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YOUNG-OF-YEAR COLORADO SQUAWFISH The YOY Colorado Squawfish Monitoring Program was designed to systematically sample reaches known as important habitat for YOY Colorado squawfish. In this report, YOY refers to age- 0 juveniles (definition according to Snyder 1976) completing their first growing season. Previous researchers (1'yus et al. 1982; Valdez et al. 1982; Archer et al. 1985; Jones and Tyus 1985) had identified two river reaches in each of the Green and Colorado rivers where most YOY Colorado squawfish were found. Although YOY Colorado squawfish were found in other areas, these four reaches contained most YOY Colorado squawfish collected from the two rivers. These important areas were between river miles (RM) 0 to 120 and 200 to 319 in the Green River and between RM 0 to 110 and 140 to 170 in the Colorado River (RM 0 =the confluence of the Green and Colorado rivers; Figure 1). These investigators had also shown that the majority of YOY Colorado squawfish were collected in backwaters or embayments-zero-velocity habitats along sand bars, islands, or the main shoreline. Fish utilized other shallow, quiet-water habitats, but were most consistently collected in backwaters or embayments (hereafter collectively referred to as backwaters)-Tyus and Haines (1991) captured about 16% of the young Colorado squawfish they handled over a 3-year period from habitats other than backwaters (i.e. shorelines, side channels, runs, and eddies). The higher catch rate in backwaters may be related, in part, to higher sampling efficiency in this type of habitat than others, but nonetheless reflects a preference for backwaters by YOY Colorado squawfish. The Service began a systematic inventory of the relative abundance of YOY Colorado squawfish in the Green River in 1979 (Tyus et al. 1982) and in the Colorado River in 1982 (Archer et al. 1985). This inventory was intended to provide an annual index of the relative reproductive success of Colorado squawfish and survival of the young fish through their first growing season (hereafter collectively referred to as reproductive success). Investigators began at the upper end of occupied reaches and proceeded downstream, sampling the first backwater they located in 5-mile sections within each reach. Early investigations also included seining of habitats adjacent to the sampled backwaters. However, the program was soon standardized to sample only backwaters because it was assumed that changes in numbers within backwaters also reflected changes in other habitats. The Biological Subcommittee used this annual inventory as the basis- for the YOY Colorado squawfish portion of ISMP. Sampling Reaches Four important nursery areas in the Green (two reaches totaling 239 miles) and Colorado (two reaches totaling 140 miles) rivers are sampled during the YOY Colorado squawfish portion of ISMP (Figure 1). These four reaches compose about 70% of the known distribution of YOY Colorado squawfish in the Green River and about 80% in the Colorado River. The following information briefly summarizes the four sampling reaches. Colorado River Reach 1-This 110-mile reach runs from Fish Ford (RM 110) downstream to the confluence of the Green and Colorado rivers (RM 0). There are two general habitat types within this reach. The upper 45 miles of the reach are higher gradient (mean drop in elevation of 4.2 ft/mi) with considerable gravel and cobble substrates. Many of the backwaters are side channels at higher flows. The lower 65 miles-beginning near Moab, Utah-have a low gradient (mean, 1.1 ft/mi) with