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Table 3. Comparison ofback-calculated post-hatch ages of larval pikeminnow using formulas developed by Haynes et al. (1985) and Muth (1990) for fish less than 22 mm. Ages based on average Total Length (mm). Total Length (mm) AGE (days) YEAR AVG (range) Westwater Moab Westwater Moab Haynes Muth Haynes Muth 1992 9.0 (11.0- 7.0) 9.0 (11.2- 5.5) 11.3 9.9 11.3 9.9 1993 8.5 (10.3- 6.9) 8.8 (10.3- 7.0) 9.2 8.4 10.5 9.3 1994 9.0 (10.0- 7.0) 10 (18.0- 8.0) 11.3 9.9 13.4 12.1 1995 9.2 (10.0- 6.0) 9.0 (11.0- 6.0) 12.2 10.4 11.3 9.9 1996 9.1 (12.0- 7.0) 9.1 (11.0- 6.0) 11.4 10.2 11.4 10.2 Each year, the distribution of collection dates revealed a series of peaks in drift density (#fish/hour), which were frequently associated with slight increases in flow, particularly in 1992 (Figure 4a-e). The distribution of estimated spawning dates also revealed a series of peaks which are similar to and offset from the collection dates by a number of days which varied yearly and between sites (Figure 6a-e). If larvae collected in Moab represented fish spawned above Westwater, and passively drifting past both sites, then collection peaks at both sites would have been identical, although temporally offset at Moab. Current velocities at the Cisco gage vary between 8 km/hour during peak flows and 3.2 km/hour during base flows (CRBRFC, 1997). At those velocities, passively drifting larvae would take 12 to 31 hours to be transported between the two sites, resulting in an approximate one day difference in collection peaks. However, collection peaks were not always identical between Westwater and Moab, or offset by one day, suggesting that although larvae are probably transported past Westwater and Moab, some additional reproduction is also occurring below Westwater. This supports the actual presence of the suspected spawning site between the two sampling sites. Collection and spawning peaks varied independently between years and sites, although there was much similarity. In 1992, both sites had four peaks in larval collection and estimated spawning dates. The collection peaks were offset from spawning peaks by 16 to 17 days. In 1993, both sites had 2 peaks, which were offset by 8 to 10 days at Westwater and 15 days at Moab. In 1994, there were three peaks at each site, offset by 19 days each. In 1995, there was one minor and one major peak at Moab, both offset by 15 days. At Westwater few fish were collected, and the collections were offset from spawning by 18 days. In 1996, there were two peaks at each site, offset by 15 days. The peaks in collection and estimated spawning could be a result of different groups of spawning fish, or the same individuals undergoing sustained periods of ripeness punctuated by activity, or larvae produced at different sites being collected at different times, or combinations of the above. In any case, the drift stations did not appear to be sampling the same populations of drifting larvae, although some overlap likely occurred. 7