8022

• UCREFRP Catalog Number: 8022
• Author: Muth, R. T., et al.
• Title: Reproduction and Early Life History of Razorback Sucker in the Green River, Utah and Colorado, 1992-1996.
• USFW Year: 1998.
• USFW - Doc Type: 34,
• Copyright Material: NO

discharge associated with the beginning of spring runoff (Figures 4 and 5). Early reproduction in
the middle and lower Green River in 1994 was related to a sharp rise in the increasing spring
discharge during late April; maximum mean daily discharge of that flow event was about 70% of
the peak runoff discharge in each river section.

Growth of Larvae

Mean and maximum TL of razorback suckers in collections from the middle or lower
Green River generally increased as sampling progressed each year, but individuals
11-12 mm TL predominated throughout the season (Figures 2 and 3). Approximately 20% of all
razorback suckers captured were larger than 12 mm TL, and the two largest specimens were 20
and 24 mm TL. Estimated mean daily growth (Table 6) of larvae collected from either river
section was lowest in 1994 (0.31 and 0.27 mm TLld for the middle and lower Green River,
respectively) and greatest in 1996 (0.35 and 0.33 mm TLld). In each year, larvae from the middle
Green River grew faster than those from the lower Green River, and differences in mean growth
rates of larvae between the river sections ranged from 6% in 1996 to 21 % in 1995.

Diet of Larvae

The diet of larval razorback suckers 11-18 mm TL in nursery habitats was comparable
among years and between river sections. Because no major or consistent annual differences in
diet were observed within either river section for fish of similar size, data were combined over all
years. For both the middle and lower Green River, the percentage of razorback suckers with food
in their digestive tracts out of the total number examined per TL interval and the mean percent
fullness of digestive tracts increased as fish length increased (Tables 7 and 8). Digestive tracts of
all fish larger than 13 mm TL contained food and averaged more than 50% full. . Principal dietary
components were early instar chironomid larvae, small cladocerans, rotifers, algae, and organic
and inorganic debris, but the relative importance of these food categories varied with fish length.
Although chironomids were predominant in guts across all TL intervals, their proportional
contribution to the diet generally increased or remained high with increasing fish length.

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