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-. . . . - . J. ~ , . 12 spawning of Colorado squawfish is initiated at approximately 20 degrees 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 delay spawning in the natural system until September. Temperature also has an effect on egg development and hatching. In the laboratory, egg mortality was 100 percent in a controlled test at 13 degrees C. At 16 to 18 degrees C, development of the egg is slightly retarded, but hatching success and survival of larvae was higher. At 20 to 26 degrees C, development and survival through the larval stage was up to 59 percent. Juvenile temperature preference tests showed preferred temperature that ranged from 21.9 to 27.6 degrees C. The most preferred temperature for juveniles and adults was estimated to be 24.6 degrees C (Miller et al. 1982). Temperatures near 24 degrees C are also 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 (Coordinating Committee) have been located in the upper Colorado River Basin: RM 16.5 of the Yampa River, and RM 156.6 of the Green River. These areas have had the common characteristic 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. These substrates are swept clean of fine sediments prior to the spawning period and maintained clean throughout the spawning and incubation periods. O'Brian (1984) studied the hydraulic and sediment transport dynamics on a cobble bar within the Yampa River spawning reach and duplicated some of its characteristics in the laboratory. He concluded that the finer sediments, primarily sand, were flushed from the coarser cobbles down to a depth of one-half the cobble diameter below the cobble surface during peak flows. He reported a range in cobble size of 50-100 mm with an average of 75 mm. At another site 2 miles downstream where Colorado squawfish have consistently congregated during the spawning season, Archer and Tyus (1984) reported the mean size of a small sample of cobbles to be 72 mm. O'Brien calculated that discharges on the order of one-half the incipient motion of the cobble bed were necessary to accomplish the observed affect at the study site. Although the location of spawning areas in the Colorado River are not well defined, the presence of larvae near the confluence of the Gunnison River, in the Loma to Blackrocks reach, and near the confluence of the Dolores River indicates that spawning occurs nearby. Investigations by Kaeding and Osmundson (1987) found that water temperatures in the Colorado River were suitable for spawning in the Grand Junction area. Miller 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 mainstem Colorado River. Miller et al. (1982) concluded from collections of larvae and young-of-year below suspected spawning sites that there is a downstream drift of larvae and young-of-year Colorado squawfish following hatching. Miller et al. (1982)