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the latest on August 13 (1995) at Westwater. Larval pikeminnow were collected for three to five weeks each year. In all but 1994 and 1996, a few pikeminnow were still being found in the drift when the study period had ended (Figure 4a-e). In 1995 we felt a portion of the pikeminnow larvae may have been missed after sampling had ended. The final day of sampling had a high catch rate, and high catch rate days were usually followed by a few days of diminishing catch rates. These hypothetical fish may have been missed at the end of the sampling in 1995 (Figure 4d). Total transport abundance of Colorado pikeminnow larvae was estimated annually. Transport abundance was higher at Moab than Westwater in all years. Abundance in #fish/hour at Westwater varied from 8,836 in 1994 to 47,116 in 1993. At Moab, for the standard dawn sets, the lowest annual transport abundance was 124,741 fish/hour in 1994, and the highest was 858,174 fish/hour in 1996. At the Moab site in 1996, during the additional collecting period from 900 to 1100 hours, nearly the same number of pikeminnow were collected as the standard set. This resulted in a higher catch rate since fewer days were sampled, making this the highest estimated annual transport abundance at 890,231 fish/hour. A summary of the sampling periods and collection data is given in Table 2. Total lengths of larval Colorado pikeminnow ranged from 5.5 mm to 18 mm, with the majority between 8 and 11 mm (Figure 5). Most of the lazger larvae were collected in 1994. Estimated spawning dates were calculated for each fish using the formula by Haynes' et al. (1985). The larvae collected were generally the same range of size and age throughout the drift season, although they tended to decrease slightly in size and therefore estimated age towazds the end of the collection period. The formulas used to back-calculate spawning and hatch dates carry assumptions which may bias the results (Haynes et al. 1985; Muth 1990; Bestgen et al. 1998; Bestgen and Williams 1994). All pikeminnow aze assumed to be 6.7 mm at hatch, and all are assumed to hatch 6 days after the spawn. However, among hatchery raised pikeminnow, there is at least a 1-1.5 day variation about the mean incubation time of 5 days, and a range of 6-7.5 mm at hatch (Hamman 1981 as cited by Nesler et al. 1988). Wild fish can be assumed to have at least that much variation if not more. A comparison of lengths and ages using the two different methods is given in Table 3. For wild Colorado pikeminnow larvae spawned in the Yampa River, Bestgen et al. (1998) estimated an average age of 6 days post-hatch for fish with an average total length of 9 mm, by counting otolith increments. In comparison, Haynes' formula estimates a 9 mm fish as 11.3 days post-hatch, and Muth's as 9.9 days. Since Haynes' method was used for this study, the estimated spawning dates for the fish in this study maybe several days eazlier than if an otolith count had been done. However we feel that the estimated spawning dates still provide a valuable relative reference for the onset of spawning, as the bias will always be the same number of days. The estimated date of the eazliest spawning activity occurred within a 5 week range, with the eazliest on June 5, 1994 at the Moab site, and the latest on July 30, 1995 at the Westwater site. In four out of five yeazs, spawning was estimated to have commenced one day to three weeks earlier above the Moab site than the Westwater site. Spawning activity continued for 5 to 8 weeks. 5