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. ~. 8 ~ ~ GRAND CANYON NATIVE FISH MANAGEMENT 7 ;iri " requisite mainstem temperatures couId be reached in earIy June, a time when tributary reproduction by native fishes typically is waning. Therefore, provision:: of a potentiaIIy important environmental cue for reproduction couId be facilitated by flows that are descending at a rate approximating the pre-dam pattern during June and July. High flows . . . couId be maintained for a shorter period, because of the decreased emphasis on retaining young fishes in tributaries, and flow reductions couId occur more rapidly to open rearing habitats following the reproductive period. >< ;: .,f ;-~: Existing knowIedge of nearshore mainstem rearing habitats, particularly backwaters, relative to river stage in Grand Canyon suggests that the number of these habitats presently is inaximized in the range of 5,000-10,000 cfs. Thus, we recommend that dam discharges be Iowered at a more rapid rate beginning in earIy July to reach this range. Decreasing mainstem flows admittedly will dewater rearing habitats at higher stages and form those at Iower stages, resuIting in some displacement of young fishes. In reality, there will be variation in the pattern of summer flows brought about by unpredictabIe tributary floods from monsoonal storms. This condition was common in the evolutionary history of these fishes, however, and we believe the negative effects of such displacements will be diminished considerably in warmer mainchannel waters. In addition, a relatively steep descent to base flows following the peak hydrograph may serve as an important spawning cue. i:;" .'~. '.\ ,;~ ?: r~: ~-' ~~.;: .' -> ;~, .' ,-:.~ Increased growth rates of young native fishes during the extended period of warm water from June-October will place these individuals at a larger size more able to cope With the return. of cold waters in November. As juvenile big-river fishes reach total lengths 100-150 mm, they begin to occupy deeper waters in eddies Iess affected by diel flow fluctuations. . This change in habitat use provides an opportunity to use hydrology during winter months '.. . ., . . . to further disadvantage non-native fishes, should this action be necessary.:We:believe that. . diel. winter flow fluctuations, of an order similar to those presently in place, couId be used . to destabilize nearshore habitats used by some non-native fishes during this period. Such fluctuations could be used to particular advantage as a tool against smalI-bodied non-natives, . such as fathead minnow and red shiner, that largeIy depend upon nearshore habitats during . their entire life cycle. We understand that these fluctuations wouId also bean advantage to hydroelectric production. '~:.\ .\~ !t.~:j i~t~ :?~ ~'#: ~. ,"'; ~! i:( ~ ."ll! ;~~~ Sediment Augmentation Element-The final ingredient to best simulate historic environmental patterns for the betterment of native fish populations wouId be to implement sediment augmentation. The best time for this application wouId be during the highest flows in spring when sediment would be most easily mobilized and distributed to the higher Ievels .of nearshore habitats Within the system. Augmentation would restore the functional roIe that sediment historically fulfilled in the temporal and structural formation of instreaIn habitats, . and would help assure that a sediment b~ce wouId be maintained in Grand Canyon. The