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13 variable between years during the optimum spawning period, but average temperatures ranged from about 22-25 °C (Tyus 1990). Others reported a peak in spawning after temperatures reach 20 °C (Haynes et al. 1984; Hamman 1982a; Holden and Stalnaker 1975; Tyus and McAda 1984). Spawning, both in the hatchery and in the wild, generally occurs from July-August but may extend into early August depending on flows and temperature. Vanicek and Kramer (1969) first suggested that discharge ,and temperature influenced spawning in Colorado squawfish. Data from 1981 to 1988 indicated that spawning occurred during the period of declining flows following spring peak runoff and increasing temperatures (Tyus 1990). This generally occurred about 26 days (range: 11-33 days) following migration. Peak discharge preceding spawn, and mean minimum temperatures during spawn were highly correlated with the spawning period, ostensibly because discharge, temperature, and the spawning period are correlated. Spawning of Colorado squawfish is considered a result of complex environmental and biological influences and is not triggered by a single flow or temperature event (Tyus 1990). As an example, flow spikes from rainstorms during spring runoff may also influence ovulation and spawning in Colorado squawfish, as recently hypothesized by Nesler et al. (1988). Radio-tracking data suggest that all adult Colorado squawfish do not spawn each year, and male and female fish may require different stimuli for gonadal maturation (Tyus 1990), some of which appear to be flow related. Breeding adults were most often concentrated in river reaches containing deep pools, eddies, and submerged cobble/boulder bars. Radio-tagged fish moved from pools or eddies to presumably spawn on bars and then returned to the former habitat (Tyus et al. 1987), behavior similar to that of spawning northern squawfish (Beamesderfer and Congleton 1981). Turbid riverine conditions precluded direct observations of a de osition• removed from the substrate during this time ofgyear are cleaneof sedbment and algae (Archer and Tyus 1984; U.S. Fish and Wildlife Service, unpublished data). There is substantial field and laboratory data showing that Colorado squawfish and other squawfish species require cleaned cobble surfaces for successful egg adhesion (Burns 1966; Patten and Rodman 1969; Hamman 1981b). Hamman (1981b) also noted hatching of Colorado squawfish larvae from cobble surfaces. The need for cleaned cobble and boulder substrates is supported by spawning of Colorado squawfish following peak flows and peak sediment transport (Tyus and Karp 1989). Spring scouring, a gradual decrease in summer flow, and a concomitant decrease in sediment load aid in preventing siltation of cobble bars. Thus, magnitude, timing, and duration of spring flows are considered potential limiting factors for successful reproduction by Colorado squawfish. Temperature also affects egg development and hatching. In the laboratory, egg mortality was 100 percent in a controlled test at 13 °C. At 16-18 °C, development of the egg is slightly retarded, but hatching success and survival of larvae were higher. At 20-26 °C, development and survival through the larval stage were up to 59 percent (Hamman 1981b).