8247

• UCREFRP Catalog Number: 8247
• Author: Hudson, J. M., K. W. Wilson, L. D. Lentsch and K. D. Christopherson.
• Title: State of Utah Stocking Plan For Endangered Fish Species of the Upper Colorado River Basin.
• USFW Year: 1999.
• USFW - Doc Type: Salt Lake City, Utah.
• Copyright Material: NO

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BACKGROUND
The razorback sucker, Xyrauchen texanus, was once abundant in warm-water reaches of
large rivers throughout the Colorado River basin (USFWS 1991; Tyus 1997), but presently it
exists naturally as only a few disjunct, aging populations or scattered individuals (Hinckley et al.
1991). Razorback sucker was listed as endangered in 1991 under authority of the 1973
Endangered Species Act (USFWS 1991). The largest extant population occurs in Lake Mohave,
Arizona, but no natural recruitment to the population has occurred in recent decades and the adult
population is declining (Marsh 1994} despite recent successful augmentation attempts (Mueller
1995, Marsh 1997). The largest riverine population occurs in low gradient reaches of the Green
River Basin (middle Green River), with the center of distribution between the mouth of the
Duchesne River to the mouth of the Yampa River (Tyus 1987; Tyus and Karp 1990). Holden
and Stalnaker (1975) found scattered razorback suckers throughout most of the upper basin and
discovered concentrations of 10 to 15 fish in the mouth of the Yampa River in 1970. During
1974-1976, twenty-seven razorback suckers were collected from the Echo Park area of Dinosaur
National Monument (McAda and Wydoski 1980). Razorback suckers are rarely found upstream
of the Little Snake River (McAda and Wydoski 1980; Lanigan and Tyus 1989; Hawkins et al.
1997).
Current Population Status and Trends
The middle Green River population was recently estimated to be 524 individuals (95%
confidence interval, 351-696) based on capture-recapture data collected from 1980-1992 (Modde
et al. 1996). An earlier study using data from 1980-1988 estimated the population at 948
individuals (95% confidence interval, 758-1,138) (Lanigan and Tyus 1989). It has recently been
suggested that with limited recruitment occurring into the population, the adult population size of
the middle Green River may be closer to 175 (1999 Colorado River Researchers Meeting).
However, the current adult population size is unknown, but has most likely decreased since 1992.
Muth et al. (1998) documented annual reproduction by razorback suckers in the Green
River through collection of larvae (N=2,175) from the middle and lower Green River during
1992-1996. These captures represent the first record of larval razorback suckers in the lower
Green River and the first larvae captured in the middle Green River since 1984 (Tyus 1987).
Most of the larvae in the middle Green River were captures downstream of a suspected primary
spawning site, adjacent to Escalante Ranch. These captures suggest that reproduction is
occurring in the middle Green River but that little or no recruitment is occurring as no juveniles
have been encountered during the Interagency Standardized Monitoring Program (ISMP)
(McAda et al. 1994a, 1994b, 1995, 1996, 1997, 1998, USFWS 1987) and very few have been
captured through other studies (Modde 1996). It is recognized that this portion of ISMP is not
designed as population monitoring for razorback sucker. As a result, basin-wide razorback
sucker monitoring was implemented to evaluate razorback sucker populations.
Most of the larvae captured in the lower Green River occurred downstream of the Green
River Valley. Chart et al. (1998) and Muth et al. (1998) suggest that the larvae may not be
transported from upstream reaches but maybe the result of localized spawning near the mouth of
the San Rafael River or within the San Rafael River itself. Other evidence for razorback sucker
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