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6 This procedure resulted in a sampling of far more sites per field trip than determined by the random selection process alone. Seine samples were taken with 3.0- x 1.2-m and 1.0- x 1.2-m seines (1.6-mm square mesh). Samples were collected according to a habitat stratification scheme designed to reflect the geomorphic, hydrologic, and ecologic variables of the study area. Habitat variables were described in detail in Haynes and Muth (1982). The width and length of each seine collection were measured with a metric tape and area sampled (m2) was calculated and recorded. Larger specimens, identifiable to species, were measured to the nearest 0.1 mm (total length, TL), counted, and released. Smaller specimens were preserved in 10% buffered formalin and returned to the Larval Fish Laboratory (Colorado State University, Fort Collins) for processing. An index of relative young-of-the-year (YOY) abundance (i.e., capture per unit effort, C/f) was developed for seine-collected YOY Colorado squawfish using basic tenets of Ricker (1975), Caughley (1978), Lackey and Hubert (1978), Southwood (1978), and Tanner (1978), i.e., that C/f is related in a constant or predictable way to population size and changes in absolute abundance over space and time will be reflected by changes in C/f. A discussion of the advantages and disadvantages of using C/f methods in open, structurally diverse systems such as the Upper Colorado River Basin was presented in Haynes and Muth (1984). C/f methods were selected in favor of other, more rigorous, abundance models because closure of the system was not possible. C/f values (numbers of YOY per 100 m2) were derived from seine samples of known area and specific habitat features. An evaluation of YOY Colorado squawfish distribution per specific habitats was made for 1981-1984 for each river and only those habitat types in which YOY had been previously collected were used for C/f calculations (i.e., backwaters, embayments, concavities, pools, isolated pools, and shallow shorelines). In this way, habitats such as riffles, which had never yielded YOY Colorado squawfish, were omitted from the calculations. Further, for any given year and river, samples taken prior to the earliest estimated spawning dates (derived from age estimates of YOY, presented later in Methods) were also excluded. In the Yampa River, only seine samples taken at or below river km 32.0 (the previously suspected upstream limit of Colorado squawfish spawning in the Yampa River) were included in C/f calculations. During July-August 1982, icythyoplankton drift-nets were deployed in the Black Rocks area, Colorado River, to determine if larval Colorado squawfish disperse via dOliUstream drift and to evaluate gear, sampling design, and methods of gear deployment. These initial trials proved successful and it was decided to extensively utilize drift-nets in the Yampa River (June-August 1983; July-August 1984) and Green River (Ju1y--August 1984). Drift-nets were deployed at two Yampa River sites in 1983 (i.e., Harding Hole, river km 32.5; and Box Elder, river km 3.1). The Harding Hole site was selected to evaluate the possibility that Colorado squawfish spawn above river km 32.0. The Box Elder site was chosen to assess larval fish drift patterns near the Yampa River mouth. In 1984 drift collections continued at Box Elder, while the Harding Hole site was dropped in favor of a Green River site approximately 9.3 km below the Yampa-Green River confluence (river km 355.3, Colorado-Utah state line). This site was included to follow progression of drifting fish larvae down the Green River.