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larvae and juveniles of all four endan- gered Colorado River fishes feed on zoo- plankton (Miller et aI., 1982) and early instars of benthic invertebrates (Muth et aI., 1998). Floodplain habitats are es- - '$. peciaIly important to razorback sucker -- 60 larvae because this species spawns on -' the ascending limb of the hydrograph ~ during the spring when small food or- ~ 40 ganisms are scarce in main channels ~ and backwaters of Upper Basin rivers CJ) (Wydoski and Wick, 1998). Therefore, emphasis is being placed on razorback sucker in recovery efforts,. but all four endangered fishes will benefit from food production in floodplain habitats. Razorback sucker larvae are 7-9 mm total length (TL) at hatching and 9-11 rom TL at swimup (Muth et al., 1998). The density of zooplankton required for larval razorback sucker survival is between 30 and 60 organisms per fish per day (Figure 9.3) (Papoulias and Minckley, 1990). This number of organisms occurred in floodplain habitats in the Upper Basin but rarely reached that density in backwaters and never reached it in the rivers (Cooper and Severn, 1994a, 1994b, 1994c, and 1994d; Grabowski and Hiebert, 1989; Mabey and Schiozawa, 1993). Thus, floodplain habitats, which produce the . highest densities of zooplankton (particularly cladocerans and copepods that are readily eaten), meet the food requirements of razorback suckers during the early life stages in the Upper Basin (Table 9.7). As larval fish become juveniles, larger food organisms become more important. Chironomids constitute a significant part of the diets of juvenile endangered Colorado River fishes (Bestgen, 1990; Grabowski and Hiebert, 1989; U.S. Fish and Wildlife Service, 1990a, 1990b, 1991). The juvenile endangered fishes inhabit low-velocity habitats such as embayments and backwaters (Figure 9.4) along the main channel where the substrate is composed of silt, detritus, and sand - substrates that are generally inhabited by chironomids (Hynes, 1970). The numbers of chironomids in the main channel of the middle Green River ranged between 3500 and 4200 organisms per m2, between 2300 and 8100 for an ephemeral side channel, and between 9000 and 31,100 for two backwaters (Wolz and Schiozawa, 1995). Floodplain habi- tats that are inundated for only a short time may contain lower densities Case Review 253 100 00 80 20 o (a) 6 12 58 115 577 1154 NUMBER OF ORGANISMS PER FISH PER DAY 30 E25 E ...... ~ 20 G Z w -' 15 -' ~ ~ 10 z <: ~ 5 25 o (b) 6 12 58 115 577 1154 NUMBER OF ORGANISMS PER FISH PER DAY Figure 9.3. Relation of survival (a) and growth (b) of razorback sucker larvae to the density of food organisms after 50 days from swimup in a laboratory. (Adapted from Table 4 of Papoulias and Minckley 1992 [survival] and Table 4 of Papoulias and Mi nckley 1990 [growth].)