Marine ecosystem perspectives on Chinook salmon recruitment: a synthesis of empirical and modeling studies from a California upwelling system

We review the suite of biophysical factors in the Northeast Pacific Ocean basin and California Current shelf ecosystem that directly or indirectly relate to central California Chinook salmon Oncorhynchus tshawytscha growth and survival upon ocean entry, a critical life-history period for this population. Our synthesis provides a framework for integrating ecosystem process studies with empirical hypothesis testing to benefit fisheries management. Our hypothesis includes seasonality (phenology) as a key element of early salmon growth and survival. The strength and location of the North Pacific High (NPH) pressure system in winter influences salmon growth and survival via 'bottom-up' productivity and retention of key prey (euphausiid crustaceans and juvenile rockfishes Sebastes spp.) in nearshore habitats prior to and during salmon emigration to sea in spring. Prey retention, which is associated with increased consumption of krill and juvenile rockfishes, and is positively correlated with juvenile salmon body condition and ocean survival, appears to set cohort strength and return rates. We examined these mechanistic relationships by reviewing the results of a biophysical model coupled to an individual-based model for salmon. Our review results in a final hypothesis stating that early salmon growth and survival are positively related to intensity of early season upwelling, and associated (forage) nekton production and retention on the shelf during spring and summer.