9540

• UCREFRP Catalog Number: 9540
• Author: Christopherson, K., G. J. Birchell and T. Modde.
• Title: Larval Razorback Sucker and Bonytail Survival and Growth in the Presence of Nonnative Fish in the Stipprup Floodplain.
• USFW Year: 2004.
• USFW - Doc Type: Salt Lake City, UT.
• Copyright Material: NO

i INTRODUCTION
Razorback sucker (Xyrauchen texanus) and bonytail (Gila elegans) are endangered
species endemic to the Colorado River Basin. Razorback sucker were listed as endangered in
~ 1991, and bonytail were listed as endangered in 1980. Wild bonytail are essentially extirpated
from the Colorado River basin, with most fish currently in the wild consisting of hatchery reared
and stocked fish. In the middle Green River however, there exists a small reproducing wild
~ razorback sucker population. Despite successful reproduction, survival beyond the larval stage
has rarely been observed (Muth et al. 1998). Lack of razorback sucker recruitment is attributed
to habitat loss, and predation on larvae or early juveniles by nonnative fish (Minckley 1983;
Minckely et al. 1991; Hawkins and Nesler 1991; Lentsch et al. 1996; USFWS 1996).
Fooodplain wetlands are believed to be important habitat for survival and recruitment of
razorback suckers (Wydoski and Wick 1998; Modde 1996 and Modde et al. 2001). In addition,
i recent studies of floodplain habitat in the lower Colorado River basin by Mueller et al. (2003)
and historical photos presented by Quatarone (1993) suggest that floodplain habitats might also
be important to bonytail. However, since the operation of Flaming Gorge Dam, the frequency of
~ floodplain inundation has been reduced (Graf 1978; USFWS 1998; Flo-engineering 1997). As a
result, access to floodplain habitats for endangered fish has been limited.

Despite intense sampling, razorback sucker survival in floodplain wetland sites was not
observed during the Levee Removal Study (Birchell et al. 2002). An extremely small population
of adult razorback suckers in the river, and the resulting low levels of spawning during the study,,
combined with large non-native fish populations, likely contributed to no observed larval
survival. Modde et al. (1996) estimated the adult razorback sucker population in the middle
Green River at only 500 fish when the study started, which has likely declined since (Bestgen et
al. 2002).