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4 Results Species composition and relative abundance A total of 674,261 fish composed of 17 different species were collected during the study period (species list, appendix, Table A2). Five species were native, the remainder were non-native. The most common species collected were three non-native cyprinids (NNC); red shiner (Cyprinella lutrensis), sand shiner (Notropis stramineous) and fathead minnow (Pimephales promelas) (Table 1). These three species comprised an average of 79.8% of the total number offish collected in the upper section, and 98.6% in the lower section. They were fairly evenly distributed, with red shiner most abundant in 1992 and 1993, and sand shiner most abundant in the last three years. Other non- native species were collected sporadically and none made up a significant proportion of the total except in the upper section, in the summer and fall of 1995. Mosquitofish (Gambusia a~nis) and green sunfish (Lepomis cyanellus) occurred in greater abundance, as a result of prolonged flooding of the Moab sloughs (Scott E. Matheson Wetlands Preserve), at RM 60 to 64. In.the lower section, Colorado squawfish were usually the most abundant of the native fishes. In the upper section, native fish relative abundance was more variable. Native fishes, mostly bluehead sucker (Catostomus discobolus), flannehnouth sucker (Catostomus latipinnis) and speckled dace (Rhinichthys osculus), reached their highest density in the summer of 1993 in both sections. Native fishes were most abundant in early summer, soon after reproduction occurred. Due to continued production of NNC throughout the summer, and mortality and habitat use changes of native fishes, the NNC soon regained complete dominance.