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DRAFT Life History Habitat Preference Colorado squawfish are adapted to rivers with seasonally variable flow, high silt loads, and turbulence. Young-of-the-year (up to 64 mm [2.5 in.] TL), juveniles (65-200 mm [2.5-8 in.]) and subadults {200-400 mm [8-16 in.]) live in shallow backwater areas, with little or no current over silt and sand bottoms (Holden 1973; Holden and Stalnaker 1975a, 1975b; Holden and Twedt 1980; Miller et al. 1982a, 1982b; Tyus et al. 1982a, 1987; Valdez et al. 1982b; Valdez and Wick 1983; Wick et al. 1979, 1981). There is a change in habitat preference at about 200 mm (8 in.) TL (Miller et al. 1982a) with larger fish selecting deeper water of at least some velocity. Adults are large-river fish, found in a variety of depths and velocities over silt, sand, gravel, and boulder substrates (Holden 1973; Holden and Twedt 1980; Holden and Wick 1982; Miller et al. 1982a, 1982b; Tyus et al. 1982a, 1984; Valdez et al. 1982b; Wick et al. 1979, 1981). Radiotelemetry studies (Miller et al. 1983; Tyus 1985, 1986; Tyus and McAda 1984; Tyus et al. 1982b; Wick et al. 1983) have provided considerable information on habitat usage of adult Colorado squawfish. Adults use various habitats depending upon season, streamflow, water temperature, and availability (Holden and Wick 1982; jyus and McAda 1984; Tyus et al. 1987; Wick et al. 1983, 1985, 1986). During peak runoff, adults move into backwater areas or flooded riparian areas where velocity is lower and water temperatures are higher than. in the main channel (Wick et al. 1983). During the decline in water level following peak runoff, spawning adult fish move into run-riffle areas and also occupy run, eddy, and pool habitats (Tyus et al. 1987). Temperature Preference The thermal tolerance of Colorado squawfish is broad, as evidenced by the range of temperatures to which the species was presumably adapted. Summer water temperatures in the vicinity of Yuma, Arizona, for example, commonly approach or exceed 35oC (95oF) and may drop to lower than lOoC (50oF) in winter (Minckley 1979). In the Upper Basin, water temperatures generally range from 25oC (17oF) during the summer to freezing (OoC [32oF]) in winter. Tyus et al. (1987) summarized the water temperatures associated with prespawning, migratory, and spawning periods for adult Colorado squawfish in the Upper Green River Basin. Spawning migrations were initiated at water temperatures of 14-20oC (57o-68oF), and spawning occurred at temperatures of 22oC (72oF) (range 15-27.5C [59o-82oF]). In the Yampa River, migrations and spawning periods varied between years. Migrations were initiated from May 12 to June 10, associated with a mean water temperature of about 14oC (51oF), and spawning occurred at 21oC (70oF) (Table 1) (Tyus and Karp 1989). However, Tyus et al. (1987) and Wick et al. (1985) cautioned that main channel. temperatures may not accurately portray temperature preferences of Colorado squawfish because the species frequently utilized habitats outside the main 6