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 m³/s, released from Glen Canyon Dam down the Colorado River through Grand Canyon, had little effect on distribution, abundance, or movement of native fishes, and only short-term effects on densities of some nonnative species. Shoreline and backwater catch rates of native fishes, including juvenile humpback chub (Gila cypha), flannelmouth suckers (Catostomus latipinnis), and bluehead suckers (C. discobolus), and all ages of speckled dace (Rhinichthys osculus), were not significantly different before and after the flood. Annual spring spawning migrations of flannelmouth suckers into the Paria River and endangered 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 tributaries during the flood. Net movement and habitat used by nine radio-tagged adult humpback chub during the flood were not significantly different from prior observations. Diet composition of adult humpback chub varied, but total biomass did not differ significantly before, during, and after the flood, indicating opportunistic feeding for a larger array of available food items displaced by the flood. Numbers of nonnative rainbow trout (Oncorhynchus mykiss) <152 mm total length decreased by ∼8% in electrofishing 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 downstream of the dam) and Hell's Hollow (314 km downstream of the dam) suggest that these young trout were displaced downstream by the flood, although displacement distance was unknown since some fish could have originated from local populations associated with intervening tributaries. Abundance, catch rate, body condition, and diet of adult rainbow trout in the dam tailwaters were not significantly affected by the flood, and the flood did not detrimentally affect spawning success; catch of young-of-year increased by 20% in summer following the flood. Post-flood catch rates of nonnative fathead minnows (Pimephales promelas) in shorelines and backwaters, and plains killifish (Fundulus zebrinus) in backwaters decreased in the vicinity of the LCR, and fathead minnows increased near Hell's Hollow, suggesting that the flood displaced this nonnative species. Densities of rainbow trout and fathead minnows recovered to pre-flood levels eight months after the flood by reinvasion 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 predators and competitors and enhance survival of native fishes.