Influence of Environmental Variability on Chinook Salmon and Coho Salmon Population Dynamics

Understanding how different populations respond to a variable environment is necessary to anticipate and evaluate population persistence as environmental conditions change, possibly through climate change. To understand specifically how atmospheric and oceanographic processes translate to fluctuations in age-structured populations requires knowledge of temporal and spatial variability patterns in both the population and environmental signals, and a mechanistic understanding of how that variability acts in age-structured populations. Recruitment and abundance in many Pacific salmon populations covary with indices of ocean productivity; however, exactly how environmental forcing interacts with population dynamic mechanisms to produce fluctuations remains unclear, which impedes effective management and conservation. This dissertation examines how patterns of ocean survival in Chinook salmon (Oncorhynchus tshawytscha ) have changed in recent decades (Chapter 1), identifies a new link between changes observed in low-frequency Pacific oceanographic and climatic conditions that affects both coho salmon (O. kisutch) and Chinook salmon (Chapter 2), and describes how the power spectrum of environmental variability in survival rates influences population persistence in an age-structured population model using spring-run Chinook salmon from Butte Creek, California as an example (Chapter 3). In Chapter 1, I show that the spatial and temporal covariability in Chinook salmon ocean survival rates has increased along the west coast of North America from 1980 to 2006. In Chapter 2, I show that the dominant mode of variability in ocean survival of both Chinook and coho salmon covaries with the North Pacific Gyre Oscillation, an important index of ecosystem productivity in the northeast Pacific Ocean that is linked to changing climatic forcing in the tropical Pacific Ocean. In Chapter 3, I show that the spectrum of environmental variability matters in understanding population variability and extinction risk of age-structured populations in terms of cohort resonance.