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I I I I I I I I I I I I I I I I I I I INTRODUCTION The razorback sucker Xyrauchen texanus is a federally designated endangered species endemic to larger rivers of the Colorado River basin (USFWS 1991). Once widespread and common, it now exists only as a few disjunct populations and scattered individuals (Minckley et al. 1991) and is the most imperiled of the basin's "large-river" native fish species with extant reproducing populations. In the upper Colorado River basin, largest populations of razorback sucker presently occur in reaches of the middle Green and lower Yampa rivers, Colorado and Utah, within or bordering Dinosaur National Monument (DING), but numbers of adults are low and recruitment to populations is minimal (Lanigan and Tyus 1989; USFWS 1991). Based on adult recapture data for 1980-1992, Modde et al. (1996) estimated an average population size of 300-600 fish for the middle Green River system. Captures of adult razorback sucker, some in ripe or recently spent condition, have been reported for reaches of the lower Green and middle Colorado rivers within or bordering Canyonlands National Park (CANY; 1975-1995 adult razorback sucker data for the upper Colorado River basin summarized by Muth 1995). Similar to other fishes with high fecundity and ecologically distinct larval and adult stages, populations of razorback sucker appear to be recruitment limited. Extremely low survival of larvae has been hypothesized as the primary reason why the species is near extinction. Probable causes of low survival are (1) alteration or loss of spawning or nursery habitats related to water and land development and invasion of exotic vegetation, and (2) predation by or competition with introduced nonnative fishes that now dominate the ichthyofauna of nursery habitats. Without sufficient survival of young and subsequent recruitment to adult stocks, wild populations of razorback sucker will become senescent and functionally extinct. Since its listing as endangered in 1991, research on razorback sucker in the upper Colorado River basin has mainly focused on reproduction and early life history during spring and early summer. Primary thrusts of many studies have included development of techniques for effective sampling of larvae and early juveniles, documentation of successful reproduction through captures of larvae, determination of larval distribution and environmental requisites, and development of procedures for removing live, naturally produced larvae from the wild for use in grow-out or future reintroduction projects. Subject of this Report In 1992, the National Park Service (NPS) obtained funding for a three-year endangered-fish program under the Natural Resource Preservation Program (NRPP). At that time, seven Rocky Mountain Region (RMR) parks or recreation areas within the Colorado River watershed had identified the need to coordinate issues related to recovery of endangered fishes. Because issues of concern varied widely among Colorado River basin parks, one of RMR's objectives was to develop coordinated policy for all Colorado River parks by establishing an Endangered Fish Program Coordinator position. Another objective was to establish close cooperation and joint funding opportunities with the multi-agency Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin to facilitate better fish management in NPS units and to encourage recovery efforts emphasizing restoration of river habitats. Program direction and project funding were guided by the NPS Colorado River Endangered Fish Advisory Group composed of natural resource managers of RMR Colorado River parks, regional scientists, and the Endangered Fish Program Coordinator (Edmund J. Wick). The advisory group directed this NRPP program toward obtaining critical information on 1