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found in the fish trap on certain days. Most of these fish were bluehead sucker <br />and flannelmouth sucker. On most days preceding and following these events, no <br />or few fish would be found in the fish trap or passageway. To demonstrate this, <br />the total number of three of the most common native fishes (bluehead sucker, <br />flannelmouth sucker, and roundtail chub) collected in the fish trap per day was <br />plotted against the mean daily discharge for each of the five years of operation. <br />Weekdays and weekends when the fish trap was not run or when the trap was checked <br />but fish were not enumerated were excluded. There were a number of these <br />instances in the five years of operation when large numbers (> 300 fish) of <br />native fishes migrated at the same time through the passageway. In 1996, 11 such <br />"spike-like" events were recorded, in 1997, 17, in 1998, 8, in 1999, 5, and in <br />2000, 5 (Figure 7). The highest single daily count of these three native fishes <br />in the fish trap was 1,179 on 21 April 1998 (Figure 7). The frequency that these <br />three native species exceeded 100 or more fish in the trap was: 1996 (24), 1997 <br />(38), 1998 (17), 1999 (22), and 2000 (14). Several occasions occurred when no <br />fish were found in the fish trap: 1996 (35 times), 1997 (27), 1998 (35), 1999 <br />(31), and 2000 (10). <br />It is unclear what compelled these large numbers of fish to migrate at one <br />time. One hypothesis is that, on some occasions, during the late-spring and <br />early-summer, the en masse movement of such large numbers of bluehead and <br />flannelmouth suckers may have been associated with an upstream spawning <br />migration. However, large numbers of these same two species were documented in <br />daily catches of the fish trap during the non-spawning period, also. While there <br />is no evidence when such large numbers of fish migrated simultaneously, fish <br />stocks were probably depleted downstream of the diversion dam. Accordingly, the <br />void created immediately downstream of the diversion dam and in the plunge pool <br />when fish migrated through the passageway, allowed fish located further <br />downstream in the Lower Gunnison River reach to move upstream to the dam, <br />replenishing fish that had migrated through the passageway. It is unknown how <br />long this process might have taken; it may have taken several days for this <br />replenishment to occur. <br />High Post-Runoff Discharge vs. Passageway Use <br />Between 31 July and 9 October 1999, Gunnison River flows recorded at the <br />USGS gaging station at Whitewater were higher than flows during the same period <br />in 1996, 1997, 1998, or 2000 (Figure 3). Much of this was attributed to the <br />unexpected earlier-than-normal rainfall that started in mid-July, rather than <br />mid-August in the region and continued into early-September. Concurrent with <br />this rainfall, Federal reservoirs upstream were maintaining water storage higher <br />than usual, anticipating high demands for water downstream from users during <br />28