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1 <br />1 <br />EXECUTIVE SUMMARY <br />This report describes the findings of a fishery study conducted to evaluate the survival of Colorado squawfish <br />(Ptychocheilus Lucius in the Green and Colorado rivers. The objectives of this ongoing study were to assess <br />survival of age-0 Colorado squawfish in the Colorado and Green Rivers and to assist the Interagency <br />Standardized Monitoring Program (ISMP). Data collected since 1987 were integrated into the analysis such <br />1 that survival of the 1987 through 1993 year classes could be determined. Each year class was sampled in the <br />fall (September) when the fish were about 3 months old, and the following spring (March) when the fish were <br />about 9 months old. <br />This study found overwinter decreases in catch per effort of; 77% (1987 year class), 62% (1988), 56% (1989), <br />7% (1990) and 62% (1991). Total numbers for the 1992 year class decreased by 26% from fall to spring, <br />' however no significant difference was noted between fall and spring catch rates. <br />The general decreases in catch per effort from fall to spring on the lower Green River suggest; (1) overwinter <br />mortality, (2) movement or transport of age-0 fish into and from the region, and/or (3) use of habitats <br />unavailable to conventional seining. Similar results were seen in the Colorado River, but the numbers of fish <br />were too low to accurately assess the magnitude of change. The overall decreases in density suggest a <br />substantial loss of fish, although the exact causative factors remain unknown. <br />Survival of age-0 Colorado squawfish in the upper basin may be low. High mortality of age-0 squawfish may <br />account for low recruitment to the population and continued endangerment of this species. Overwinter <br />' decreases of age-0 squawfish CPE, observed in this study, averaged approximately 60% for six year classes. <br />We feel a large portion of this decrease is due to mortality during the winter months. Downstream transport <br />and use of habitats other than backwaters were recognized as important considerations, but did not account <br />for substantial decreases. <br />1 <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br />iii <br />1 <br />