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<br />--- <br /> <br />C~) <br />(~) <br />f-' <br />CD <br />.~ <br /> <br />. <br /> <br /> <br />. <br /> <br />significant. For red shiner, seasonal abundance differences were not significant, but <br />differences among years were. Seasonal, reach, and annual comparisons did not yield <br />significant differences for fathead minnow. Seasonal and annual (reaches combined) <br />differences in speckled dace abundance were significant as was that among years <br />(seasons combined). Flannelmouth sucker abundance relationships were similar to those <br />for speckled dace. Only annual comparisons (reaches combined) of blue head sucker <br />abundance yielded significant differences. Abundance of west em mosquito fish was <br />significantly different among years (reaches or seasons combined). Abundance of red <br />shiner, fathead minnow, tlannelmouth sucker, and speckled dace generally declined (both <br />summer and autumn) from 1993 through 1997. Abundance of blue head sucker and <br />westem mosquitofish were variable with no evident pattern. <br /> <br />Total species abundance was not a good predictor of total autumn abundance. <br />However, summer abundance of fathead minnow, speckled dace, flannelmouth sucker, <br />blue head sucker, and western mosquitofish was a good predictor of their autumn <br />abundance. Autumn abundance of red shiner and channel catfish was not related to their <br />summer abundance. <br /> <br />An array of biotic and abiotic factors influenced the seasonal abundance of fishes <br />in secondary channels of the San Juan River. Elevated flows during spring runoff made <br />most secondary channels accessible to all fish residents of the river, particularly large- <br />bodied individuals. Large-bodied fishes may enter secondary channels to forage, spawn, <br />and avoid higher velocity water of the primary channel. Habitat preferences and life <br />history strategies likely influenced the longitudinal abundance distribution oflarge- <br />bodied fishes in secondary channels. The rarity of Colorado pikeminnow and razorback <br />sucker in secondary channels during spring was likely more a consequence of their <br />overall rarity rather than avoidance of secondary channels. During summer and autumn, <br />fish assemblages of secondary channels are numerically dominated by small-bodied <br />fishes and young oflarge-bodied fishes. Among these species, nonnative fishes were <br />typically the most abundant. There was generally a positive relationship between <br />summer abundance of several common species, including red shiner, and elevated spring <br />discharge. Although a natural flow regime was mimicked, spring flows during the study <br />were never as high as occurred in some years prior to completion of Navajo Dam. Thus, <br />elevated spring flows evidently were not sufficient to displace some nonnative species, as <br />documented in several other southwestern systems having a natural hydrograph. <br />Elevated flows during summer, however, generally had negative effects on autumn <br />abundance of nonnative fishes and positive or no effect on native fishes. Abundance of <br />all commonly collected species was less in 1997 than in 1993. The low spring runoff and <br />low summer and autumn flows of 1996 depressed the abundance of all species, native <br />and nonnative. Abundance of most increased slightly in 1997 with higher spring runoff <br />and higher summer and autumn flows, but not to levels found prior to 1996. Although <br />the abundance of most species changed with mimicry of a natural hydro graph, it is likely <br />changes will continue as flows continue to mediate species relationships and interactions <br />and habitats change in response to a mimicked natural hydrograph. <br /> <br />iv <br /> <br /> <br />\v'-I <br />IV-a <br />~ <br />~l <br />\i;-I <br />~'''I <br />~ <br />~-=t <br />~ <br />~ <br />~:a <br />2 <br />~ <br /> <br />