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I I I I I I I I I I I I I I I I I I I EXECUTIVE SUMMARY A three-year study was conducted to learn more about the factors that limit the size of the Colorado squawfish population of the upper Colorado River. These studies, conducted in the river reach between Lake Powell, Utah, and Palisade, Colorado, included investigations of movement of adult fish and of the. relationship between this movement and the occurrence of spawning areas. Additional studies involved estimation of the relative abundance of post-larval, age-O squawfish--an index of reproductive success--as well as preliminary investigations of three non-flow management options (habitat development, use of hatchery-reared fish, and passage structures) often suggested as means to recover Colorado squawfish. Humpback chub investigations were conducted at Black Rocks, a one-mile- long reach near the Utah-Colorado border. These two-year studies were directed toward increasing our understanding of the reproductive ecology of humpback chub and that of roundtail chub, a closely related species with which humpback chub is assumed to interbreed under conditions of habitat alteration. Important observations resulting from our investigations include the following: 1. In contrast with observations on Colorado squawfish in the Green River system, the presumed spawning migrations of adult Colorado squawfish in the Colorado are relatively short. Most movements were less than 40 mi. It was hypothesized that long-distance movement to spawning areas in the Colorado is unnecessary because suitable spawning areas occur in several reaches distributed throughout the river. 2. Although Colorado squawfish larvae are rare in the Colorado River, concentrations of these larvae were found in several river reaches distributed throughout the study area. This observation supported the above hypothesis that squawfish spawning areas are widespread. 3. The onset of Colorado squawfish spawning occurs when seasonal river temperatures reach about 20 C, as seasonal runoff recedes. Abnormally large runoff in 1983 and 1984 delayed the seasonal increase in river temperature, which in turn delayed spawning of Colorado squawfish and resulted in relatively small age-O squawfish at the end of the growing season during those years. 4. Data collected as part of the present study were used to develop the hypothesis that temperature, as it affects growth rate of Colorado squawfish, might be an important factor limiting the Colorado squawfish population of the upper Colorado. Slow growth of Colorado squawfish results in late age of first reproduction; the immature fish are therefore exposed to the causes of mortality for a longer period and fewer of them reach maturity. The result is greatly reduced potential for population increase. x