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The regression coefficients are all highly positive when relative percent <br />survival is regressed against W'~A brown trout fry habitat. All the regression <br />coefficients are highly negative when relative percent survival is regressed <br />against mean discharge/month. <br />The years of good to excellent recruitment (1977, 1978, and 1981) were <br />extremely dry years for both snow pack the previous winter as well as for <br />stream-swelling summer rains. Conversely, the years 1976, 1979, and 1980 <br />(years of very poor recruitment) were years of near record snowfall. The years <br />1982 and 1983 (years of moderate recruitment) were years of ,moderate to <br />heavy snowfall and Moderately high run-off levels. Indeed, the coefficients <br />of determination (r in Table 2), indicate 80% to 98% of the variation in <br />relative percent survival can be accounted for by the variation in mean <br />discharge/month. <br />3 For th~ majority of the discharge hydrograph (except for flows below 1.13 <br />m /s (40 ft /s) there is a strong highly negative exponential relationship <br />(r = -0.962) between discharge and brown trout fry weighted usable area. A <br />plot of the WiJA as percent of the total stream area versus the discharge <br />hydrographs for 1977 (lowest flow year 1976-1983) and 1979 (the highest flow <br />year 1976-1983) reveals an interesting comparison. In 1977 (the dry year) <br />brown trout fry habitat was at a maximum for much of the year except for a <br />short period of a week or two in May and again in June. In 1977 we had the <br />highest relative percent survival for the ent=ire study period. Conversely, in <br />1979 (the wet year) brown trout fry habitat was reduced by 75-97% from April <br />17 through July 19 and it was reduced by 84-97% from May 19 through July 4. <br />The year 1979 was the year of poorest YOY brown trout survival of the study. <br />Ottaway and Clarke (1981) indicate brown trout fry pass through a post-emergence <br />period when they are extremely vulnerable to downstream displacement by high. <br />water velocities. We believe this is what happens during spring run-off on <br />the South Fork of the Rio Grande. <br />CONCLUSIONS <br />Brown trout fry survival in this study is most certainly not density <br />dependent as found by some investigators (McFadden 1961; LeCren 1962). The <br />years when we found the largest numbers of YOY brown trout were also the years <br />of the best percent survival on brown trout fry. Brown trout YOY survival in <br />the South Fork of the Rio Grande is strongly correlated with the amount of fry <br />WUA available in the spring-early summer months (April-July). WUA brown <br />fry habitat is inversely correlated with stream discharges in an exponential <br />manner at all levels above the basal minimum winter flow. Thus during years <br />of low spring run-off (1977, 1978, and 1981) fry survival was maximized. <br />Parent spawner density was not correlated with the number of YOY brown trout <br />observed in the subsequent fall electroshocking. We have good indications <br />that the same relationships between fry survival and spring stream discharge <br />levels observed on the South Fork of the Rio Grande also exist on many other <br />major trout streams in Colorado. However, at the present time we have not <br />accumulated enough population data to make an air-tight case. Biologists and <br />40 <br />