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8 essential behavior patterns, such as breeding, feeding, and sheltering. Osmundson and Burnham (1996) summarized the status and trend of the Colorado River population of Colorado pikeminnow. Thee found that numbers were low but new individuals were actively recruiting to the adult population, and recruitment largely occurs in pulses from infrequent strong year classes. These investigators concluded that low adult numbers and infrequent pulsed recruitment make this population vulnerable to extirpation over time from both natural fluctuations in numbers as well as from continued changes in habitat. Life History The life-history phases that appear to be most critical for the Colorado pikeminnow include spawning, egg hatching, development of larvae, and the first year of life. These phases of Colorado pikeminnow development are tied closely to specific habitat requirements. Natural spawning of Colorado pikeminnow is initiated on the descending limb of the annual hydrograph as water temperatures approach or exceed 20° C (Vanicek and Kramer 1969, Hamman 1981, Haynes et al. 1984, Tyus 1990, McAda and Kaeding 1991). Temperature at initiation of spawning varies somewhat by river: in t:he Green River, spawning begins as temperatures exceed 20-23° C; in the: Yampa River, 16-23° C (Bestgen et al. 1998); in the Colorado River, 18-22° C (McAda and Kaeding 1991) Spawning, both in the hatchery and under natural riverine conditions, generally occurs in a 2-month time frame between late June and late August. However, in the natural system, sustained high flows during wet years may suppress river temperatures and extend spawning into September (MCAda and Kaeding 1991). Conversely, during low flow years, when the water warms earlier, spawning may commence in mid June. Temperature also has an effect on egg development and :hatching success. In the laboratory, egg development was tested at five temperatures and hatching success was found to be highest at 20° C, lower at 25° C, and mortality was 100 percent at 5, 10, 15, and 30° C. In addition, larval abnormalities were twice as high at 25° C than at 20° C (Marsh 1985). Experimental tests of temperature preference of yearling (Black and Bulkley 1985a) and adult (Bulkley et al. 1981) Colorado pikeminnow indicated that 25° C was the most preferred temperature: for both life phases. Additional experiments indicated that optimum growth of yearling Colorado pikeminnow also occurs at temperatures near 25° C (Black and Bulkley 1985b). Although no such tests were conducted using adults, the tests with yearlings supported the conclusions of Jobling (1981) that the final thermal preferendum provides a good indication of optimum growth temperature, i.e., 25° C.