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

lower Green River, our collections oflarvae and captures of adults in spring (e.g., Chart et al.
1997) at or immediately downstream of the mouth of the San Rafael River are suggestive of
localized spawning. Other possible recent evidence for razorback sucker reproduction in the
lower Green River includes the collection oftwo early juveniles (36,6 and 39.3 mm TL) from a
backwater at RK 89.5 on 30 July 1991 (Gutermuth et al. 1994) and the capture of 15 larvae
(13-16 mm TL), presumably produced in the Green River, from the Colorado River inflow to
Lake Powell on 22 June 1993 (Muth and Wick 1997). However, regardless of the source, the
occurrence of razorback sucker larvae in the lower Green River obligates resource agencies to
consider management actions for both the middle and lower Green River to enhance survival of
early life stages.
Razorback suckers in the Green River system reproduce in spring (April through June) at
increasing and highest runoff flows and warming water temperatures, as evidenced by our
observations on larvae and those of other investigators on adults (e.g., McAda and Wydoski
1980; Tyus 1987; Tyus and Karp 1989, 1990). In comparison, reproduction by razorback suckers
in the lower Colorado River basin generally occurs during January through April (Medel-Ulmer
1983; Minckley 1983; Langhorst and Marsh 1986; Mueller 1989) but may extend from
November into May (Bozek et al. 1991). Annual initiation of razorback sucker spawning in the
Green River during our investigation was probably triggered by a suite of interacting
environmental cues that could not be detected by our analysis of single water temperature and
discharge parameters. Modde and Wick (1997) concluded that initial movement of adult
razorback suckers to the Escalante spawning site was influenced primarily by increases in river
discharge and secondarily by increases in water temperature. Our estimated spawning periods in
the middle or lower Green River, respectively, encompassed a wide range of mainstem mean
daily discharges (78-623 m3/s or 134-696 m3/s) and instantaneous daily water temperatures
(8.0-19.50C or 10.0-21.00C). The predominance of razorback sucker larvae 11-12 mm TL in
our collections throughout the season each year suggests continuous spawning and larval
production. Tyus and Karp (1990) associated razorback sucker reproduction in the middle Green
River during the low-to-average runoff years of 1987-1989 with discharges of 150-250 m3/s
(estimated from graphs) and water temperatures of9-1 rc. Spawning by razorback suckers in

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