UC Davis

Anadromous salmonids utilize a combination of physical, chemical, and biological cues when migrating from ocean foraging grounds as mature adults to freshwater spawning grounds to complete their lifecycle. The biological processes behind homing behavior (migrating to their stream of natal origin to spawn) and straying behavior (migrating to non-natal habitats) have been well studied since the 1950’s. However, the abiotic environmental factors that partially drive these behaviors have received comparatively less attention. One aspect that is particularly understudied is the influence of localized channel hydraulics on habitat selection in migrating adults. The first chapter of this dissertation includes an investigation into migratory microhabitat selection at a major river confluence by California Central Valley fall-run Chinook salmon in response to localized hydraulic conditions, as well as temperature and turbidity. Conveyance (depth*velocity magnitude) was found to be the strongest hydraulic predictor of micro-scale habitat selection, and acts in conjunction with temperature to facilitate rheotactic swimming behavior. Chapter 2 includes modeled simulations of migratory movement under varying drought conditions, utilizing the same confluence used in Chapter 1 as a study system. These simulations indicate that both discharge magnitude in each river, as well as the ratio of discharge magnitudes between them, are significant drivers of the availability and distribution of preferred hydraulic microhabitats at the confluence, potentially influencing migratory routing. These findings support observed patterns in Chinook salmon escapement data in response to historical flows at this confluence. Chapter 3 includes a literature review and meta-analysis assessing the extent to which abiotic factors have been investigated in the existing literature on homing and straying behavior in anadromous salmonids. Only 70 out of 169 articles included at least one abiotic study component. This phenomenon was surprisingly consistent across study method types, study locations, and study species, with none of these attributes having statistically significant differences in frequency of abiotic components among attribute classes. The research presented in this dissertation provides valuable advancements in the field of fisheries ecology, particularly with respect to the topic of homing and straying in anadromous salmonids. Novel approaches to both field data collection and riverine habitat modeling were developed, and key results link open channel hydraulic processes to adult migratory behavior, filling an existing knowledge gap in the basic life history of salmonids.