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further guide the fish to specific sites in the spawning reach. Colorado squawfish larvae may imprint on some home-site olfactory bouquet and recognize such odors when they return as migrating adults (Wick et al. 1983; Tyus 1990). The potamodromous migrations and homing behavior of Colorado squawfish mandates protection of known migration routes, since feasibility of fish passage facilities has yet to be demonstrated for this species. Migrations of Colorado squawfish are vulnerable to stream blockage (Tyus 1984) as evidenced by the recent loss of 80km of occupied habitat in the White River due to blockage (Martinez 1986). Lack of access to spawning grounds is implicated in the decline of Colorado squawfish (Joseph et al. 1977; Tyus 1984). Spawning Two Colorado squawfish spawning areas have been identified in the Green River basin by tracking radio-tagged migratory fish. These include riffle and pool habitats in Yampa Canyon of the lower Yampa River and Gray Canyon of the Green River (Tyus and Karp 1989; Tyus 1990). As indicated in Figure 5, the annual spawning period (as indicated by presence of migrating radio-tagged fish on spawning grounds, collections of ripe fish, or calculated dates of larval emergence in spawning reach) lasts about 4 to 5 weeks. An optimum spawning period was also calculated by averaging the dates in which radio-tagged fish and ripe fish were present in the spawning reach and back-calculated dates of egg deposition. The length of the estimated optimal spawning period (about 26 days) was similar in both spawning areas all years. Spawning generally occurred earlier in low-water years, 1981, 1987 and 1988, and later in high-flow years of 1983 and 1984. Discharge was variable between years during the optimum spawning period, but average temperatures ranged from about 22-25oC (Tyus 1990). Vanicek and Kramer (1969) first suggested that discharge and temperature influenced spawning in Colorado squawfish. FWS data from 1981 to 1988 indicated that spawning occurred during the period of declining flows following spring peak runoff and increasing temperatures (Figure 6; Tyus 1990). This generally occurred about 26 days (range: 17 - 33 d) following migration. Peak discharge preceding spawn, and mean minimum temperatures during spawn were highly correlated with the spawning period (r = 0.84, r = 0.88, respectively; P < 0.05), ostensibly because discharge, temperature, and spawning period are correlated. Spawning of Colorado squawfish was considered a result of complex environmental and biological influences and is not triggered by a single flow or temperature event (Tyus 1990). As an example, flow spikes from rainstorms during spring runoff may also influence ovulation and spawning in Colorado squawfish, as recently hypothesized by Nesler et al. (1988). Radiotracking data suggests that all adult Colorado squawfish do not spawn each year, and male and female fish may require different stimuli for gonadal maturation (Tyus 1990), some of which are presumably flow-related. Breeding adults were most often concentrated in river reaches containing deep pools, eddies, and submerged cobble/boulder bars. Radio-tagged fish moved from pools or eddies to presumably spawn on bars and then returned to 18