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<br />4 <br /> <br />CLARlCSON ET AL. <br /> <br />restoration of the Missouri River (Hesse and MestI 1993) and Green River fish communities <br />(Linde1l1992). <br /> <br />Benefits to native fishes from such a discharge pattern include the recreation of historic <br />interactions among hydroIogy ,aquatic habitat and native fish life histories, and suppression <br />of non-native fish populations. Continued reIease of perennially coId waters will largeIy <br />precIude successful mainstem reproduction by native fishes, and may restrict the potential <br />of recommended flows to improve conditions for increased growth and survival of progeny. <br />With this precaution, we offer the following recommendations for hydroIogy in the absence <br />of water temperature modification. <br /> <br />~ <br /> <br />We recommend replacing daily fluctuations with seasonal adjustments in discharge to <br />produce a singIe peak hydrograph. Beginning in March, discharge levels shouId be raised <br />above autumn-winter low flows. These base flows preferabIy shouId be held at a minimum <br />of S ,000 cfs. The ascension of spring flows should be scheduIed to arrive at a peak as high <br />as practicalIy possible (near 30,000 cfs) in Mayor June, so that high, but not necessarily <br />maxiriuim, flows are present through the period of major reproduction by native fishes in <br />tributaries. ..- Even higher flows might be desirabIe, but we recognize there are legal <br />constraints..on annual relea~5 and that a sediment balance must be maintained. <br /> <br />We believe:that increasing spring flows to a sustained maximum will serve four purposes: <br />(1) provide an environmental cue for gonadal maturation and aggregation ofmainstem fishes <br />prior to their ascension into tributaries for reproduction; (2) back up the waters of canyon- <br />bound tributaries forming ponded rearing habitats in their mouths and reduce the transport <br />of early life stages from tributaries into the mainstream; (3) create and maintain nearshore <br />rearing habitats forearIy life stages entering the mainstem, and; (4) reduce the populations. _ <br />of mainstem non-native fishes through downstream displacement, disruption of reproduction; <br />and lowering survivorship of earIy life stages. <br /> <br />Minckley (1991) suggested that the key to reestablishment of the native fish fauna of Grand <br />Canyon lies with the suppression of non-native fishes. This objective can be accomplished <br />in part.by cxpIoitingthe inability of non-native fishes to cope with large floods carrying <br />heavy sediment loads (Minckley and Meffe 1987). . We anticipate that the effectiveness of <br />controlled floods to displace non-native fishes will be a function of both magnitude and <br />duration; Thus, in years when water releases in excess of 8.23 MAP are scheduled and <br />sufficient sediment is available, we advocate that additional supplies be programmed into <br />spring releases to further exploit the negative effects of floods on non-native fishes. We <br />recogi1ize that this effect is temporary, but the limitation can be offset by periodic repetition <br />of the treatment. <br /> <br />\ <br />