of Colorado squawfish (400-550 mm total length) during spring 1994. Values are overall vol- umes of each food item as a percentage of total volume (100%) and are averages from all fish containing food (see text); UI = unidentified; MOD = miscellaneous organic debris. omponent Reach Lower Upper (N = 16) (N = 17) Habitat Back- waters Channel (N = 30) (N = 9) UI fish parts 23.6 35.8 30.2 20.8 UI fish 24.5 2.9 11.5 11.5 Fathead minnow 27.2 28.2 35.3 27.6 Red shiner 0.0 11.1 6.8 0.0 Sand shiner 13.7 0.0 1.3 25.1 Roundtail chub 9.3 0.0 3.3 5.8 Green sunfish Lepomis cyanellus 0.0 5.8 3.3 0.0 White sucker 0.0 10.9 6.2 0.0 Annelids 0.9 0.7 1.0 0.7 MOD 0.8 4.5 1.0 8.6 significantly higher than in all other strata, and stratum 6 CPUE was higher than that in stratum 5. Diet The modified Seaburg stomach sampler proved effective in removing stomach contents from Col- orado squawfish smaller than about 550 mm. For larger adults, the method was abandoned due to persistent difficulties associated with (1) handling the fish while attempting to manipulate the flush- ing tube, (2) passing tubes of sufficient length and diameter (to encapsulate large prey) down the esophagus, and (3) producing sufficient flushing pressure with the squeeze bulb for forcing items through the large tubes. Efforts to encourage large fish to regurgitate their stomach contents by using tartar emetic (after Jernejcic 1969) were also aban- doned because of apparent stress to the fish. Diets of Colorado squawfish 400-550 mm cap- tured in the upper reach were similar to those of fish captured in the lower reach and were similar between fish captured from main channel and backwater habitats (Table 4). Fish in stomach sam- ples were generally small: lengths of fathead min- nows, the most common prey, averaged 51 mm (SL) whereas roundtail chub and white sucker av- eraged 85 and 70 mm (SL), respectively. Of stom- achs containing identifiable fish (N = 27), 63% contained one fish; 26% contained two or three fish; 4% contained five fish; and 7% contained six or seven fish. Feeding rate may be reduced in this species prior to spawning: significantly more (test of proportions, P = 0.002) sampled fish 400-550 mm (TL) had empty stomachs from 26 May to 20 June (93%, N = 14) than fish sampled before 26 May (41%, N = 58) in the early spawning year of 1994. Percentages of empty stomachs before 26 May were similar (P > 0.05) between lower (38%, N = 26) and upper reaches (44%, N = 32). Temperature Annual thermal units for growth decreased in an upstream direction as expected (Table 5). As a percentage of thermal units in stratum 1, thermal units in stratum 3 were 93%; in stratum 5, 81%, in stratum 6, 71%; in stratum 7, 62%; at Cameo, 54%; and at Rulison, 42%. Thermal units de- creased between sites in strata 1 and 3 by an av- erage of 0.08 units/rkm; between strata 3 and 5 sites by 0.17/rkm; between strata 5 and 6 sites by 0.15/rkm; between strata 6 and 7 sites by 0.23/ rkm; between stratum 7 and Cameo sites by 0.24/ TABLE 5.-Total annual thermal units for Colorado squawfish growth at seven temperature-monitoring stations along the Colorado River. Bold italics indicate missing values that were estimated. Missing values for stratum 1 were estimated by using the relationship between thermal growth units there and at the stratum 3 or stratum 5 sites; missing values for strata 3 and 6 were estimated from stratum 5 values; missing values for the Rulison (R) site were estimated from the Cameo (C) site values. No thermograph was located in stratum 7 during the 1992-1996 period; all yearly totals are estimates based on the relationship between thermal growth units there and at the Cameo site (a ratio of 1.165:1) developed during 1986-1991 (not shown); Rkm = river kilometers upstream of the confluence with the Green River. Stratum or site Location Rkm 992 993 Year 1994 995 996 ean (SE) 1 87.4 100.9 68.3 97.6 53.1 85.2 81.0 (9.0) 3 158.2 85.3 53.5 96.7 47.2 77.2 72.0 (7.5) 5 214.6 73.6 46.2 87.4 39.2 65.9 62.5 (7.5) 6 264.7 63.9 41.3 75.9 34.6 59.4 55.0 (8.8) 7 292.8 60.4 34.0 68.8 29.7 49.3 48.4 (9.4) C 321.4 51.9 29.2 59.1 25.5 42.3 41.6 (6.4) R 369.9 40.4 22.8 46.6 19.1 33.8 32.5(5.2)