Exploring the Migratory Life-Histories of Pacific Salmonids

Variation in life-history traits within and across species is known to reflect adaptations to different environmental drivers through a diversity of mechanisms. Trait variation can also help buffer species and populations against extinction in fluctuating environments and against anthropogenic disturbances. This recognition has led to a growing interest in applying evolutionary principles to inform eco- logical restoration actions. In this dissertation, I explored how widespread variation in migratory life-histories in Pacific salmonids (Oncorhynchus spp.) is driven by a suite of environmental factors with the goals of shedding light on what drives the frequency of these migratory strategies and creating a restoration framework to aid in the restoration of these imperiled species. In Chapter 1, I examined how anthropogenic disturbance in the form of stream modification influences the frequency of anadromy in steelhead (O. mykiss) across California. I found that with an increasing number of instream barriers, the frequency of anadromy was reduced. I then developed an evolutionary restoration framework to guide managers and encourage broader consideration of in situ evolution during the development of habitat restoration projects. In Chapter 2, I quantified distinct juvenile migratory strategies of Chinook salmon (O. tshawytscha) and modeled the environmental drivers that influence their frequency. I found two distinct migratory strategies were widespread across these populations and their frequencies were driven a juvenile density, rearing habitat, and flow. In Chapter 3, I examined what environmental and demographic factors influenced spring juvenile migratory strategies in coastal coho salmon (O. kisutch). I found that juvenile density, flow, and latitude are strong drivers of these migratory behaviors. As in Chinook salmon, a density-dependent mechanism also appears to operate in coho salmon populations, suggesting a form of bet-hedging might be common across out-migrating Pacific salmon. My dissertation links life-history evolution, environmental variation, and anthropogenic modifications to the landscape, demonstrating that alternative life-history strategies are common in Pacific salmonids and suggesting restoration approaches that can be used to conserve this variation