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Study objectives In this study, we examined the relationship between total body length and overwinter mortality of age-O Colorado pikeminnow in the Green River, Utah, by comparing first year circuli counts from scales of age-O fish with those of adults and by examining quantile-quantile plots oflength-frequency distributions. We also examined associations among year class length and size in fall and spring and temperature and flow regimes. Our main hypothesis was that overwinter survival by age-O Colorado pikeminnow was greater for larger sized fish. To address this hypothesis, we determined: 1) if Colorado pikeminnow formed a first year growth check; 2) the usefulness of scale analysis in identifying size-dependent overwinter mortality and 3) if size- dependent overwinter growth and mortality of age-O Colorado pikeminnow could be discerned using empirical length- frequency distributions. We further hypothesized that differences in the temperature and flow regimes of the Green River may result in different growing season lengths and quality, thereby reducing growth and increasing overwinter mortality of age-O Colorado pikeminnow in some areas or in some years. In addition, we investigated whether greater degree-day accumulation and flows in the lower Green River during summer and winter seasons were associated with larger fish or greater survival in the lower Green River compared to the middle Green River. METHODS Fish sampling Sampling for age-O Colorado pikeminnow was part of ongoing seasonal and annual research and monitoring projects in the Upper Colorado River conducted by BIOIWEST, Inc. (Logan, Utah), Utah Division of Wildlife Resources and U.S. Fish and Wildlife Service (Vernal, Utah) (USFWS 1987). Age-O Colorado pikeminnow were seined from backwaters and near- shore, low-velocity habitats in the Green River in Utah during fall and spring from fall of 1987 through spring of 1995 covering 8 winter seasons. Fall sampling occurred during late September or early October and spring was during late March or early April. Early life-stage sampling took place in two areas that contain known concentrations of nursery habitat (Figure 1). For this discussion, these areas are referred to as the lower Green River (RK 0 to 200) above its confluence with the Colorado River (1987 to 1995; 8 seasons) and the middle Green River (RK 360 to 550) from Split Mountain to Sand Wash (1989 to 1995; 6 seasons). Two seine hauls were taken in each of the first two backwaters every five miles according to standardized monitoring protocol (USFWS 1987). Relative abundance was recorded as geometric mean catch per effort by seine haul henceforth referred to as CPE. Information on adult fish was acquired from the U.S. Fish and Wildlife Service database for endangered Colorado River fish in the upper Colorado River basin maintained in Grand Junction, Colorado. These data represent the combined efforts of private, state and federal agencies collected for various studies of Colorado pikeminnow ecology from 1978 to 1989. These fish were juveniles and adults greater than 200 nun TL which, for purposes of this report, 3