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American Fisheries Society Symposium 15:127-135, 1995 A Program for Maintaining the Razorback Sucker in Lake Mohave GORDON MUELLER National Biological Service, Post Office Box 25007, Denver, Colorado 80225, USA Abstract.-Lake Mohave, Arizona-Nevada, supports the last large population of the Endangered razorback sucker Xyrauchen texanus. Razorback suckers successfully spawn in Lake Mohave; however, predation by nonnative fish appears to restrict recruitment. Most razorback suckers are believed to be more than 40 years old and nearing the end of their life span. The population is expected to perish within the next few years unless steps are taken to ensure survival and recruitment of young. Concerned biologists from seven state and federal agencies formed the Native Fish Work Group (NFWG) to maintain the razorback sucker in Lake Mohave. The NFWG has developed a program to replace the aging population with young adults that reflect the genetic divergence of the reservoir population. Larval fish spawned naturally in the reservoir are collected and stocked by themselves into small, isolated nursery areas. Fish are grown in these predator-free environments to 25 cm, a length believed large enough to evade most predators. The NFWG plans to release a minimum of 10,000 young adult razorback suckers back into Lake Mohave and monitor their survival. This represents the first step in a long-term management commitment. Populations of razorback suckers Xyrauchen texa- nus are now restricted to less than 25% of their former range (Minckley et al. 1991). Nearly all ra- zorback suckers collected during the past 2 decades have been adults. These fish are nearing extinction in the upper Colorado River basin (Tyus 1987; Os- mundson and Kaeding 1991; USFWS 1991). The largest remaining population of razorback suckers is found in Lake Mohave, Arizona-Nevada, a reg- ulatory reservoir formed by Davis Dam. These fish are old, and, as in the more pristine upper Colorado River basin, reproduction occurs but few young survive to reach adulthood. The Lake Mohave pop- ulation has declined 60% (59,500 to 23,300) in the past 5 years (Marsh 1994). Populations elsewhere are also declining and extinction will occur this decade unless steps are taken immediately to aug- ment populations (Minckley et al. 1991; Burdick 1992). The species decline is attributed to habitat deg- radation and competition and predation by non- native species. Razorback suckers do successfully spawn at several locations throughout the basin. Eggs incubate and hatch and larval razorback suck- ers are produced, but young longer than 25 mm total length (TL) rarely are found. Today young razorback suckers are vulnerable to a large host of predators not present a century ago. Over 40 non- native fish species have been successfully intro- duced into the Colorado River basin. Implications of these introductions are difficult to quantify; how- ever, Marsh and Brooks (1989) showed that small razorback suckers stocked in Arizona streams were eaten by channel catfish ktalurus punctatus and other nonnative species. Predation on larval razor- back suckers, although not quantified, is considered the single most important factor limiting recruit- ment in Lake Mohave (Minckley 1983; Marsh and Langhorst 1988; Minckley et al. 1991). Efforts to recover the razorback sucker and other native fishes began in 1976 with the creation of the Colorado River Fishes Recovery Team (K D. Miller 1982). The Colorado River Fishery Project was initiated by the U.S. Fish and Wildlife Service (USFWS) in 1979. Its purpose, along with a sub- stantially larger recovery program for the upper Colorado River basin that followed in 1987, was to recover the razorback sucker, bonytail Gila elegans, humpback chub G. cypha, and Colorado squawfish Ptychocheilus lucius in a manner that allows further water development. The cost for the remaining 10 years of the 15-year recovery program is estimated to be US$84-134 million (USFWS 1993). A coop- erative agreement between USFWS and Arizona Game and Fish Department delayed listing of the razorback sucker in order to reintroduce nearly 12 million small razorback suckers into Arizona streams between 1980 and 1989 (Johnson 1985; Minckley et al. 1991). This massive stocking attempt was unsuccessful in reestablishing self-sustaining populations, and the razorback sucker was listed as Endangered in 1990. Unfortunately, 18 years of recovery effort have failed to slow, let alone reverse, the decline of this native fish (Tyus 1987; Marsh and Brooks 1989; Langhorst 1989; Osmundson and Kaeding 1991; 127