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33 <br />between age groups were tested by an analysis of variance, but <br />adequate analysis of the data was hindered by the small sample sizes. <br />The only significant difference between age groups of fish of the <br />same sex from the same river occurred between age groups VI and VII <br />of females from the Colorado River. <br />Male razorback suckers from the Colorado River were significantly <br />larger than males from the Yampa River (p<0.10) and females from the <br />Colorado River were significantly larger than females from the Yampa <br />River (p<0.01). Yampa River females of age group V were significantly <br />larger than males at this age. Females from the Colorado River were <br />significantly larger than males from the same river in age groups V <br />(p<0.05) and VI (p<0.0005). In 1975, fish in age groups V (n=20) <br />and VI (n=35) were dominant in both rivers. Few fish were collected <br />in 1976, so documentation of a strong year class could not be made. <br />The data do indicate that spawning was more successful in certain <br />years than in others. <br />Reproduction <br />Spawning period <br />Prior to the-construction of large'dams, razorback suckers made <br />extensive spawning migrations in early spring. Jordan [1891] reported <br />razorback suckers ascended the Rio Animas in the spring of 1891 for <br />spawning [in Hibbs and Mitler, 1953] and Chanbertain[1904] cited <br />reports by early settlers in Arizona that razorback suckers congregated <br />in tributary streams for spawning. With:the damming of the rivers, <br />these migrations were blocked and many razorback suckers were forced <br />to spawn in reservoirs.