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14 Larvae and Postlarvae Colorado squawfish eggs hatch in 4-5 days at 20-2i °C. Sac-fry emerge from cobble bars and drift downstream with declining flows (Tyus et al. 1982b; Haynes et al. 1984; Tyus and Haines 1991) to concentrate in shallow backwater habitats in the Green River (Tyus et a]. 1982b an'd~1987)~.: Newly emerged Colorado squawfish fry drift to the mouth of the Yampa River from the midpoint of the spawning grounds, river mile 16.5-18.2, in about 6 days after hatching (Tyus and Haines 1991). Nesler et al. (1988) also noted rapid downstream transport of larvae (3-15 days) following hatching. From 1979 to 1988, peaks in abundance of young Colorado squawfish occurred in the Green River about 100 miles downstream of the Yampa River spawnimg reach (Tyus et al. 1982b and 1987; Tyus and Haines 1991). Young fish presumably use river transport for dispersal from upstream spawning grounds to downstream nursery habitats (Tyus and McAda 1984; Tyus 1986; Nesler et al. 1988; Tyus and Haines 1991; Paulin et al., in prep.). Gradually decreasing flows following spring runoff create productive nursery habitats which persist with summer-winter base flow conditions. Availability (quality and quantity) of these habitats in the Green River is considered essential for successful recruitment of the species. Flow, river temperature, availability of backwater habitat, and predator abundance contribute to mortality of drifting larvae. Berry (1988) noted that larval Colorado squawfish acclimated to about 22 °C are adversely affected by cold water of 10 °C and 15 °C. Young Colorado squawfish are routinely collected in isolated pools in the Green River system (U.S. Fish and Wildlife Service, unpublished data). These pools form when decreasing flows strand bodies of water from the main channel. Natural fluctuations in river level are usually slow enough to make this a gradual process and thereby allow entrapped fish to escape. Abrupt fluctuations in river level, as is characteristic of the Green River and other regulated systems, could increase mortality of small fish by cutting off escape routes and thereby increasing potential for competitive interactions and exposure to terrestrial predation. Herons (Ardeidae), raccoons ( on iotor), garter snakes (Thamnophis spp.), and other animals were observed feeding on fish trapped in isolated pools (Erman and Leidy 1975; U.S. Fish and Wildlife Service, unpublished data). Age-0 Colorado squawfish are most abundant in shoreline backwaters when water temperatures are the same or greater than the main river channel. Mark- recapture studies indicated a diel movement of postlarval (age-0 and age-1) fish between backwaters and the mainstream river in April, October, and November (Tyus 1991). Postlarval squawfish appear adaptable to changing conditions. During the winters of 1985-1986, 1987-1988, and 1988-1989, a significant overwintering mortality of age-0 Colorado squawfish was not detected (Tyus and Haines 199 1 . In addition, low flows in 1988 did not reduce standing crops (T.7 fish/100 m seined) or growth of young Colorado squawfish. Total length of the fish in October 1988 (41.3 and 45.0 mm, upper and lower Green River, respectively) was similar to other years of best growth (Tyus and Haines 1991). Additionally, high spring catches of age-0 fish (6.5-28.9 fish/100 mZ seined) showed good overwinter survival in some years. Laboratory studies (Thompson 1989) found that all age-0 Colorado squawfish survived simulated winter conditions when