Multi-scale environmental forcing of pacific salmon population dynamics

Understanding how environmental forcing governs the productivity of marine and anadromous fish populations is a central, yet elusive, problem in fisheries science. In this thesis, I use a cross-system comparative approach to investigate how environmental forcing pathways could link climatic and ocean processes to dynamics of Pacific salmon (Oncorhynchus spp.) populations in the Northeast Pacific Ocean. I begin by showing that phytoplankton phenology and ocean current patterns are both strongly associated with inter-annual changes in salmon productivity, suggesting that two alternative environmental pathways may contribute to changes in salmon productivity: one mediated by vertical ocean transport and subsequent phytoplankton dynamics and the other mediated by horizontal ocean transport and subsequent advection of plankton into coastal areas. The relative importance of these pathways, however, may vary over large spatial scales because the magnitude and direction of the estimated environmental effects on productivity were conditional on the latitude of juvenile salmon ocean entry. I then use a probabilistic network modeling approach to show that changes in climatic and ocean processes can impact salmon productivity via multiple concurrent environmental pathways, including multiple pathways originating from the same climatic process. Finally, I use policy analysis to demonstrate why efforts to integrate highly migratory species, such as Pacific salmon, into ecosystem-based management policies need to explicitly account for mismatches between the scale of ecosystem services provided by these species and the scale at which human activities and natural processes impact those services. Collectively, my thesis provides empirical evidence that accounting for spatial heterogeneity and the relative importance of simultaneously operating environmental pathways may be critical to developing effective management and conservation strategies that are robust to future environmental change.