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<br />further guide the fish to specific sites in the spawning reach. Colorado <br />squawfish larvae may imprint on some home-site olfactory bouquet and recognize <br />such odors when they return as migrating adults (Wick et al. 1983; Tyus <br />1990). <br /> <br />The potamodromous migrations and homing behavior of Colorado squawfish <br />mandates protection of known migration routes, since feasibility of fish <br />passage facilities has yet to be demonstrated for this species. Migrations of <br />Colorado squawfish are vulnerable to stream blockage (Tyus 1984) as evidenced <br />by the recent loss of BOkm of occupied habitat in the White River due to <br />blockage (Martinez 1986). lack of access to spawning grounds is implicated in <br />the decline of Colorado squawfish (Joseph et a1. 1977; Tyus 1984). <br /> <br />Spawning <br /> <br />Two Colorado squawfish spawning areas have been identified in the Green <br />River basin by tracking radio-tagged migratory fish. These include riffle and <br />pool habitats in Yampa Canyon of the lower Yampa River and Gray Canyon of the <br />Green River (Tyus and Karp 1989; Tyus 1990). <br /> <br />As indicated in Figure 5, the annual spawning period (as indicated by <br />presence of migrating radio-tagged fish on spawning grounds, collections of <br />ripe fish, or calculated dates of larval emergence in spawning reach) lasts <br />about 4 to 5 weeks. An optimum spawning period was also calculated by <br />averaging the dates in which radio-tagged fish and ripe fish were present in <br />the spawning reach and back-calculated dates of egg deposition. The length of <br />the estimated optimal spawning period (about 26 days) was similar in both <br />spawning areas all years. Spawning generally occurred earlier in low-water <br />years, 1981, 1987 and 1988, and later in high-flow years of 1983 and 19B4. <br />Discharge was variable between years during the optimum spawning period, but <br />average temperatures ranged from about 22-2SoC (Tyus 1990). <br /> <br />Vanicek and Kramer (1969) first suggested that discharge and temperature <br />influenced spawning in Colorado squawfish. FWS data from 1981 to 1988 <br />indicated that spawning occurred during the period of declining flows <br />following spring peak runoff and increasing temperatures (Figure 6; Tyus <br />1990). This generally occurred about 26 days (range: 17 - 33 d) following <br />migration. Peak discharge preceding spawn, and mean minimum temperatures <br />during spawn were highly correlated with the spawning period (r = 0.84, r = <br />0.88, respectively; P < O.OS), ostensibly because discharge, temperature, and <br />spawning period are correlated. Spawning of Colorado squawfish was considered <br />a result of complex environmental and biological influences and is not <br />triggered by a single flow or temperature event (Tyus 1990). As an example, <br />flow spikes from rainstorms during spring runoff may also influence ovulation <br />and spawning in Colorado squawfish, as recently hypothesized by Nesler et al. <br />(1988). Radiotracking data suggests that all adult Colorado squawfish do not <br />spawn each year, and male and female fish may require different stimuli for <br />gonadal maturation (Tyus 1990), some of which are presumably flow-related. <br /> <br />Breeding adults were most often concentrated in river reaches containing <br />deep pools, eddies, and submerged cobble/boulder bars. Radio-tagged fish <br />moved from pools or eddies to presumably spawn on bars and then returned to <br /> <br />18 <br />