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<br />be associated with different size-selective overwinter mechanisms between the middle and lower <br />reaches or among years. One exception to this was a dominant mechanism of size-selective <br />growth occurring only in the lower Green River where temperatures were warmer (1987-88 and <br />1990-91). Fish growth did not appear to be related to mainstem degree-day accumulation which <br />suggests limitations of age-O Colorado pikeminnow growth may have been due to differences in <br />specific habitat type quantity or quality between the two areas or possibly differential food, <br />predation or competition pressure. This contention is further supported by a lack of a detectable <br />association between age-O year class size and length in fall and physical flow attributes, such as <br />timing and magnitude of peak flows, average summer flows, and summer degree-day <br />accumulation. A notable but insignificant association between degree-day accumulation and size <br />of age-O fish in fall in the lower Green river (which did not show up in the middle Green River) <br />suggests limiting factors of early life-stage survival may be different at different sites. In fact, <br />age-l year class size and length in spring was best predicted by age-O year class size and length <br />in the fall and was not related to average winter flows or winter degree-day accumulation. The <br />largest and most abundant fall year classes were also the largest and most abundant spring year <br />classes. Such a finding lead us to the conclusion that limiting factors on age-O Colorado <br />pikeminnow recruitment to age-l generally occurs before fall of their first year, not overwinter as <br />has been previously hypothesized. <br />We recommend further investigations between size-dependent overwinter growth and <br />mortality to quantify limitations on age-O Colorado pikeminnow survival and more information <br />on factors limiting pre-winter recruitment of age-O Colorado pikeminnow. <br /> <br />LIST OF KEYWORDS <br /> <br />Colorado pikeminnow, overwinter growth, overwinter mortality, size-dependent, age-O, scale <br />analysis <br /> <br />IX <br />