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n 1 1 1 a mean of only 2 organisms per liter for backwater sites. 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 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. 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 (USFWS 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). Relation of Food to Larval Fish Survival. Year-class strength of marine fisheries is often related to the Match or Mismatch Principle where environmental conditions such as water temperature as well as the quality and quantity of food organisms must match the hatching of larval fishes to realize high survival (Hjort 1914, 1926). In other words, the density, size, time and duration of availability of zooplankton must "match" the timing of the swim-up stage of fish larvae. When the yolk sac absorbs, food organisms of the right size must be available to fish larvae or they will starve. This principle was suggested by Marsh and Langhorst (1988) for wild razorback sucker larvae in Lake Mohave and documented for the razorback sucker larvae in the laboratory (Papoulias and Minckley 1990). Razorback sucker larvae of about 10 mm total length were maintained in the laboratory at 18°C. Unfed razorback larvae died in 10 to 30 days. Razorback larvae must find food of the right size and density in 8 to 19 days to survive. The "point of no return" when the fish died even though sufficient food of the right size was available was between 19 and 23 days for razorback sucker larvae. Papoulias and Minckley reported that the quantity of food required for survival of the razorback larvae was 20 brine shrimp nauplii per fish or high mortality occurred. Food Availability for Larval Endangered Fishes in the Upper Colorado River Basin. The larvae of all endangered Colorado River fishes feed on zooplankton early in life (USFWS 1987, 1990a, 1990b, 1991). The first foods of larval razorback suckers in ponds were diatoms, detritus, algae, and rotifers (Papoulias and Minckley 1992). Soon afterward, the razorback larvae began to select larger organisms such as zooplankton (primarily cladocerans). The density of zooplankton needed for larval razorback sucker survival (20 organisms per liter of water; Papoulias and Minckley 1990) occurred in flooded bottoml-and habitats along the Green River but rarely reached that density in backwaters, and never 12