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78 Environmental Enhancement of Water Projects INTRODUCTION The razorback sucker (Xyrauchen texanus) is a large catostomid endemic to the Colorado River system of western North America. Once common in the mainstem and major tributaries from Wyoming to Mexico, the species is now reduced to remnant populations in the Green River, Utah; the Upper Colorado River Basin, Colorado- Utah; and Lake Mohave, Arizona-Nevada, where most of the remaining fish are found (Minckley et al., 1991). The development of water resources and associated environmental impacts, combined with the introduction of non-native fish, has greatly reduced the range and number of razorback suckers. Because of dwindling numbers and continuing threats to its survival, the razorback sucker was Federally listed in 1991 as endangered under the protection of the Endangered Species Act of 1973. Efforts are currently being made to designate a large portion of the Colorado River, including Lake Mohave, as critical habitat. Although the razorback sucker is considered a large riverine species, the vast majority of remaining suckers, about 60,000, are found in Lake Mohave, a regulatory reservoir on the Colorado River. This and other small populations are comprised of old fish believed to exceed 30 years of age. Research has documented successful spawning, egg incubation, and hatching. However, larval suckers disappear before reaching 1 inch (25 mm) in length. Most scientists agree that predation by non-native fishes is the single-most important factor responsible for recruitment failure in Lake Mohave. Unless young fish survive to adult life stages, biologists believe the "sun set" population will die off within this decade (Minckley et al., 1991). Razorback suckers are easily propagated in hatcheries; however, efforts to reintroduce suckers have generally failed. A ten year stocking program (1980-1990) of the Fish and Wildlife Service (FWS) and Arizona Game and Fish Department (AGFD) reintroduced approximately 11.5 million larval and juvenile razorback suckers into historic waters (Minckley et al., 1991). Follow-up evaluations indicated these young suckers also disappeared. Predation studies conducted by Marsh and Brooks (1989), indicated stocked suckers were quickly eaten by catfish and other non-native species. They recommended stocking larger suckers to reduce potential predation, which might improve survival. Unfortunately, hatchery space is often limited and Razorback Sucker Program 79 costs associated with rearing and hauling fish increase exponentially with size. The culturing of large numbers of subadult suckers is feasible, but quite costly. Biologists conducted an experiment in Lake Mohave during 1985 which illustrated that razorback suckers could successfully spawn and rear young in an environment free of other fish. Fish were removed from an isolated backwater that formed during low reservoir elevations. All fish were removed from the backwater and replaced with adult razorback suckers just prior to spawning. Spawning was successful, and because of abnormally low reservoir elevations the backwater remained isolated for the entire year. Juvenile suckers had grown to 8 inches (20.4 cm) in length when they were collected in December. Fishery biologists began to discuss the possibility of on-site spawning and rearing as a low cost alternative to hatchery produced fish. If this concept is successful with razorback suckers, it might also provide a management option for other long lived native fish. PROPOSED PLAN The Bureau of Reclamation (BOR) initiated the formation of the Native Fish Work Group (NFWG) in 1990. The task force is comprised of technical representatives from the FWS, AGFD, National Biological Survice (NBS), National Park Service (NPS), Arizona State University (ASU), and Nevada Department of Wildlife (NDOW). The goal of the NFWG is to develop, implement and test an on-site rearing program designed to sustain the existing razorback sucker population. Our ultimate objective involves the production of subadult suckers under "semi-natural" conditions, (i.e., spawning in waters free of predacious fish and without supplemental feeding). A Research and Management Plan was developed that called for the construction of isolated, on-site backwaters to allow razorback suckers to spawn and grow unmolested. Small coves ranging from 2-5 acres (0.8-2 hectares) would be isolated from the main reservoir using temporary and permanent barriers. Exotic fish would be removed and the backwaters would be stocked with spawning razorback suckers or their young for the