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was also a trend for better overwinter survival for larger fish in fall. Relationships of non-native fish densities to survival were inconclusive. Hypotheses We attempt to link overwinter survival of age-O Colorado pikeminnow to (1) length of fish in falL (2) river flow variation, (3) river temperature, (4) backwater depth, and (5) density of non- native fishes in backwaters. We assessed overwinter survival index of age-O Colorado pikeminnow in two reaches of the Green River by comparing fall and spring densities for the same cohort (i.e., fi sh hatched in late summer were sampled in fall and the following spring). It is assumed that age-O Colorado pikeminnow remain in backwaters during winter, although sampling in winter has not been conducted to confirm this assumption. The following hypotheses were tested: "0 1: Larger age-O Colorado pikeminnow have significantly higher overwinter survival than smaller individuals. This hypothesis is based on the work of Thompson et al. (1991) who determined in the laboratory that overwinter survival of age-O Colorado pikeminnow appears to be determined by the size of fish going into the winter period; larger fish had a higher fat content and hence, were better tit to survive food shortages and slowed metabolism during winter. Ho 2: Overwinter survival of age-O Colorado pikeminnow is significantly higher at stable, non-fluctuating winter flows. Overwintering age-O Colorado pikeminnow are believed to remain in nursery backwaters throughout winter. Fluctuating flows that inundate or desiccate these backwaters can force the fish to move in search of other backwaters. During this period, the fish are along the mainstem shoreline, where they are more vulnerable to predators and where velocities and low production inhibit sutlicient food supplies. Backwaters that are persistently watered with sufficient depth during winter are likely to enhance survival of age-O fish. "03: Overwinter survival of age-O Colorado pikeminnow is significantly higher in winters of very warm or very cold water temperatures. Very warm water temperatures in winter preclude ice formation and maintain a free-flowing ri\'er. with similar conditions to the antecedent fall season. Very cold water temperatures quickly f<.mll a thick, stable ice cap over the river that insulates and keeps the river relatively warm and protected from frazil and jam ice formation. During years of moderate temperatures, frazil and jam ice formation cause ponding of the river and erosion of retaining backwater sand bars, causing flow- through the nursery backwaters. Valdez and Cowdell (1999) found that about 50% of backwaters were rendered unsuitable as nursery habitat as a result of ice jams and collapsing ice lenses. 2