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ill I I I I I I I I I I I 1 I I I I I I I EXECUTIVE SUMMARY This report describes the findings of a fishery study conducted to evaluate the survival of Colorado squawfish (Ptvchocheilus lucius) in the Green and Colorado rivers. The objectives of this ongoing study were to assess survival of age-O Colorado squawfish in the Colorado and Green Rivers and to assist the Interagency Standardized Monitoring Program (ISMP). Data collected since 1987 were integrated into the analysis such that survival of the 1987 through 1991 year classes could be determined. Each year class was sampled in the fall (September) when the fish were about 3 months old, and the following spring (March) when the fish were about 9 months old. This study found overwinter decreases in catch per effort of; 77% (1987 year class), 62% (1988),56% (1989), 7% (1990) and 62% (1991). This suggests (1) overwinter mortality, (2) movement or transport of age-O fish into and from the region, and/or (3) use of habitats unavailable to conventional seining. Similar results were seen in the Colorado River, but the numbers of fish were too low to accurately assess the magnitude of change. The overall decreases in density suggest a substantial loss of fish, although the exact causative factors remain unknown. Sampling in 1990 was scheduled to extend upstream to more accurately account for movement and transport of young through the system. IT these young fish are moving or being transported downstream during winter or spring runoff as part of an evolutionary dispersal mechanism, many may be transported annually into Lake Powell. This reservoir normally supports little nursery habitat and a variety and abundance of predators, that very likely results in low survival of age-O Colorado squawfish. Understanding the factors that limit survival of these fish in the upper basin may lead to management strategies that enhance recruitment into adult stocks and establishment of self-sustaining populations of this endangered species.