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Transactions of the American Fisheries Society 132:1251-1256, 2003 © Copyright by the American Fisheries Society 2003 Decline of the Razorback Sucker in Lake Mohave, Colorado River, Arizona and Nevada PAUL C. MARSH,* CAROL A. PACEY, AND BRIAN R. KESNER Arizona State University, School of Life Sciences, Tempe, Arizona 85287-4501, USA Abstract.-The razorback sucker Xyrauchen texanus is an endangered fish endemic to the Colorado River basin in the western United States. Once widely distributed and common throughout the basin, the species has been eliminated from most of its former range by establish- ment of nonnative fishes and water development, and remaining numbers have dwindled precipitously from historical levels. Although Lake Mohave, Arizona and Nevada, supports the largest and genetically most di- verse remaining population, razorback sucker abundance in the lake plummeted from historical numbers in the hundreds of thousands to only 44,000 in 1991 and fewer than 3,000 in 2001. This population is limited primarily to large, old adults because predation on their larvae by nonnative fishes has precluded measurable recruitment for nearly half a century. At the current rate of decline, extirpation is anticipated within this decade because, at this time, there is no practicable method to remove the continuing threat of nonnative predators to razorback sucker larvae. As such, the persistence of razorback suckers in Lake Mohave depends on a repatriation pro- gram begun in 1991, which uses wild-produced larvae that are reared in captivity and returned to the lake at a size that is less susceptible to predation. The razorback sucker Xyrauchen texanus is among a suite of now-endangered, endemic "big- river" fishes of the Colorado River basin of the western United States. Historically, it was widely distributed and abundant to common in the main- stream and major tributaries from Wyoming south through Colorado, Utah, and Nevada, into Arizona and California, and to the Gulf of California in Sonora and Baja, Mexico (Minckley 1973). Fed- eral listing in 1991 was a result of population de- clines and range contractions due largely to im- pacts of nonnative (introduced) fishes and habitat alterations associated with regional water devel- opment (USFWS 1998). Riverine populations per- sist only in the upper Colorado River basin up- stream from Glen Canyon Dam, but these are small and isolated (McAda and Wydoski 1980; Modde et al. 1996). Populations in all but one lower-basin reservoir also are small, while a few individuals are occasionally encountered in other highly mod- * Corresponding author: fish.drCCasu.edu Received August 16, 2002; accepted April 29, 2003 ified lower Colorado River reaches (Marsh and Minckley 1989; Minckley et al. 1991; Holden et al. 2001). The largest remaining razorback sucker population occurs in Lake Mohave, a mainstream impoundment in Arizona and Nevada. The long-term effective female population of razorback suckers in Lake Mohave was estimated via molecular techniques to be in the hundreds of thousands (Garrigan et al. 2002; Minckley et al. 2003), and these fish likely were derived from a historical population that numbered in the millions of individuals basinwide. The Lake Mohave pop- ulation today consists of fish that were produced in the riverine environment during the early 1950s, when young were abundant (Winn and Miller 1954). In the present impoundment, most remain- ing adults are estimated to exceed 50 years old (McCarthy and Minckley 1987). Natural recruit- ment into the population is precluded by predation on early life stages by nonnative fishes (Marsh and Langhorst 1988): juveniles are absent from col- lections, and there is only one record of natural recruitment since the early 1950s (Marsh and Minckley 1989). Abundance estimates for the Lake Mohave razorback sucker population de- clined dramatically from 60,000-75,000 in the 1980s to fewer than 25,000 in the mid-1990s (Marsh, unpublished data), and extirpation was an- ticipated by the early 2000s (Minckley 1983; Minckley et al. 1991; Pacey and Marsh, in press). We present the most recent population abun- dance estimate and trend data, plus a population viability analysis (PVA) for razorback suckers in Lake Mohave, based on mark-recapture records for the 12-year period, 1991-2002. Methods Razorback sucker population abundance moni- toring was conducted in Lake Mohave, a105-km- long, 11,400-ha, run-of-the-river reservoir im- pounded on the mainstream Colorado River be- tween Hoover Dam to the north and Davis Dam to the south. Recent work was part of a regional management and recovery effort by an ad-hoc Na- tive Fish Work Group (NFWG; Mueller 1995) and 1251