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<br />squawfish, and that extreme hydrologic events are unfavorable to fishes <br />introduced into the system from more mesic environments. <br /> <br />METHODS <br />Study area. The Green River basin is located in eastern Utah, <br />northwestern Colorado and southern Wyoming. The primary study area <br />included 3 km of the Yampa River and the lower 552 km of the mainstream <br />Green River downstream of the confluence of the Yampa River (Figure 1). <br />Two nursery areas were delineated by a high catch of age-a Colorado <br />squawfish (> 0.01 fish/m2 seined; Tyus and Haines, In Review): a lower <br />(35-211 RK) and upper area (340-515 RK) (Figure 1.) Both nursery areas <br />were characterized as low gradient (0.2-0.4 m/km) reaches with sand and <br />silt substrates, each occurring approximately 60 km downstream of two <br />known spawning areas (Tyus 1985, Tyus and Karp 1989). <br />Field collections. Larval fish (< 30 mm TL) were sampled with <br />drift nets and seines in July and August. Larval drift was sampled in <br />the Yampa River July 3-August 14, 1986, July 6-July 31, 1987 and July 18- <br />August 5, 1988, and in the Green River July 2- August 14, 1986. These <br />dates corresponded to larval emergence and downstream drift of Colorado <br />squawfish. Drift netting was conducted about 25 km below the two <br />spawning sites in the lower Yampa River near its confluence with the <br />Green River, and the Green River immediately below Gray Canyon (stratum <br />B; Figure 1). Drift nets were 1 m2 diameter ichthyoplankton nets with <br />0.76 mm2 mesh wall and 0.51 mm2 cod end. Each net was attached to metal <br />fence posts and suspended in shallow shorelines of the river channel. <br />Nets were set for about one-half hour duration at 0600, 1200, 1800 and <br /> <br />4 <br />