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INTRODUCTION Colorado pikeminnow (Ptychocheilus lucius) has been federally recognized as endangered since 1967 and was one of the original species protected under the Endangered Species Act of 1973 (Federal Register 39(3):1175; e.g. Tyus 1991). This cyprinid is endemic to the Colorado River Basin (Miller 1961) and the largest in North America. Colorado pikeminnow was historically distributed throughout warmwater alluvial reaches of the Colorado River basin, north into Wyoming, east into Colorado and south into Mexico (e.g. Miller 1961; Minckley 1973; Holden and Wick 1982). Colorado pikeminnow is now considered extirpated from highly developed reaches downstream of Glen Canyon Dam (Minckley 1973). Decline of the Colorado pikeminnow has been attributed to water development, nonnative fish introductions, fish harvesting, piscicides and water pollution (Miller 1961; Minckley and Deacon 1968; Seethaler 1978; Behnke and Bensen 1983; Miller et al. 1982; Holden 1991; Quartarone 1993). The system of dams on the Colorado River Basin alters habitat conditions, fragments fish populations and makes many areas inaccessible, which is especially problematic to potadramous spring spawners, like the Colorado pikeminnow (Behnke and Benson 1983; Tyus . 1985; Tyus 1991). Nonnative fishes may pose as great a threat through predation, competition, and introduced disease (Behnke and Benson 1983; Haines and Tyus 1990; Tyus 1991; Muth and Snyder 1995). Although these ecological changes have long been identified, recruitment limitations are, as yet, hypothetical. Overwinter growth and mortality One hypothesized limitation to recruitment is the overwinter mortality of age-O Colorado pikeminnow (Kaeding and Osmundson 1988; Thompson et al. 1991). Overwinter mortality of age-O fish, as noted through density reductions from fall to spring, has been documented in the Green River system (Tyus and Haines 1991; Valdez and CowdellI996). Several studies have investigated possible factors limiting overwinter survival of age-O Colorado pikeminnow, like temperature, habitat and food (Black and Bulkley 1985; Osmundson 1987; Kaeding and Osmundson 1988; Thompson 1989; Valdez and Masslich 1989; Tyus and Haines 1991; Crowl et al. 1995; Valdez and CowdellI994). Most conclude that the relationship between size of age-O fish in fall and survival through winter is crucial to understanding early life-stage recruitment, yet this relationship remains speculative. Lack of a clear understanding of this relationship is partially due to possible confounding effects of overwinter growth. For example, in addition to overwinter density changes, a shift in the body length distribution to overall longer fish, from fall to spring, has been reported (Tyus and Haines 1991; Valdez and CowdeIl1996). It is not known, however, if the shift in average size from fall to spring is due to growth of larger individuals or mortality of smaller individuals. For 10 years, researchers have debated the significance of overwinter growth and mortality and which of these mechanisms causes the noted shift in size from fall to spring (Kaeding and Osmundson 1988; Tyus and Haines 1991; Thompson et al. 1991; Osmundson et al. 1995). Such a shift could be the result of either size-selective forces or their interaction, depending on annual changes in 1