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80 TYUS AND HAINES FIGURE I. - Map of the Green and Yampa river study areas. Strata A-G delineate stream reaches sampled in the Green River. Black dots indicate U.S. Geological Survey gaging stations at Green River (stratum B), Jen- sen (stratum F), Deerlodge (Yampa River), and Maybell (Yampa River); RK means river kilometer from the Col- orado River. conducted in the Green River upstream from its junction with the Yampa River in 1979-1981 and in the lower 225 km of the White River in 1981. The Green River was divided into seven strata (A-G in Figure 1) between river kilometers (RK) 35 and 552. Strata A, B, E, and F are low-gradient (0.2-0.4 m/km) reaches with sand and silt sub- strates. Strata C, D, and G are relatively high- gradient reaches (1.3-2.1 m/km) dominated by boulder and cobble substrates. Methods Larval collections. - Hatching dates, growth, and distribution of larval Colorado squawfish (< 25 mm total length, TL) in the Green and Yampa rivers, 1981-1988, were determined by collec- tions with seines and drift nets. Larval seining was conducted in the Green, Yampa, and White rivers in 1981 to identify potential spawning grounds. Similar collections were made in the Green River in 1984, 1985, 1986, and 1988 to determine the distribution oflarva in July and August. Sampling was done by dividing the river into 8-km reaches and seining the first two backwaters in each reach. Sampling in 1984 and 1986-1988 was expanded to include seining at weekly intervals during July and August below Yampa Canyon and Gray Can- yon spawning sites. Seine samples were taken with I-m-long x I-m-deep seine with 0.63-mm2 mesh. Drift netting was conducted about 25 km below the two spawning sites in June and July 1986- 1988: in the Yampa River near the confluence of the Yampa and Green rivers, and in the Green River immediately below Gray Canyon (Figure 1). Icthyoplankton nets were about 4 m in length and had rectangular openings of 0.5 x 0.3 m. Each net was suspended in the river channel by attach- ment to metal fence posts. Nets were set for about 30 min duration at 0600, 1200, 1800, and 2400 hours. Water velocity was recorded at each loca- tion and sampling period with a Marsh-McBimey Model 2011 current meter. All larvae were preserved in 5% buffered for- malin solution and sent to the Larval Fish Lab- oratory at Colorado State University, Fort Col- lins, Colorado, for identification. Postlarval collections. -Postlarva1 Colorado squawfish (25-70 mm TL) were sampled with 5-m- long x I-m-deep seines of 3.2 x 4.8-mm woven mesh in September and October 1979-1985, and with 3-m-long x 1-m-deep seines in 1987-1988 by use of a systematic sampling design. A stratum was selected at random and the first two back- waters encountered in each 8-km reach of that stratum were seined. Sampling then proceeded downstream to each of the remaining 8-km reach- es. Colorado squawfish were counted and mea- sured to the nearest millimeter total length. All other fish species were counted and placed in 10- mm length-classes. The area seined, depth ofwa- ter, and location (RK) of each backwater sampled was recorded. In 1979-1981, runs, eddies, side channels, and shoreline habitats were also seined. Runs were defined as stretches of the main chan- nel that were relatively deep and fast with laminar flow. Eddies were portions ofthe stream with dis- tinct countercurrents and were usually deeper than the adjacent channel. A side channel was defined as a smaller channel in a braided river section that carries appreciable flow during high water. A shoreline habitat consisted of shallow water next I Mention of trade names does not indicate endorse- ment of commercial products by the Fish and Wildlife Service. , I .