Dispersal of Larval Fishes in a Regulated River Tributary

We investigated longitudinal distributions, nearshore movements, and drift of larval native fishes (humpback chub Gila cypha , speckled dace Rhinichthys osculus , bluehead sucker Catostomus discobolus , and flannelmouth sucker Catostomus latipinnis ) in the Little Colorado River, a tributary to the regulated Colorado River in Grand Canyon, Arizona, to determine spawning sites, larval dispersal patterns, and amount of drift into the mainstem Colorado River. Larval distributions and drift indicated native fishes spawned throughout the terminal 14.2 km of the Little Colorado River. In addition, distribution, drift, and trap data suggest an active component to dispersal for all four native species. Drift of larval native fish was greater near shore than midchannel, and except for speckled dace larvae, which were prone to drift at night, larval native fish did not exhibit diel periodicity in drift. During a 46‐d period in 1993, we estimated that over 370,000 native fish larvae drifted out of the Little Colorado River into the Colorado River. Regulated discharge from Glen Canyon Dam has all but eliminated spring–summer ponding of tributary mouths that occurred when ascending flows in the Colorado River coincided with descending and base flows in tributaries; thus, drifting larvae are allowed to pass directly into the Colorado River. Survival of larvae now transported into the Colorado River is probably poor because of perennially cold water temperatures and instability of nearshore habitats.