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. Colorado squawfish, and we believe our interpretation was therefore <br />correct. <br />We were unable to adequately assess overwintering mortality of young <br />Colorado squawfish. Catches of age-1 in the spring (fish/1000 m2) was <br />greater in half of our samples than that of the same cohort the previous <br />autumn. We assume that differences in habitat use made the fish more <br />vulnerable to capture in the spring during some years. <br />Age-1 fish were generally larger in the spring than the previous <br />autumn (x = 42.3 mm, N = 1243 autumn; x = 45.2mm, N = 1,194 spring), but <br />we were unable to determine if this was the result of growth or a size- <br />selective mortality. Thompson (1989) observed that age-0 fish actively fed <br />and survived in laboratory aquaria under simulated winter conditions (3- <br />4°C), but they grew very little. We have collected age-0 fish in November <br />and December and most (>80%) had food in their gut (unpublished data, U.S. <br />Fish and Wildlife Service, Vernal field office). However, we believe that <br />small size of the fish in the autumn could elevate overwinter mortality in <br />times of limited food, as speculated by Thompson (1989). Young fish can, <br />and presumably do, move freely between backwaters and other main-channel <br />habitats in response to temperature differences, and perhaps other <br />environmental variables. Although our catches were low in the winter of <br />1983-84, autumn samples demonstrated a small-size young fish (x = 29.4, N <br />= 31)-and spring samples suggested that survival was only about 3% (one <br />fish, 34 mm). Smaller fish may not be able to exert the needed energy to <br />seek acceptable habitats, and may not have energy reserves to survive long <br />harsh winters. Shuter et al (1980) noted that more optimal summer <br />17 <br />