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1. INTRODUCTION The Colorado River Fisheries Project, funded primarily by the Bureau of Reclamation, initiated a monitoring program for evaluating the reproduc- tive success of Colorado squawfish (Ptychocheilus lucius) in the Green River Basin in 1979. This program, which is part of a more comprehensive study has been expanded and continued through the present (FY 85). A major part of the program includes seiningl/ Age-O Colorado squawfish in their preferred habitat (shallow ephemeral shoreline embayments) in the fall (September and October). Catch data are .expressed as total catch- per-area seined, or catch-per-unit-effort (C/E) thus allowing'a comparison of relative abundance in this habitat between years and river strata (Figure 1). In addition, the relative success of Colorado squawfish spawning each year-was evaluated from research studies conducted by Colorado Division of Wildlife (Haynes et al. 1984, Haynes and Muth 1984) and FWS (Archer and Tyus 1984, Tyus et al. 1982). The conclusions reached in these combined studies form the basis of this report. ABUNDANCE, SIZE, AND DISTRIBUTION OF LARVAL COLORADO SQUAWFISH Colorado squawfish spawning areas and optimum nursery areas in the Green River Basin are provided in Figure 2. The two confirmed spawning sites identified to date are located in canyon areas characterized by deep pools and turbulent riffles. The two major.nursery areas are located in river reaches with an abundance of ephemeral backwater habitat. Spawning activity occurs at water temperatures of 20-240C. as reported by several investigators (Toney 1974, Hamman 1981, Haynes et al. 1984), when river discharge is decreasing. The length of.the_spawning period is reported as 2-2 2 weeks (Haynes and Muth 1984) although this can be altered greatly by the water temperature pattern present. Survival and percent- age hatch of Colorado squawfish eggs was reported highest at 2000 and egg mortality occurs within 48-60 hrs. at 150C (Marsh. and Pisano 1982). Spawning success at the Yampa River site was at least as high or higher in 1983 than the preceding two years, since C/E values for very young larvae presented by Haynes and Muth (1984) indicated an increase in numbers in 1983.- These data and.general flow characteristics of the respective years are presented in Table 1. Haynes and Muth (1984) suggested that spawning success may be enhanced by high flows, if accompanied by an accumulation of a. sufficient number of degree days. Radiotelemetry observation of Colorado squawfish at the spawning site (Archer and Tyus 1984) also indicated that spawning. in 1983 was at least as successful as the previous two years. l/In 1979 both 1/4 and 1/8 inch diamond mesh seines were used. A comparison t of sampling efficiency indicated no difference between the two mesh sizes.. After 1980 an Ace style 3/16 inch effective mesh size was used. In all cases a 4.5 m (15 ft) seine was used.