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1 I'. lJ 1969). Mabey and Schiozawa (1993) reported that the most comprehensive studies have been made of the plankton communities in the Orinoco River, Venezuela. Mean densitites of cladocerans and copepods (the most abundant taxa) were 421 organisms per liter in the Laguna la Orsinera. Welcomme (1989) summarized zooplankton densities in floodplains in a range of 270 to 10,000 organims per liter. Mabey and Shiozawa (1993) documented zooplankton densities in the middle Green River as 0.3 to 1.3 organisms per liter, 1.5 to 7.1 in the Ouray backwater, 63.4 at Intersection Wash (another backwater), and 206 to 690 in Old Charlie Wash (Woods Bottom) on the Ouray National Wildlife Refuge, located downstream from Vernal, Utah. Grabowski and Hiebert (1989) reported 0 to 20 planktonic crustaceans (cladocerans and copepods) per liter in the middle Green River channel and 0.02 to 17 organisms per liter in backwaters during 1987 and 1988. In an open water habitat of the Moab Slough, immediately upstream on the Colorado River from Moab, Utah, the density of planktonic crustaceans (cladocerans and copepods) averaged about 36 organisms per liter during the summer of 1993 (Cooper and Severn 1994a). Cooper and Severn reported a mean of 2 organisms per liter for backwater sites and a mean of 36 organisms per liter for open water wetlands. Samples of planktonic crustaceans (cladocerans and copepods) from the Escalante Ranch site on the middle Green River, upstream from Jensen, Utah, contained 0 organisms per liter for the main channel, a mean of 41 organisms per liter for backwaters, and a mean of 71 organisms per liter for an open water wetland (Cooper and Severn 1994b). The mean number of cladocerans and copepods from a backwater of the Gunnison River at the Escalante State Wildlife Area, about 5 miles downstream from Delta, Colorado, was 11 organisms per liter of water and the mean number from an open water wetland was 25 organisms per liter (Cooper and Severn 1994c). Zooplankton (cladocerans and copepods) samples from a bottomland (Old Charlie Wash) on the Ouray National Wildlife Refuge contained a mean of 31 organisms per liter of water (Cooper and Severn 1994d). Samples taken from the middle Green River and a backwater on the refuge did not contain any cladocerans or copepods. C. Major Limiting Factor Affecting Endangered Colorado River Fish Populations. The major limiting factor affecting the populations of endangered Colorado River fishes is the lack of successful recruitment (U.S. Fish and Wildlife Service 1987, 1990a, 1990b, 1991). In the Lower Colorado River Basin, studies have demonstrated that predation by non-native fishes and the lack of food may be limiting the survival of larval and juvenile endangered fishes (Marsh and Langhorst 1988; Papoulias and Minckley 1990, Papoulias and Minckley 1992). Similar conclusions have been reached for the endangered Colorado River fishes in the upper basin (Miller et al. 1982; Maddux et al. 1993). 26