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UD3J95 (mid April-mid June) for mark-recapture population estimates: this task was met. Background.-- In 1996, a report was completed by Osmundson and Burnham (later published in 1998 in Transactions of the American Fisheries Society 127:957-970) outlining the current status and trend of the Colorado pikeminnow population in the Colorado River mainstem. The authors used a four-year mark-recapture effort to estimate the size of the subadult and adult population. Strong year classes in 1985 and 1986 recruited to the adult population during their 1991-1994 study, allowing the authors to document the large effect that a couple of strong year classes can have on adult population size. In the first year of the study (1991) most adults were found concentrated in the upper reach of the river (upstream of Westwater Canyon) and this population consisted of only about 200 individuals. By 1994, catch rates there had doubled. The 1994 point estimate for the upper reach was about 330 fish; though not double, it was substantially higher than that estimated three years prior. Also, an additional 300 or so young or soon-to-be adult fish were estimated to reside in the lower reach (downstream of Westwater Canyon). Thus, in four years the river-wide population of individuals > 450 mm TLincreased from somewhere around 200-250 fish to around 465 fish. - In a 1997 report (later published in 1998 in Transactions of the American Fisheries Society 127:943-956), Osmundson et al. documented the dispersal of these young, recruiting adults in the lower reach and showed that many of these young fish moved to the upper reach. Based on body condition in the lower reach that declined as the fish grew and later improved upon arrival in the upper reach, along with differences in forage between the two reaches, the authors concluded that these upstream movements were related to an inadequate supply offood for adult pikeminnow in the lower reach. New informationfrom 1998-2000. - In 1998, a second mark-recapture study was begun to monitor the status of this dynamic population - this time a three-year effort was conducted instead ofa four-year effort. In 1998-2000, the same protocol was used as before: three passes, or capture efforts, were made through the upper reach and two passes through the lower reach. With each pass, trammel-netting of backwaters and flooded canyon mouths was used to capture fish during the run-off period. Shoreline electrofishing was also used during periods when backwaters were not sufficiently flooded. Utah DWR and Colorado DOW captures during the sampling periods were included. Captures for the three years were as follows: Pass Upper Reach Lower Reach '" 1998 1 32 31 2 67 65 3 . 43 - 22-a-2-2