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The YOY Colorado squawfish collected probably hatched in early to _~- late July 1987. These dates were determined through the use of equations developed from the culture of squawfish in a hatchery ~ (Haynes and Muth 1984). Hamman (1981) reported an incubation time for Colorado squawfish embryos of 90-121 hr at 20-24oC. If an incubation period of 3-5 days is assumed, then these YOY squawfish were probably spawned sometime in early to mid-July 1987. Flows during that time period ranged between 7,270 and 5,380 cfs (July mean flow=4,945 cfs; Figure 9). Water temperatures during July were: mean=19.8oC; ~ maximum=23.6oC, and minimum=17.9oC (U.S.G.S. Bluff, Utah gaging station). Estimation of the site where these fish were spawned is not possible since our understanding of the movements (distances, diel patterns, etc.) of young Colorado squawfish is limited. Backwater habitats were dominated by exotic species. Throughout ~ the area sampled by UDWR, red shiner and fathead minnow were the most abundant fishes. Mosquitofish, another exotic, were common. Red shiners constituted 51% of the total number of fishes collected in all first seine hauls combined; fathead minnow constituted 25%; native catostomids 6%, and mosquitofish 8% (Table 10). Similar to upstream river reaches, backwaters in the river-lake transition zone were ~ dominated by exotic species. Fathead minnow, red shiner, and mosquitofish were the most abundant species. A difference between the _ backwater fish communities of the river-lake transition zone and upstream river reaches was that fathead minnow (56%) were more abundant than red shiner (39%) in backwaters in the river-lake _ transition zone (Table 10). r The dominance of exotic species in backwater habitats exists throughout the Upper Colorado River Basin. Backwaters in 1987 in the Colorado and Green rivers had fish communities dominated by red shiner, fathead minnow and sand shiner. Overall, in the Colorado, Green and San Juan rivers, red shiner was the most abundant fish in ~ samples of backwaters (Table 11). Relative abundances (CPUE) of red shiner, fathead minnow, and sand shiner did vary within and between rivers. Red shiner were less abundant in the San Juan River than in the Colorado or Green rivers (Table 11). The relative abundance of sand shiner in the San Juan River was less than in the Colorado River, but the San Juan River and Green River relative abundances of sand ~ shiner were similar (Table 11). Relative abundance of fathead minnow in backwaters of the San Juan River was generally similar in the Colorado and Green rivers (Table 11). - 29 -