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r ' w INTRODUCTION The Colorado squawfish (Ptychocheilus lucius) is the largest cyprinid native to North America. It is endemic to the Colorado River basin and historically was widely distributed in the Colorado River and its major tributaries. Following water developments on the Colorado River system, (Nicola 1979) squawfish populations declined and the species is now listed by the U. S. Fish and Wildlife Service as endangered (USFWS, 1974). One of the many facets of the recovery effort has been the collection of larval squawfish to determine distribution and abundance in the Upper Colorado River basin. These data have also been used with other 1 information to determine spawning date, location, and success (Miller et al., 1984; Wick 1983). For example, in the Colorado River from its confluence with the Green River to the confluence of the Gunnison River, no larvae were collected in 1981, but made up as much as 30% of collections in 1982 and 1983. In the Yampa-River, collection of 12-larvae in 1982 and 104 in 1983 in the lower Yampa River helped locate a possible spawning site. Fish were collected in late August both years indicating that spawning took place about August 10. Although squawfish larvae made up as much as 10% of the Yampa River samples in 1983, they were scarce (n=7) in the Green River samples the same year. High flows limited sampling efficiency and backwater habitat, but speculations that the 1983 year class was below normal were later confirmed when numbers of fingerlings from the Green River were below those expected in the fall (Archer et al., 1984). Surveys from 1979 to 1982 had indicated that the Green River below Jensen, Utah was good nursery habitat for Age-0 squawfish that were normally found throughout the river at catch rates of about 3 fish /10 m2. However, in 1983, no fish were found at about half of