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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INTRODUCTION

The razorback sucker Xyrauchen texanus is endemic to the Colorado River basin (Miller
1959; Minckley et al. 1986) and was once widely distributed in warm-water reaches of larger
rivers from Mexico to Wyoming (Jordan and Evermann 1896; Minckley 1973; Behnke and
Benson 1983; Bestgen 1990). Adults of this unique fish are distinguished by a pronounced bony
dorsal keel ("razor") arising immediately posterior to the occiput and may attain maximum total
length (TL) of about 1 m (commonly 400-700 mm), weigh 5-6 kg (commonly less than 3 kg),
and exceed 40 years of age (Minckley 1983; McCarthy and Minckley 1987). Larvae are
generally 7-9 mm TL at hatching and 9-11 mm TL at swimup (Minckley and Gustafson 1982;
Marsh 1985; Snyder and Muth 1990; R. T. Muth, personal observation). In rivers, larval
razorback suckers presumably enter the drift after emerging from spawning substrates (Mueller
1989; Paulin et al. 1989) and are transported downstream into off-channel nursery habitats with
quiet, warm, shallow water (e.g., tributary mouths, backwaters, and floodplain wetlands),
Transition to the juvenile period (sensu Snyder 1976) occurs at 27-30 mm TL (Snyder and Muth
1990), and, generally, fish greater than 350 mm TL are sexually mature (Minckley 1983;
Hamman 1985). Estimates of the total fecundity of wild females ranged up to 144,000 ova/fish
(Minckley 1983).
Today, the razorback sucker is one of the most imperiled fishes in the Colorado River
basin and is listed as federally endangered under provisions of the 1973 Endangered Species Act,
as amended (USFWS 1991). The historic widespread distribution of razorback suckers has been
fragmented by dams and water diversions and reduced by over 75%; the species presently exists
naturally as only a few disjunct, aging populations or scattered individuals (Minckley et al.
1991). Although there is evidence of reproduction in at least the two largest extant populations,
natural survival of fish beyond the larval period appears low or nonexistent, and wild stocks are
primarily composed of older fish and continue to decline in abundance (Lanigan and Tyus 1989;
Marsh and Minckley 1989). Lack of recruitment sufficient to sustain populations has been
mainly attributed to the cumulative effects of habitat loss and modification (including
degradation of water quality) caused by water and land development, and predation on eggs,
larvae, or early juveniles by nonnative fishes, many of which have well-established and abundant

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