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Table 10. Average non-native cyprinid (NNC) densities (number/100 m3) from the upper and lower stations on Colorado River (Trammell and Chart, 1998). Year Class RUNOFF SUIVIMER #/100 m3 FALL #/100 m3 SPRING #/100 m3 1993 HIGH 943 2473 576- 1994 1995 HIGH 1075 1829 548- 1996 1992 LOW 1866 4373 1748- 1993 1994 LOW 4065 7926 6115- 1995 1996 INTER. 3570 12375 NoData - 1997 Mean Total 2304 5795 2247 Mean HIGH 1009 2151 562 Mean LOW 2965 6149 3931 INTER. 3507 12375 NoData Non-native cyprinid density estimates varied seasonally. Densities peaked in the fall indicating recruitment of young fish during the summer period. Densities were reduced in the spring indicating over-winter mortality (Table 10). In the two high flow years (1993 and 1995) summer (post-runoff) densities were less than in the spring, presumably because the runoff flooded preferred habitats (backwaters) which reduced habitat availability during high flows. In the other three years (1992, 1994 and 1996) NNC density increased in the summer indicating peak flows of those years did not negatively impact habitat availability and/or temperatures suitable for reproduction were reached sooner. The reduced NNC density in 1993 and 1995 is therefore attributed to runoff conditions. Higher spring than fall NNC density in 1994 (low year) indicates there can be a large range in variability between years. Since data was not collected in the spring of 1997, it is not known how over-winter mortality would have affected year-class strength of the abundant NNC population identified in the 1996 fall. There was a negative correlation between larval Colorado pikeminnow drift density estimates at Loma, Colorado and the average densities for YOY Colorado pikeminnow summer and fall in Utah backwaters (Trammell and Chart 1998). However, the relationship was very weak with the coefficient of determination (RZ) of 0.20. Again the data suggests that year-class strength determined in Moab was not related to reproductive success as measured by number of larvae in the drift at Loma. The reason the relationship was negative was due to the fact that the best year for larval abundance was the worst years of backwater abundance (1995) and the worst year for larval abundance was the best year for backwater abundance (1996). The 1995 results may be explained by the theory that high summer flows stimulated increase spawning efforts and larval production. However, Colorado pikeminnow larval size was small and survival poor in 1995 because of a short growing season. Therefore even though many larvae were produced, few survived. However the 1996 data does not lend itself to flow related explanations. 1996 had the lowest total larvae production including Colorado pikeminnow, other native and non-native species at Loma. This indicates either reduced spawning (adult numbers) or larval survival for all species. In contrast, 1996 was the best year for both YOY Colorado pikeminnow and NNC 19