Near-census ecohydraulics bioverification of Oncorhynchus mykiss spawning microhabitat preferences

Abstract

ABSTRACTThere is increasing scientific and management interest in Oncorhynchus mykiss habitat use, as this species has become rare and elusive due to anthropogenic impacts on regional populations. Past research defined general physical habitat conditions utilized by spawning O. mykiss, but modern science and management require spatially explicit predictive capability. Meanwhile, ecohydraulics prediction in general lacks objective, transparent bioverification, defined as assessment of the complete performance of a physical habitat model relative to observed biological utilization. This study developed a robust framework for ecohydraulics bioverification and applied it to improve the understanding of O. mykiss spawning. The testbed was 35.2 km of the regulated gravel–cobble lower Yuba River, California. Using two-dimensional hydrodynamic modelling, substrate mapping, and a two-year survey of O. mykiss redds, microhabitat representations were tested for their ability to predict spawner preference and avoidance. O. mykiss redds showed a strong preference for mean water column velocities of 0.36–0.69 m/s and depths of 0.38–0.84 m. The substrate range preferred for O. mykiss spawning was within 32–90 mm. O. mykiss spawning microhabitat predictions passed multiple bioverification tests enabling development of a habitat area versus discharge relation that showed that the lower Yuba River has ample O. mykiss spawning habitat.