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ol:;.> f>""" enhance spawning success of native species and inhibit that of non-native species, Haynes et al. (1984) reported that fish species, such as Colorado sQuawfish, that evolved under highly fluctuating flow conditions. were better able to survive and successfully recruit under those conditions than the introduced species. The decline of endemic Colorado River fishes seems to be partially related to competition or other behavioral interactions with nonnative species, which perhaps have been exacerbated by alterations in the natural fluvial environment. Valdez (1990) reported that densities of three nonnative cyprinids (red shiner, sand shiner, and fathead minnow) in the Colorado River were much lower following high-flow years and increased three to four times in a 2-year period during normal- and low-water years. Platania (19901 noted that, during the 3 years of studies on the San Juan River, spring flows and Colorado sQuawfish reproduction were highest in 1987. He further noted catch rates for channel catfish were lowest in 1987. Recent studies also found catch rates for young-of-year Colorado squaw fish to be highest in high water years, such as 1993 (Buntjer et al. 1994, Lashmett 1994). o '-:-) (." .. \ .~ The life-history phases that appear to be most critical for the Colorado squawfish include spawning, egg fertilization, and development of larvae through the first year of life. These phases of Colorado squawfish development are tied closely to specific habitat requirements. Natural spawning of Colorado sQuawfish is initiated on the descending limb of the annual hydrograph as water temperatures approach 20' Celsius (C). Spawning, both in the hatchery and in the field, generally occurs in a 2-month timeframe between July 1 and September 1, although high flow water years may suppress river temperatures and extend spawning in the natural system into September. Conversely, during low flow years when the water warms earlier, spawning may occur in late June. A natural hydrograph with a large spring peak; a gradually declining/descending limb into early summer; and low, stable flows through summer, fall, and winter are thought to create the best habitat conditions for endangered fishes while maintaining the integrity of the channel geomorphology. Tyus and Karp (1989) pointed out the importance of peak flows (spring runoff) associated with reproductive activities of Colorado squawfish. They further stated that alteration of this hydrological event may affect initiation of Colorado squawfish migration and spawning. Additionally, maintenance of low stable flows in summer and fall are necessary far growth and survival of young Colorado squawfish. Temperature also has an effect on egg development and hatching. In the laboratory, egg mortality was 100 percent in a .controll.ed test at 13" C. At 16' to 18" C, dev~lopment of the egg is slightly retarded, but hatching success and survival of larvae was higher. At 20' to 26' C, development and survival through the larval stage was up to 59 percent (Hamman -1981). Juvenile temperature preference tests showed that preferred temperatures ranged from 21.9' to 27.6' C. The most preferred temperature for juveniles and adults was estimated to be 24.6' C: Temperatures near 24' C also are needed for optimal development and growth of young (Miller et al. 1982). Miller et al. (1982) concluded from collections of larvae and young-of-year below known spawning sites that there is a downstream drift of larval Colorado squawfish following hatching. Extensive studies in the Yampa and upper Green Rivers have demonstrated downstream distribution of young Colorado squawfish from known spawning areas (Archer et al. 1986; Haynes et al. 1985). Miller et al. (19821 also found that young-of-year Colorado squawfish, from late summer through fall, preferred natural backwater areas of zero velOCity and less than 1 ,5-foot depth over a silt substrate. Juvenile Colorado 6