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Post-larval fishes. Similar trends were evident in fish densities in <br />backwaters during fall. Again, densities of native species were generally <br />positively correlated with peak flows, while introduced species were <br />negatively correlated (Table 3). In general, r values were lower than for <br />the larvae data. This is perhaps not surprising because by the time fall <br />sampling occurs other mortality factors have had time to operate and fish <br />abundance by then is less influenced by reproductive success alone. Howev- <br />er., the correlation for Colorado squawfish young-of-the-year (YOY) in the <br />18-mile reach was very similar to that for larvae; abundance was highly <br />correlated with peak discharge (r = 0.974; r2 = .925; P - .026; indicating <br />that production of young and/or survival of young until fall was higher <br />during years of increased peak flow (Fig. 5). We can thus conclude with <br />97.4% confidence that 92.5% of the annual variation in Colorado squawfish <br />YOY densities in the 18-mile reach during fall can be explained by varia- <br />tion in annual peak flow. <br />Discussion. Of the four years in which larvae were collected, 1986 was <br />the year of highest catch rates of larval Colorado squawfish and bluehead <br />sucker, and of lowest catch rates of larval red shiner, sand shiner, and <br />fathead minnow. Colorado squawfish YOY abundance was also highest during <br />1986. Peak flows during this year were 32,800 cfs at the State line gage, <br />an estimated 30,928 cfs in the 18-mile reach, and 22,742 cfs at the top of <br />the 15-mile reach. <br />Why high spring flows are correlated with reproductive success of various <br />species is not well understood. McAda and Kaeding (1989) cautioned that <br />these correlations do not necessarily establish cause and effect relation- <br />21