SYSTEMATIC LARVAL FISH SURVEYS AND ABIOTIC CORRELATES CHARACTERIZE EXTANT NATIVE FISH ASSEMBLAGE REPRODUCTIVE SUCCESS IN THE COLORADO RIVER, WESTERN GRAND CANYON, ARIZONA

Abstract Systematic larval fish surveys increase the probability of detecting rare species and provide ecological insights for more common species that can be difficult to infer from surveys of older life-stages. To characterize the reproductive success of the extant fish assemblage in the western Grand Canyon portion of the Colorado River, we conducted systematic larval fish surveys in 2014 and 2015. The effort identified endangered razorback sucker (Xyrauchen texanus) as initiating spawning in February with a continuation through July. We collected recently hatched protolarval fish throughout the study area including the most upstream sample sites, suggesting fish spawned throughout and upstream of the study area. Our collection of a protolarval endangered humpback chub (Gila cypha) supports the hypothesis that the population increased in range within the Grand Canyon from its dramatic 2001 contraction into the Little Colorado River and suggests localized spawning. Back-calculating hatching dates indicated humpback chub began hatching in late April and continued through mid-May. Native fishes numerically dominated larval fish collections (97.6 and 99.3% for 2014 and 2015, respectively) and recruitment to the early juvenile life-stage was documented for all fish species captured except razorback sucker. Abundance, measured as catch per unit effort and frequency of occurrence, was different between years for each native species captured (n = 5). Interannual differences in abiotic factors (discharge, diel fluctuations in discharge, and water temperature) were significantly different for most months between years. A redundancy analysis, evaluating the relationship between abiotic environmental factors and catch rates, suggested both discharge and water temperature were associated with speckled dace (Rhinichthys osculus), bluehead sucker (Catostomus discobolus), and flannelmouth sucker (Catostomus latipinnis), with a positive correlation to water temperature and a negative correlation to discharge. We were unable to identify associations between abiotic factors and the endangered fishes, which may have been due to the lack of variability in those species' catch rates. Improving our understanding of the relationship between reproductive success and abiotic factors would enhance management of the system to benefit native fishes.