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INTRODUCTION In the last four years, light trapping for razorback sucker larvae in backwater habitats on the lower Green River has produced tremendous variability in the spring of the year. Results of trapping performed by Ed Wick (National Park Service, Ft. Collins) and Bob Muth (C.S.U. Larval Fish Laboratory) in the spring of 1993 yielded catches as high as 100 larvae per light unit in a single night. In response to these numbers, the Utah Division of Wildlife, Department of Natural Resources (UDWDNR) in Moab began a two year investigation in the early spring of 1994. Their purpose was to see if razorback suckers were spawning at the mouth of the San Rafael river and to see if squawfish were spawning near the Millard Canyon backwater. Data from this study revealed catches of 50, 1 and 200 razorback larvae per light unit over a period of three nights. If there is in fact a breeding population of razorback suckers in the lower Green river, why are larval numbers so variable from year to year? A possible explanation is that the hydraulic alteration of the Green river has resulted in the loss of primary breeding and nursery habitat. These historic, seasonally inundated floodplain and wetlands habitats most likely supported a rich community of zooplankton, aquatic insects and plants and allowed larval suckers to thrive. When rising water levels obliterated these habitats, larval fish were forced to seek out secondary nursery habitats where food availability, competition and predation are likely more severe. The UDWDNR has identified three backwater habitats on the lower Green river to trap larval fish. These sites are distinct entities associated with either side channels or intermittent streams and are connected to the main river channel during periods of high flow. Because they are still water sites, they offer protection from the turbulent main river channel. However, it is unknown if the timing and abundance of food resources in these backwater habitats is adequate to sustain growth and survival of endangered razorback sucker larvae. There is no data in the literature which quantifies zooplankton densities in the lower or upper Green river during the critical period in high water when razorbacks are hatching. There has been a lot of research aimed at quantifying benthic invertebrate densities on both the upper and lower Green river (Grabowski and Hiebert 1987, Mabey 1993, Jordan 1996, Wolz and Shiozawa 1995), but only during periods oflow flows in the summer months. The objective of this study therefore, was to determine the density of planktonic and benthic invertebrates in three backwater habitats on the lower Green river in late spring and early summer when early life stages of razorback suckers were present. 1