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o o o 1\1 CJl W Mr. Calvin Joyner 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 I, 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. 8 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 (Miller et al. 1982). Although the location of spawning areas in the San Juan River are not well defined, the capture of young-of-year specimens in 1987 and 1988 documents that successful Colorado squawfish reproduction does occur when conditions are favorable (Platania 1990). Young-of-year squawfish were collected from three main areas--in the vicinity of the Mancos River confluence in New Mexico; in the viCinity of the Montezuma Creek confluence near Bluff, Utah; and in the upstream portion of the San Juan River inflow to lake Powell (Platania 1990). 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, (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. 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 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). The highest documented reproduction for Colorado squawfish occurred in 1987 concurrently with high spring flows that were near historic levels, Catch rates for channel catfish, a nonnative predator, were lowest in 1987 (Platania 1990).