An Evaluation of Survival of Chinook Salmon (Oncorhynchus tshawytscha) from Klamath-Trinity River Hatcheries in Their First Year at Sea Reveals a Unique Response to Local Oceanographic Conditions

Developing reliable predictive models of adult abundance is challenging for Chinook salmon (Oncorhynchus tshawytscha) fisheries due to very high interannual variability, which is strongly related to marine survival during the first year at sea.  Research focusing on the factors affecting early marine survival of Chinook salmon has been largely neglected for California populations. 

We evaluated 28 years of estimated first-year marine survival rates for Chinook salmon released from Klamath-Trinity River (KT) hatcheries to identify oceanographic and atmospheric conditions that may affect survival, and to identify periods within the first year that most strongly affect survival. Unlike previous studies, we found that marine survival of KT Chinook was positively related to the Pacific Decadal Oscillation (PDO).  However, KT Chinook appear to respond to El Niño and La Niña conditions similarly to other Pacific salmonids.  Conditions that strongly affected survival include winter preconditioning and conditions encountered during ocean entry and the first winter at sea for subyearlings released in June, and conditions during the first winter at sea and the following spring for subyearlings released in October. These results provide important insights concerning oceanographic factors that influence first-year marine survival of KT Chinook; however, predictive models were not adequate for management purposes.