7834 - Laserfiche WebLink

Home Browse Search

INTRODUCTION General . The razorback sucker (Xyrauchen texanus) is one of several native fishes that were formerly common and widespread throughout warm-water reaches of the Colorado River Basin, primarily in the mainstem and large tributaries from Wyoming to Mexico. Razorback sucker numbers have declined appreciably since human alteration of the aquatic environment began in the basin near the turn of the century. The decline of the razorback sucker is attributed to changes in physical and biotic factors leading to very low recruitment because of high mortality during the larval and juvenile life stages. Although the causes are not known, recruitment failure may be due to deterioration of water quality and/or predation on eggs, larvae, and juvenile razorback suckers by nonnative fish. Reduction of high spring flows has altered the natural flooding cycle, and some investigators (McAda 1977; Tyus and Karp 1989; Osmundson and Kaeding 1991) attribute lack of recruitment, in part, to the reduced availability of inundated floodplains which historically provided spawning and feeding for adults and nursery habitat for young. Because of its precarious position, the U. S. Fish and Wildlife Service (FWS) listed the razorback sucker as an endangered species (effective 22 November 1991; Federal Register, Vol. 56, No. 2105, 23 October 1991) under authority of the Endangered Species Act of 1973 (FWS 1973). Most investigators (e.g., Tyus et al. 1987; Osmundson and Kaeding 1989; Bestgen 1990) consider the razorback sucker to be much more rare than the endangered Colorado squawfish (Ptychocheilus lucius). In the Lower Colorado River, downstream of Glen Canyon Dam, a substantial population of razorback sucker persists in Lake Mohave but occurs only sporadically in riverine reaches (Marsh and Minckley 1989). In the Upper Colorado River Basin, the largest population of adult razorback sucker are found in the Green and Yampa rivers (Holden and Wick 1982; Lanigan and Tyus 1989). Total numbers of razorback sucker collected from the Upper Colorado River have declined dramatically since 1976. In the Upper Colorado River, only 25 adult razorback sucker were captured between 1980 and 1990 (Valdez et al. 1982) from riverine habitats; only seven adult fish have been captured since 1990 (Osmundson and Kaeding 1991; Burdick 1992; unpublished FWS data). The last adult razorback sucker in the Gunnison River was captured in 1981 (Holden et al. 1981). About 125 adult razorback sucker have been collected recently from lentic habitats in human-made gravel-pit ponds. The majority of these fish were captured in Etter Pond near Debeque, Colorado. These fish were determined from genetic analyses to be progeny from a few adults that gained access to Etter Pond during the high spring flows of 1983 and 1984 and subsequently became trapped in Etter Pond. In the Gunnison River, wild razorback sucker are thought to be extirpated, and the present population in the Upper Colorado River is considered functionally extinct because so few fish remain to sustain stable populations in the riverine system. Remaining razorback sucker are composed exclusively of old adults in the Upper Colorado River. Neither spawning or recruitment have recently been documented. If limited recruitment is occurring, it is not sufficient to maintain populations in either river. Only a few juvenile razorback suckers pos-