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

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the Yampa River in 1975,1981,1988, and 1989 was believed to occur at discharges ranging
from 70 to 400 m3/s (estimated from graphs) and water temperatures averaging 150C (Tyus and
Karp 1989). The observed wide ranges in discharges and water temperatures during spawning
suggest that the reproductive strategy of razorback sucker is adapted to a variable and fluctuating
environment.
Mean catch rates of razorback sucker larvae, which may be an index of annual larval
production, were highly variable among years and reaches, Temporal or spatial variations in
catch data were expected because of inherent variability in biological and physical processes.
However, we are uncertain if our CPUE estimates were true indicators of population abundance
or if they were biased by differences in sampling efficiency, For example, annual CPUE for
light-trap collections in the middle Green River was lowest in the wet year of 1995 and highest in
the dry year of 1994, but sampling in 1995 was probably comparatively less effective due to
greater availability of flooded quiet-water habitats and wide dispersal of larvae. Wick (1997)
hypothesized that the low catch rate in 1995 was indicative of poor reproductive success due to
increased sedimentation of cobble and gravel substrates at the Escalante spawning bar. He
associated increased sedimentation with unnaturally high releases from Flaming Gorge Dam too
early in the spring-runoff period. Snyder and Meismer (1997) reported that 13-51% of razorback
sucker larvae or early juveniles confined within 0.5 m of light traps in laboratory experiments
entered those traps within 1 h. However, they suggested that the usefulness of light traps to
approximate the natural density or abundance of fish early life stages should be addressed
through additional experiments designed to be more representative of natural conditions.
Regardless of observed annual variations in CPUE, whether real or sampling artifact, razorback
sucker larvae apparently disappeared from Green River nursery habitats by early or mid-July each
year, suggesting extremely low survival. Furthermore, only six early juvenile razorback suckers
have been collected from the Green River within recent years (Gutermuth et al. 1994; Utah
Division of Wildlife Resources, unpublished data). Thus, recruitment sufficient to sustain wild
populations may not be occurring.

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