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. ", 6 BioloQV 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 tefllleratures approach 200 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. Teflllerature also has an effect on egg development and hatching. In the laboratory, egg mortality was 100 percent in a controlled test at 130 C. At 160 to IBo C, development of the egg is slightly retarded, but hatching success and survival of larvae was higher. At 200 to 260 C, development and survival through the larval stage was up to 59 percent (Hamman 1981). Juvenile teflllerature preference tests showed that preferred temperatures ranged from 21.90 to 21.60 C. The most preferred temperature for juveniles and adults was estimated to be 24.60 C. Temperatures near 240 C also are needed for optimal development and growth of young (Miller et al. 1982). Only two Colorado sQuawfish confirmed spawning sites, as defined by the Upper Colorado River Coordinating Committee, have been located in the Upper Colorado River BasiR--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 suspe~ zone for discharges less than bank.full. 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 sand reservoir is again filled. Without cobble movement. sand will be scoured only to a depth of one-half to one median cobble diameter below the cobble bed surface.