Population dynamics of Chinook salmon Oncorhynchus tshawytscha relative to prey availability in the central California coastal region

Mortality during the first period at sea is thought to be a primary determinant of salmon productivity and return rates. Here, we test this hypothesis by linking variation in prey resources during the initial phase at sea with measurements of central California Chinook salmon Oncorhynchus tshawytscha diet, condition, and later adult abundance. Specifically, we investigate linkages between the distribution and abundance of krill and other prey with juvenile Chinook salmon diet and body condition. Hydrographic features of the Gulf of the Farallones during May and June were related to the abundance and spatial organization of Chinook salmon prey. When upwelling was reduced, there were fewer krill on the inner Gulf of the Farallones shelf, thereby less available to outgoing juvenile Chinook salmon smolts. Notably, we found a 1 yr lag in the relationship between the abundance of adult Thysanoessa spinifera and the volume of krill in the diet of juvenile Chinook salmon. Body condition of juvenile Chinook salmon was positively related to the abundance of adult krill the year before and specifically to the proportion of T. spinifera in the diet. In turn, the condition of juvenile Chinook salmon was correlated to the abundance of mature Chinook salmon returning from the same cohort the next year. This information may be useful for fisheries management by improving sibling-based forecasting models as well as informing escapement goals.