Physiological Correlates of Habitat Utilization by Juvenile Chinook Salmon in the Columbia River Estuary

Habitat restoration projects in the lower Columbia River often focus on increasing habitat connectivity for the conservation of Pacific salmonids (Oncorhynchus spp.).  These projects rely on the assumption that juvenile salmonids benefit from access to diverse habitats during rearing and emigration, leading to populations that are more resilient to disturbance.  However, there has been little quantification of the physiological benefits of habitat restoration for individual juvenile salmon.  We evaluated the physiological correlates (nutritional condition, growth, and smoltification) of habitat utilization (main-channel vs. off-channel) by juvenile Chinook salmon (O. tshawytscha) during emigration.  Plasma based indicators of nutritional condition (plasma protein and triglyceride) fluctuated both between habitat types and across the sampling period, suggesting a correlative relationship between habitat utilization and fish condition.  Growth potential (RNA:DNA ratio) did not vary temporally or by habitat, and the osmoregulatory capacity (gill Na⁺,K⁺ -ATPase activity) of fish captured in the study indicated that all individuals were undergoing smoltification and actively emigrating.  While continued research is required to elucidate the functional mechanisms linking the quality of restored habitats to organismal condition in emigrating salmon, integration of physiological monitoring into restoration plans can provide action agencies with quantitative measurements of the success of restoration activities.