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Last modified
7/14/2009 5:02:35 PM
Creation date
5/17/2009 10:51:31 PM
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UCREFRP
UCREFRP Catalog Number
9410
Author
Wydoski, R. S. and E. J. Wick.
Title
Ecological Value of Floodplain Habitats to Razorback Suckers in the Upper Colorado River Basin.
USFW Year
1998.
USFW - Doc Type
Denver.
Copyright Material
NO
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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 />
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