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Post-Larval Sampling A standardized sampling program for post-larval YOY Colorado squawfish has been carried out annually since 1978 in the Green River and in the 2 Colorado River since 1982. Between 8,400 and 13,900 m , have been sampled each year to provide the most reliable information available on yay Colorado squawfish abundance throughout the Upper Colorado River Basin. Some difficulties have been encountered which have increased variation in the data making the results less reliable for comparisons between rivers and years. Major environmental variables which have affected the data have been the unusually high discharges and cold wa ter temperatures which reduced the prevalence of backwaters, delayed spawning and slowed growth of the young Colorado squawfish. These circumstances probably reduced the catchability of the young Colorado squawfish by reducing their size, making them more difficult to catch in small seines and to identify macrospically. Also, with fewer backwaters they were probably more dispersed and less catchable. The data clearly suggests that the Green River, with its 555 Km (345 mi) of quality nursery habitat, represents the major area of Colorado squawfish production during most years. While mean catch rates may not be compared without some restraint they have to be considered reliable indicators of the relative importance of the different river reaches. Fran 1979-1982 catch rates in the Green River ranged between 1l8-227 post-larval Colorado 2 2 squawfish per 1000 m and 5.0-7.5 per 1000 m during the high flow years of 1913 and 1984 (Table 7). Colorado R.1ver poat-larval YOY catch rates have been less variable but considerably lower except for 1983 and 1984, 2 ranging between 18.75 and 50.65 post larval YOY per 1000 m . The past five year's data suggests that Colorado squawfish YOY production in the Green River has been good during the years 1979-1981 but very poor during 1983 and 1984. Catch rates were consistently and substantially lower throughout the study period on the Colorado suggesting that production of YOY in that river syst_ maybe a major problem. 25