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24 I I I I I I I I I I I I I I I I I I I which suggests some spawning taking place below Westwater. Similar annual trends in Colorado pikeminnow drift and higher densities at Moab than Loma suggest that larvae produced in Colorado could be contributing to larval numbers sampled in Moab. However, since the Westwater data do not support a progressively increasing downstream drift and since there are opposing results between study sites in 1996, the significance of larvae produced in Colorado are not strongly apparent. Tyus and Haines (1991) reported it took about 6 days for the transport of newly emerged ( sac fry) Colorado pikeminnow from the spawning grounds (RK 26.4 to 29.1) to the mouth of the Yampa (RK 0). Eventually young Colorado pikeminnow were observed about 160 km downstream in the Green River and were believed to have originated from the Yampa River spawning reach (Tyus and Karp 1991). In the Colorado River it appears that spawning is more wide spread than in the Yampa River. Since spawning begins earlier in downstream reaches, the source of larvae sampled in the Moab reach could have originated from fish that spawned a relatively short distance upstream to spawning sites located 160 km upstream in Colorado. Colorado Pikeminnow YOY Size and Survival (Obiective 2) The second study objective was to assess the impacts of flow on the growing season using size of Colorado pikeminnow young-of-year as a qualitative indicator. Presumably larger young-of-year fish are more fit for over-winter survivorship and larger size fry result in years that have more benevolent conditions for growth. The type of data generated by sampling fish larvae is not suited for such an analysis, since all larvae sampled were less than 12 nun, about 15 days post hatch. Mean size of Colorado pikeminnow larvae in the drift (Loma, Colorado) was similar (9.3 and 9.5 mm) between years, but the date larvae were first collected and the median collection date were variable between years and apparently related to water temperatures. Mean length of yay in summer backwater samples was highly variable between years and negatively related to water temperatures with greater size in low runoff years and reduced size in high years. The year with the smallest mean was in the year with the slowest warming (1995) and the year with the largest mean YOY size in summer backwaters was in the year with the earliest warming (1994). Colorado pikeminnow larvae were sampled in backwater habitats at sizes less than 11 mm, but were not sampled in the drift at lengths over 11 mm. The fact that pikeminnow larvae over 11 rom were not collected by drift nets strongly indicates that either this sized larval were not vulnerable to the drift net sampling gear or they abandon the drift to begin to use or reside in backwater habitats at that size. If larvae over 11 mm are drifting but are not being sampled, transport abundance would be underestimated at downstream sites. Otherwise the data indicates that larvae are quickly transported downstream ofLoma. The best year (1994) for over-winter survival rate for Colorado pikeminnowwas also the best year for NNC over-winter survival. The four years of spring NNC and