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8 <br />Although sample size of older fish was too small (one to seven fish) for <br />a realistic approximation of incremental growth, average annual growth <br />increments for the first 15 years ranged from 17.2 to 53 mm TL. Growth was <br />greatest in the first year and declined subsequently. Fish attained average <br />TL > 400 mm at age 9 and average lengths >599 mm by age 17. Mean back- <br />calculated TL at each annulus indicated that growth in the Green River system <br />was much less than that of channel catfish in the midwestern and southwestern <br />United States (reviewed by Carlander, 1969 and Allen and Roden, 1978). <br />However, the length-weight relationship in the Green River basin (log WT = <br />3.12 log TL - 5.41) was similar to other locations (Carlander, 1969), <br />suggesting the fish were healthy. <br />The long life span (maximum 22 years) of our fish was remarkable. Even <br />though ages of almost 40 years are reported (Moyle, 1976), a maximum of 6 to <br />10 years is usual (Davis, 1959; Moyle, 1976; Robison and Buchanan, 1988). <br />Twenty-four percent of the fish we aged (n = 89) were judged older than 10 <br />years. <br />Slow growth of channel catfish in the Green and Yampa rivers may be due <br />to limited resources, suboptimal temperature regimen, short growing season, <br />and other unfavorable riverine conditions. Channel catfish do not grow well at <br />water temperatures below 21 to 22°C (Canfield, 1947; McCammon and LaFaunce, <br />1961; Andrews and Stickney, 1972; Randolph and Clemens, 1976), and water <br />temperature in the Green River exceeds 21°C only from mid-July to early <br />September (ReMilliard et al., 1988). We assumed that little or no growth would <br />occur from October through May and that growth may be slight in years when <br />average daily water temperatures do not reach 21°C or do so for only a few <br />days. In addition, because the fish may actually experience weight loss below <br />