Investigators: Correigh Greene, Corey Phillis (NOAA), Rene Henery, Jack Williams, Rob Masonis (Trout Unlimited)
Studies across the Pacific Northwest point to the importance of life history variation (e.g., variation in size, outmigration timing, and age structure) in influencing the productivity and resilience of salmon populations in response to environmental variation. Much of this variation is produced in freshwater before salmon enter marine waters, but the consequences of this variation in marine waters (and Puget Sound in particular) is not known. Understanding the extent to which variation in growth, migration, and survival of different life history types in freshwater and marine habitats is important to determine the extent to which changes in survival within Puget Sound affect adult returns.
This research activity used limiting factors analysis to test the effects of juvenile life history diversity on marine survival. This analysis consists of estimating habitat capacity available to juvenile salmon life history types, determining the number of adults produced by each life history type, and developing a model framework to identify factors limiting production potential. Models were constructed using long-term data on Chinook salmon from the intensively-monitored Skagit River watershed. This watershed has shown strong density dependence in life history variation, particularly upon fry migrants, which bypass rearing in estuary habitats. Results suggested that estuary restoration can improve adult returns by providing more rearing area for migrating fry. This increases marine survival by allowing a greater proportion of the population to increase their body size at outmigration. These findings demonstrate the connectedness of freshwater conditions and marine survival for Chinook within the Skagit River system, and provide a framework to investigate life history diversity effects on salmon survival in other Puget Sound watersheds.