8233 - Laserfiche WebLink

Home Browse Search

The historical analysis of YOY pikeminnow total lengths in relation to the suspected spawning bar at Millard Canyon (RK 53.9) failed to reveal conclusive evidence of a shift in size in the lower Green River. Young of the year pikeminnow collected in the lower 53 km of the Green River during fall ISMP have been both significantly larger and smaller than YOY collected upstream depending on the yeaz. The significant differences in size typically were recorded during low flow years. During the low flow years, conditions are better for YOY growth, and the overall size of the cohort is more variable in September. When average size of YOY pikeminnow (1986-1996) was analyzed longitudinally (six consecutive 32 km sections) throughout Reach 3 (RK 193-0.0), it became appazent that the size of individuals was never consistent from Green River, Utah, to the confluence. Clusters of large YOY (relative to the fish collected in the other 32 km sections within a given year) were found randomly from yeaz to yeaz. In high water years (1986, 1993, and 1995), YOY grew less overall and there were not significant differences in the size of pikeminnow collected above vs. below Millard Canyon. A plausible interpretation of this analysis might be that pikeminnow reproduce successfully at Millard Canyon only during low flow years. However, based on the variability in size of YOY seen throughout the lower Green River during low flow years this data set would not support that claim. Since this study, eight Colorado pikeminnow larvae (7.8-10.2 mm TL) were collected near Bonita Bend (RK 50) between 10 June and 25 July 1996. Bestgen et al. (1998) concluded that the reproductive window for Colorado pikeminnow in 1996 for the lower Green River (as determined by sampling below the Three Fords spawning azea) extended from 1 S June to 26 July. 'The time of capture, the size of these larvae, and their capture location again suggests that spawning could have occurred somewhere in the lower Green River that year. The indications of spawning occurrences for razorback sucker and to a lesser extent Colorado pikeminnow in the lower Green River persist. Larval razorbacks collected in this study were likely produced in the lower Green River rather than transported there from upstream areas. Lack of adult captures was indicative of extremely small reproductive groups, which may have been exacerbated by sampling inefficiency. Based on the combined adult and larval data sets we assume there is a small group of razorback suckers spawning in the lower Green River near the confluence with the San Rafael River. Similarly, if pikeminnow spawn near Millard Canyon it represents a small, and perhaps dwindling component of that species production in the lower Green River. 9