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<br />I <br /> <br />I <br />I <br /> <br />of time with the result that larvae, surv~v1ng starvation, are apparently <br />completely consumed by piscivorous fishes in Lake Mohave (Horn 1996; <br />Minckley et al. 1991). Shifts in survival during early life stages of <br />fish populations most often result in a decline of populations in altered <br />aquatic habitats (Houde 1987; Lasker 1981; Lawler 1965). <br /> <br />I <br /> <br />The average size of 1,735 razorback sucker larvae with a mean age of 12-17 <br />days from the middle Green River was 12 mm TL (range 8 - 24) and the <br />average size of 440 larvae from the lower Green River was 13 mm TL with a <br />range between 10 and 20 mm (Muth et al. 1998). Muth et al. reported that <br />only 20% of the razorback larvae collected from the Green River, Utah were <br />larger than 12 mm TL with the two largest larvae at 20 and 24 mm TL. <br /> <br />I <br /> <br />I <br /> <br />Larger, faster-growing razorback sucker larvae are more likely to be <br />captured in light traps because they are more mobile but may not be <br />representative of the actual size structure among all larvae (Horn 1996) . <br />Razorback sucker larvae are 7-9 mm TL at hatching and 9-11 mm at swimup <br />(Muth et al. 1998). Most razorback sucker larvae captured in Lake Mohave <br />were less than 20 days old and averaged 11.4 mm TL while larvae reared in <br />the laboratory reached a length of about 15 mm in 20 days at 18 C and <br />nearly 16 mm in 20 days at 14 C (Horn 1996). Horn's estimates of <br />mortality from starvation in Lake Mohave were between 23% and 78% between <br />1992 and 1995, depending upon the year of capture and nutritional index <br />used. Therefore, most razorback sucker larvae collected in both the Upper <br />and Lower Colorado River Basins are of a size (11-12 mm TL) when they are <br />just converting to exogenus feeding during their critical period when they <br />disappear from samples. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The larvae and juveniles of all endangered Colorado River fishes feed on <br />zooplankton (U.S. Fish and Wildlife Service 1987, 1990a, 1990b, 1991). <br />The first foods of larval razorback suckers in ponds were diatoms, <br />detritus, algae, and rotifers (Bestgen 1990; Papoulias and Minckley 1992) . <br />Soon afterward, razorback larvae begin to select larger zooplankton <br />organisms,primarily cladocerans. Marsh and Langhorst (1988) reported that <br />razorback sucker larvae, less than 21 mm TL, fed on rotifers, cladocerans, <br />and copepods in the open water of Lake Mohave and that the diet of <br />razorback larvae in backwaters included larval chironomids and <br />trichopterans. Older and larger razorback sucker larvae in Lake Mohave <br />occupied small inlets or bays that were shielded from wind and wave action <br />where a broader spectrum of small prey occur and are eaten by the larvae <br />(Horn 1996) . <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Muth et al. (1998) reported that small food items generally composed <br />between one-half to two-thirds (range 49-68%) of the total volume in the <br />digestive tracts of razorback sucker larvae between 11 and 14 mm TL from <br />the middle and lower Green River and that early instar chironomids <br />composed between 32 and 51% of the total volume. Most of these razorback <br />larvae were collected in light traps from shallow backwaters. The volume <br />of digestive tracts composed of chironomids in Muth et al. (1998) may be <br />an artifact because free-swimming invertebrates are attracted to light <br />traps (Ervin and Haines 1972) and chironomids are larger than zooplankton <br />so a few organisms would compose a large percentage of the total volume of <br />food. It is likely that razorback sucker larvae were opportunistic in <br />eating the early instar benthic organisms that were attracted to and <br />accumulated in light traps. Gradually, late larvae and early juveniles <br />feed on larger benthic organisms when they are available. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The diet of adult razorback suckers in reservoirs consisted largely of <br />zooplankton that they obtained from the water column (Marsh 1987) but, in <br />river habitats, the diet of adults consists of benthic materials, mainly <br />immature Chironomidae, Ephemeroptera, and Trichoptera along with algae and <br />detritus (Bestgen 1990). The density of zooplankton required for larval <br /> <br />I <br /> <br />I <br /> <br />19 <br /> <br />I <br />