Effects of a test flood on fishes of the Colorado River in Grand Canyon, Arizona

A beach/habitat-building flow (i.e., test flood) of 1274 m(3)/s, released f rom Glen Canyon Dam down the Colorado River through Grand Canyon, had littl e effect on distribution, abundance, or movement of native fishes, and only short-term effects on densities of some nonnative species. Shoreline and b ackwater catch rates of native fishes, including juvenile humpback chub (Gi la cypha), flannelmouth suckers (Catostomus latipinnis), and bluehead sucke rs (C. discobolus), and all ages of speckled dace (Rhinichthys osculus), we re not significantly different before and after the flood. Annual spring sp awning migrations of flannelmouth suckers into the Paria River and endanger ed humpback chub into the Little Colorado River (LCR) took place during and after the flood, indicating no impediment to fish migrations. Pre-spawning adults staged in large slack water pools formed at the mouths of these tri butaries during the flood. Net movement and habitat used by nine radio-tagg ed adult humpback chub during the flood were not significantly different fr om prior observations. Diet composition of adult humpback chub varied, but total biomass did not differ significantly before, during, and after the fl ood, indicating opportunistic feeding for a larger array of available food items; displaced by the flood. Numbers of nonnative rainbow trout (Oncorhyn chus mykiss) < 152 mm total length decreased by similar to8% in electrofish ing samples from the dam tailwaters (0-25 km downstream of the dam) during the flood. Increased catch rates in the vicinity of the LCR (125 km downstr eam of the dam) and Hell's Hollow (314 km downstream of the dam) suggest th at these young trout were displaced downstream by the flood, although displ acement distance was unknown since some fish could have originated from loc al populations associated with intervening tributaries. Abundance, catch ra te, body condition, and diet of adult rainbow trout in the dam tailwaters w ere not significantly affected by the flood, and the flood did not detrimen tally affect spawning success; catch of young-of-year increased by 20% in s ummer following the flood. Post-flood catch rates of nonnative fathead minn ows (Pimephales promelas) in shorelines and backwaters, and plains killifis h (Fundulus zebrinus) in backwaters decreased in the vicinity of the LCR; a nd fathead minnows increased near Hell's Hollow, suggesting that the flood displaced this nonnative species. Densities of rainbow trout and fathead mi nnows recovered to pre-flood levels eight months after the flood by reinvas ion from tributaries and reproduction in backwaters. We concluded that the flood was of insufficient magnitude to substantially reduce populations of nonnative fishes, but that similar managed floods can disadvantage alien pr edators and competitors and enhance survival of native fishes.