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I I I I I I I I I I I I I I I I I I I from field readings taken during this, investigation. Flow records are from routing models from flow gauges at Green River (for Green River) and Cisco (for Colorado River), NOAA Colorado Basin River Forecast Center. Further analyses are needed to determine if spawing time of Colorado squawfish in the Cataract Canyon Region is established be water temperature, flow, or both. The histograms in Figure 3 indicate that spawning occurs during an increase in water temperaturres and a decrease in flows. It is hypothesized that spawning time may be established by occumulated temperture degree days, but that analysis has not been Operformed on these dates. Also, a relationship between temperature, flow and spawning time may not exist for ~{aY captured in the Cataract Canyon Region, because these fish probalby originate from dif- ferent areas and spawning events that may be influenced by envi ronmental factors other than those recorded in Cataract Canyon. HL Survival of Year Classes. Kaeding et al. (1985) hypothesized that squawfish in the Colorado River, hatched relatively late in the summer, enter the winter smaller and less able to survive into the following spring. The analysis of hatching dates, as presented in Figure 3, combined with lengths and CPE for yay in fall compared to the following spring before runoff ,provides data to partly test this hypothesis. Unfortunately the current investigation is not scheduled to collect much data on these fish in the spring, and so, CPE and lengths of the previous year class are based on sampling in late June and July. As the first phase of this analysis, we established the peak hatching dates from back-calculations presented in Figure 3. Although hatching dates for the three years examined generally occurred over a two-month period, the peak of hatching occurred in early July in 1985, in early August in 1986, and in late July in 1987. The second phase of this analysis deals with the relative lengths of the yay. A comparison of average lengths of yay captured during comparable sample dates in 1985, 1986, and 1987 (Table 21, Figure 4), shows that the individuals of the 1985 year class (hatched early) were larger than those of the 1986 year class (hatched late). yay collected during comparable trips 3, 4, and 5 showed an increasing difference in average lengths of up to 16.5 rom. The 1985 sample for trip 6 (n=8) is considered too small to provide reliable average. This comparison suggests that the progeny of the 1985 year class entered the winter as larger fish than the 1986 year class. A comparison of the 1985 and 1986 year classes with the 1987 year class shows a similar correlation between average length of yay and relative hatching time. The peak hatching time in 1987 was intermediate between the two previous years classes, as were the average lengths of yay. Since the sample dates differed slightly for the three years, average lengths of yay are plotted in Figure 4. The third phase of this hypothesis deals with CPE of yay in the early summer of the year after hatching (Le., the fish are 10-11 months of age). Ideally, this sampling should be conducted in the pre-runoff period, during March-April (Le., the fish would be 7-8 months of age) when the yay are using the rela- tively warmer backwater before these habitats are inundated by runoff. 21