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~ t~. ~ ~ ~ ,~ Colonel John N Reese ORA F T 7 Temperature also has an effect on egg development and hatching. In the laboratory, egg mortality was 100 percent in a controlled test at 13' C. At 16' to 18' C, development 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 (Hiller et al. 1982). Hiller 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). Hiller et al. (1982) 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 squawfish habitat preferences are similar to that of young-of-year fish, but they appear to be mobile and more tolerant of lotic conditions away from the sheltered backwater environment. Hiller et al. (1982) and Archer et al. (1986) demonstrated that Colorado. squawfish often migrate considerable distances to spawn in the Green and .Yampa Rivers, and similar movement has been noted in the main stem San Juan River. A fish captured and tagged in the San Juan Arm of lake Powell in April 1987 was later recaptured in the San Juan River approximately 80 miles upstream in September 1987 (Platania 1990). Only two Colorado squawfish confirmed spawning sites, as defined in the Colorado Squawfish Recovery Plan, have been located in the Upper Basin: river mile 16.5 of the Yampa River and river mile 156.6 of the Green River. These areas have the common characteristics of coarse cobble or boulder substrates forming rapids or riffles associated with deeper pools or eddies. It is believed that a stable, clean substrate is necessary for spawning and incubation. Substrates are swept clean of finer sediments by high flows scouring the bed prior to the spawning period. O'Brien (1984) studied the hydraulic and sediment transport dynamics of the cobble bar within the Yampa River spawning site and duplicated some of its characteristics in a laboratory flume study. Based on field observations, he reported: "On the rising limb of the hydrograph, sands are deposited in the cobble interstices. These sands are interchanged between the bed' and the suspended zone for discharges less than bankfull. Depending on the supply-capacity relationship, either deposition or scour could be occurring. When the cobbles move, the sand, of course, is washed from the interstices and may be completely removed from around the cobbles. Rearrangement of the cobbles will result in more stability of the armor layer. On the falling limb, the armor layer becomes a trap for sands.until finally, the