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generally exceeding 14 °C (range: 14-22°C in the Green River from May 23-July 27; Tyus, in review). Homing behavior in Colorado squawfish is indicated by long-distance movement patterns and repeated recaptures of the same fish on spawning grounds in subsequent years (Wick et al. 1983; Tyus 1985, Tyus, in review). The absence of any fish using more than one spawning area also supports the concept of spawning fidelity in Colorado squawfish. Different genetic stocks may be separated by homing fidelity to spawning areas (reviewed by Smith 1985). Colorado squawfish spawning areas are considered unique and critical to the conservation of the species. The Groundwater Seepage Hypothesis, proposed for other species by Harden- Jones (1981), may be implicated as a possible homing mechanism for Colorado squawfish (Tyus 1985).. Migrating adult Colorado squawfish pass through many kilometers of potentially good spawning habitat (e.g., canyon-bound cobble bars in Split and Whirlpool canyons in the Green River, and Cross Mountain and Juniper Mountain canyons in the Yampa River) to reach specific spawning grounds in Yampa and Gray canyons. Observations at the two confirmed spawning grounds in upper Green River basin suggest that migrating individuals may be orienting to these areas in response to freshwater inflow from spring-fed tributaries (e.g., Florence Creek, Green River; Warm Springs Creek, Yampa River), sandstone-limestone seeps (e.g., at Coal Creek, Green River; at Cleopatras Couch, Yampa River), and chemical inputs from runoff, snowmelt and inundated shorelines (Lake 1967; Tyus, in review). Thus, significance of groundwater and surface inflows in these areas, relative to possible orientation mechanisms in Colorado squawfish, should be further investigated. 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. 1917; 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, in review). 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, ar calculated dates of larval emergence in spawning reach) lasted 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 22-25°C. 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, in 17