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(Tyus 1991, McAda and Kaeding 1991). Back calculating hatching dates from total length <br />of larvae, McAda and Kaeding (1991) found that timing of spawning during 1982-1985 <br />varied among years but generally occurred when water temperatures were 1-8-22 C and <br />river flow was 15-30% of the maximum discharge for the year. M re recently, larval drift <br />studies from 1992-1996 revealed that spawning began as early as June 5 (1994) and as late <br />as July 11 (1995); spawning began 1-4 weeks after runoff peaked and shortly after main- <br />channel temperatures reached 17-18 C (Trammel and Chart 1999a, Anderson 1999). In <br />general, spawning occurred earlier during years of low runoff and later in years of high <br />runoff. This phenomenon has also been observed in the Green River sub-basin (Tyus and <br />Haines 1991, Bestgen et al. 1998). During years of high runoff, flows stay high for an <br />extended period and temperatures do not rise sufficiently until flows subside, pushing the <br />spawning period later into the summer. <br />Spring flows are thus important in triggering adult physiology and behavior <br />associated with reproduction. Rising water levels in spring along with increasing photo- <br />period no doubt triggers the resumption of vitellogenesis following a winter dormancy of <br />developing eggs (Tyus 1990). Warm water found in flooded backwaters, tributary mouths <br />and bottomlands during April, May and June may hasten the gamete maturation process <br />(Valdez and Wick 1983). Declining water levels following the spring peak may cue fish to <br />seek spawning sites. After fish reach spawning sites and begin to congregate in staging <br />pools, some time may elapse while fish await final physiological changes to take place (final <br />egg maturation, ovulation, etc.) and for the requisite mix of environmental factors to occur. <br />What these environmental factors are is unclear, but probably include some range of suitable <br />depths and velocities over a substrate that has been sufficiently cleaned of fine sediment. <br />Females likely provide cues to males that ovulated oocytes are ready to be oviposited and <br />fertilized. How long individual fish participate in spawning is unknown, but larval-drift data <br />indicate that overall spawning activity may last 5-8 weeks, with spawning at downstream <br />sites beginning 1 day to 3 weeks earlier than at upstream sites (Trammel and Chart 1999a). <br />If runoff is extended too long and temperatures remain low, it has been suggested that a <br />decreasing photoperiod (after June 22) may prompt fish to spawn even though optimum <br />temperatures have not been attained (C. McAda, personal communication) <br />30