LAUSR

We investigated how extreme drought conditions influenced the abundance, growth, movement, and seawater readiness of juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) in a small central California coastal lagoon. In 2015, the seasonal sandbar at the mouth of Scott Creek formed over 2 months earlier than average, effectively trapping fish in the lagoon for 7 additional months (mid-May through December) before outmigration opportunities eventually resumed. Monthly mark–recapture sampling demonstrated that juvenile coho salmon and steelhead were able to persist in the lagoon during extended periods of high water temperature and low dissolved oxygen concentration. Both salmonid species exhibited similar temporal trends in abundance, growth, and Na+-K+-ATPase activity levels during lagoon residence; however, abundance and growth rates were consistently higher for steelhead. Stationary passive integrated transponder tag antenna detections revealed recurrent movement of individuals between the warm lagoon and cooler lower mainstem creek, suggesting individuals regulated key physiological processes by moving between the adjacent habitats. Our study provides new insight concerning the consequences of drought for imperiled salmonid populations and underscores the importance of life-history diversity during extreme climatic events.