Abstract:
Highly variable recruitment and declines in productivity and abundance of Chinook Salmon Oncorhynchus tshawytscha have created economic and cultural hardships for communities throughout Alaska. Although pre- and post-smolt growth are important for determining brood-year (BY) survival and productivity for Pacific salmon through size-mediated mortality, these relationships remain unclear for Chinook Salmon. As a result, it is necessary to better understand the relationships between environmental and biological factors that influence freshwater and marine growth, smolt outmigrations, and recruitment success. This study used retrospective growth to identify the importance of annual growth in determining BY survival and recruitment, determine if growth dependency between growth zones was present, and examine growth differences among age classes for Chinook Salmon in the Chilkat (BYs 1985 - 2007) and Stikine (BYs 1991 - 1998 and 2000 - 2007) rivers. Biological and environmental factors were also assessed to determine their influence on freshwater smolt production, smolt outmigration, and marine survival. Greater first-year marine growth was correlated with higher BY total return and productivity for Chinook Salmon from the Chilkat River and higher BY marine survival for Chinook Salmon from the Stikine River. Daily smolt outmigration of Chilkat River Chinook Salmon was positively correlated to water temperature and negatively correlated to discharge (Deviance explained = 68.5%), while timing of the start of outmigration was influenced by nearshore sea surface temperatures (R² = 0.57) and timing of the mid and end points were positively related to smolt length (R² = 0.72 and 0.34, respectively). Freshwater smolt production was negatively correlated to parr length and fall discharge and positively correlated to spring temperature and discharge (R²adj= 0.52). Marine survival of Stikine River Chinook Salmon was significantly related to smolt size (R² = 0.26), while Chilkat River Chinook Salmon were positively related to migration timing and smolt length and negatively related to discharge (R² = 0.5). These results support the importance of the early marine period in determining year-class strength and highlight the variation in mechanisms that influence recruitment success of Chinook Salmon stocks.
