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<br />Nesler et al. (1988) found agreement in the literature that temperature and photoperiod combine <br />as mechanisms influencing gonad maturation, particularly in cyprinids, but that the final phase <br />of maturation and the release of gametes also may be controlled by water velocity and quality, ^ <br />substrale;,barometric pressure, or pheromones. <br /> <br />For the Grand,Canyon population,:gonadaldevelopment of humpback chub estimated to be three <br />to four years of age or older began 'in December and February and increased rapidly until April; <br />.during April and May the gonadosomatic index (relationship of gonad weight to whole-body <br />weight} quickly declined indicating spawning had occurred (Kaeding and Zimmerman 1983). <br />From specimens collected from 1967 to 1976, including one fromjust below Glen Canyon Dam,' . <br />Suttkus and Clemmer (1977) judged that spawning occurred during June and July. Based on <br />collections of larvae, AGFD found spawning from April to July (Angradi et al. 1992; AGFD <br />. 1993). In the upper basin, Black Rocks populations have spawned from May to July (Valdez <br />and Clemmer 1982; Archer et al. 1985; Kaeding ilt al. 1990) and Yampa Canyon populations <br />from mid-May to mid-June (Tyus and Karp 1989). <br /> <br />The foregoing research generally found the humpback chub to spawn when the hydrograph of <br />the river was at its peak or descending. Streamflow was often variable between years at the <br />same site. Maximum daily water temperatures reported by the above researchers were at Black <br />Rocks 11.5 to 23 .C, at Yampa Canyon 16 to 23 .C, and in the LCR 18 to 22 .C. Suspected <br />spawning areas in Yampa Canyon were shoreline eddy and run habitat (Tyus and Karp 1989). <br /> <br />Caution should be used when evaluating reported spawning conditions because some physical <br />attributes of sexually mature humpback chubs such as tubercles are often not closely related to <br />the actual spawning event (Archer et al. 1985) and required temperatures may be more a <br />function of degree-days and allowable fluctuations of temperature rather than daily maximum <br />temperatures. A further problem in identifying spawning requirements is that the actual <br />spawning site for humpback chub has not been documented. <br /> <br />Hamman (1982) found humpback chub eggs adhered to rock substrate in the study raceway and <br />hatched in 5-7 days at 19 to 20 .C with 84 % success. Hatching success diminished at lower <br />temperatures (16 to 17.C = 62%; 12 to 13.C = 12%) and higher temPeratures (21 to 22.C <br />= 79%). Comparative conclusions with an optimum temperature of 20 "C were found by <br />Bulkley et al. (1982) and Marsh (1985). Temperatures colder than optimum increase hatching <br />time; for example, Marsh (1985) reported 372 hours at 15 .C versus 166 hours at 20 .C and <br />significant sublethal effects such as stunted or deformed larvae with the increased hatching time. <br /> <br />Larvae and Juveniles <br /> <br />Life history information is limited for larval humpback chub in the wild compared to other life <br />stages. In the hatchery, larvae were about 7 mm total length at hatching, swam up in 3 days, <br />and begin feeding on. zooplankton near the surface for two weeks (Hamman 1982). Larval <br />humpback chub have been collected from the mainstem in total lengths as small as 16 mm <br />(AGFD 1993). In the LCR, guts of40 humpback chub from 13 to 35 mm total length were <br />examined during May 1990 and chironomid larvae were most frequent (62.5%) (Angradi et al. <br /> <br />IS December 1994 fmal biological opinion 2-21'-93-F-167 <br /> <br />10 <br />