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1 1 1 1 1 1 1 1 t ~l eddy-run interfaces. Preliminary data from winter studies suggested that adult Colorado squawfish overwintered in the upper Yampa River, using large backwaters (ephemeral along-shore embayments), runs, and eddies, but were most common in ice covered, low velocity shoreline areas, where large schools of minnows were observed (Wick and Hawkins 1987), and presumably feeding. Adult Colorado squawfish used backwater habitats in spring and early summer during years of low flow (e.g. 1981), but were common in flooded bottomlands in high flow years (e.g. 1983 and 1984). (NB:: High, average, and low flow designations for type of water year are defined in Butler (1988)). Radiotracking efforts in late spring/early summer in the Yampa River indicated high use of shoreline backwater habitats in the 1981 low flow year (66%, N=6 individual fish). Conversely, during the 1983 high flow year, adults exhibited a high use for flooded bottomlands (40%, N=10; none of the 10 fish contacted during this period were located in backwater habitat). Wic'-c et al. (1983) similarly found that in 1982, adult Colorado squawfish used flooded areas in spring, but moved to backwater habitats as river level dropped. Radiotracking data for adult Colorado squawfish in Green River, 1983 and 1984, generally support these findings (Tyus et al. 1987). Spawning: Two major spawning migrations have been identified by FWS in the Green River basin by tracking radiotagged Colorado squawfish. One migration was discovered in the Yampa and upper Green River in 1981 (Tyus and McAda 1984), confirmed in 1982 (Wick et al. 1983) and again from 1983-1987 (this report, Tyus et al. 1987). Movement patterns of fish migrating to the Yampa River spawning ground are presented in Figure 5. In May and early June, 1981- 1987, Colorado squawfish initiated downstream migrations in the Yampa River 9