Assessing seasonal and biological indices of juvenile Chinook Salmon for freshwater decision triggers that increase ocean survival

Decision triggers, used in adaptive management frameworks to decide when a specific management action will be implemented, are often informed by monitoring data. The identification and application of decision triggers is highly relevant to endangered fishes migrating through regulated rivers, as examined in the current study. The main goal was to determine whether seasonal patterns of behavioral, physical, and physiological indices of juveniles were related to subsequent smolt-to-adult return (SAR) survival and, if so, to determine whether these indices could be used to guide decisions related to the mitigation strategy of the juvenile fish transportation program in the Federal Columbia River Power System (Pacific Northwest, USA). Hatchery yearling Chinook Salmon (Oncorhynchus tshawytscha Walbaum in Artedi, 1792) were collected over the migration season at 3 dams in the hydrosystem and measured for fork length, wet mass, Fulton’s K (or condition factor), Na+/K+-ATPase (NKA) activity (or smoltification index), and % dry mass (or index of energetic reserves and smoltification). We estimated SAR survival from passive integrated transponder-tagged fish representative of our field samples and assessed its relationship to our fish indices, as well as indices of transported vs run-of-river passage and distance of sampling site to ocean. SAR survival was associated to interaction effects between juvenile fish transportation and % dry mass or NKA activity. Transported hatchery Chinook Salmon with dry mass <23% of whole fish wet mass and NKA activity >7 µmol ADP mg protein−1 h−1 showed greater SAR survival than their run-of-river counterparts. Fish with the highest predicted SAR survival had been transported and had fish indices consistent with smolts that were more developed (i.e., lower % dry mass and higher NKA activity). Furthermore, our results on % dry mass provided support for the hypothesis that greater lipid content increases fish buoyancy leading to greater susceptibility to predation. The buoyancy effect is expected to be greatest in hatchery fish. Overall, this study shows that decision triggers based on biological indices of migrating fish are potentially useful tools for in-season management.