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,,~~ 12 Miaration The initiation of spawning migration is an important companent of the reproductive cycle of the Colorado squawfish. Based on radio-tracking data (Wick et al. 1983: Tyus and McAda 1984: Tyus 1990), fish in the Green and Yampa Rivers initiated spawning migrations around the summer sa1stice: Green River fish initiated migrations about June 21 (range: May 23-Ju1y 22) and Yampa River fish migrated about June 15 (range: May 26-Ju1y 13). These mavements included downstream migratians in the Yampa and White Rivers and upstream and downstream migratians in the Green River (Tyus et a1. 1987; Tyus 1990). Flaws and water temperatur.es were highly variable during Calorado squawfish spawning migrations, and migratians were initiated earlier in low-water years (e.g., 1981) and later in higher water years (e.g., 1983). The earlier the spring peak flow occurred, the longer the interval before migration began (Tyus 1990). Initiation of spring migration occurred about 28 days after peak flow in the Yampa and Green Rivers (Tyus and Karp 1989). Migration was associated with water temperatures of at least 9 'C (average 14 'C). Haming behavior in Colorado squawfish is suggested by long-distance movement patterns and repeated recaptures of the same fish on spawning graunds in subsequent years (Wick et a1. 1983; Tyus 1985 and 1990). Fish use of mare than one spawning area was not detected (Tyus 1990), supporting the idea of spawning fidelity in Colorada squawfish. Colorado squawfish spawning areas thus should be protected as unique habitats that are essential ta the canservation of the species. The migrations and homing behaviar .of Colorado squawfish mandates pratection of known migration routes. Migrations of Colorado squawfish are vulnerable to stream b1ackage (Tyus 1984) as evidenced by the recent loss of 50 miles of occupied habitat in the White River due to blockage (Martinez 1986) at Kenny Reservoir. lack of access to spawning grounds was implicated in the decline of Co10rada squawfish (Joseph et a1. 1977; Tyus 1984). SDawnina Males generally mature at age 5 or 6 and females at age 6 or 7 (Hamman 1981b; Vanicek and Kramer 1969; Seetha1er 1979). Mast mature fish in the Green and Yampa Rivers migrate in spring to two locations identified as spawning areas. These include riffle and pool habitats in Yampa Canyon on the lower Yampa River and Gray Canyon on the Green River (Tyus and Karp 1989; Tyus.1990). The annual spawning period (as indicated by the presence of migrating radio- tagged fish on spawning grounds, collections of ripe fish, or calculated dates of larval emergence in spawning reaches) lasts about 4 to 5 weeks. An optimum spawning period also was 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 for all years. Spawning generally occurred earlier in low-water years (1981, 1987, and 1988) and later in high-flow years (1983 and 1984). Discharge was