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.

Abstract: In California (USA), seasonal lagoons provide important oversummer rearing habitat for juvenile steelhead trout (anadromous Oncorhynchus mykiss). However, key water quality parameters such as temperature and dissolved oxygen concentration can periodically approach or exceed the physiological tolerances of steelhead during the protracted dry season. A field study employing distributed temperature sensing technology, water quality monitoring, habitat mapping, and mark-recapture sampling was conducted to examine how shifting environmental conditions affected the performance and behavior of juvenile steelhead in the Scott Creek estuary/lagoon (Santa Cruz County). Abiotic conditions were driven by episodic inputs of seawater to the typically freshwater lagoon. During midsummer, the water column was vertically stratified which reduced suitable lagoon rearing habitat by approximately 40%. Nevertheless, steelhead abundance, growth, and condition factor were high during the summer and decreased in autumn following lagoon destratification and cooling. Unlike previous work, this study identified limited emigration from the lagoon to riverine habitat during the summer. Instead, juvenile steelhead exhibited crepuscular movement patterns within the lagoon, with peaks in upstream (to upper lagoon habitat) and downstream (to lower lagoon habitat) movement occurring at dawn and dusk, respectively. This study underscores that habitat complexity and connectivity are critical for juvenile steelhead production and persistence and provides insight into steelhead habitat use and behavior in seasonal lagoons.