Effects of early-life stage and environmental factors on the freshwater and marine survival of Chinook salmon (Oncorhynchus tshawytscha) in rivers of Southeast Alaska

Highly variable recruitment and declines in productivity of Chinook salmon (Oncorhynchus tshawytscha) have created economic and cultural hardships throughout Alaska. Therefore, it is necessary to understand the factors influencing these declines. In this study, principal component anal¬sis and regression were used to determine how biological and environmental factors have influenced freshwater survival, smolt production, and marine survival of Chinook salmon in the Chilkat River (brood years 1999–2009) and marine survival in the Stikine River (brood years 1998–2009). Smolt production in the Chilkat River was higher in years when parr had shorter average mid-eye fork lengths, river discharge was low in the fall (P=0.05), and river temperatures and discharge were high in early spring (P=0.03), indicating density-dependent growth in fresh water and the importance of river conditions to annual growth and abundance of this species. Marine survival of fish in the Stikine River was positively related to smolt length, indicating higher survival in years when smolts were larger (coefficient of multiple determination [R2]=0.26). Greater marine survival of salmon in the Chilkat River occurred in years when smolt migrations ended later in the month of May, smolts were larger, and discharge was lower (R2=0.5) than in other years. These results indicate the importance of the early marine period in determining year-class strength and highlight the variation in mechanisms that influence survival of stocks of Chinook salmon.