| Summary: | Thesis (Master's)--University of Washington, 2020 Complex ecological processes determine whether and how fish species survive in the ocean. For some fishes such as Pacific salmon (Oncorhynchus spp.), these processes are thoroughly studied and modeled. For eulachon (Thaleichthys pacificus), however, these processes are more mysterious despite their trophic link to salmon and similar anadromous life history. The ocean ecology of eulachon was identified by the National Oceanic and Atmospheric Administration (NOAA) as a key knowledge gap and research priority for eulachon, which were listed under the Endangered Species Act as a threatened species in 2010 after suffering significant declines throughout the southern portion of their range. Eulachon are a culturally and historically important species for Native American and First Nations people, as well as a critical component of freshwater, estuarine, and marine food webs. So, a decline in their abundance has motivated researchers to better understand what drives fluctuations in their populations, despite a lack of data in the marine environment. The marine ecology of eulachon may be relatively understudied, but data collected and compiled for salmon can partially fill this gap. NOAA developed ocean ecosystem indicators (physical, chemical, and biological factors) that are used to predict salmon returns. I applied these indicators to a new question: are the indicators used for salmon also predictive for a trophically related and ecologically similar species? And what environmental and biological factors in the ocean drive fluctuations in eulachon abundance in a major spawning basin, and when? Using multivariate analyses, I found that ocean ecosystem indicators in years of ocean residency are correlated with eulachon abundance in the Columbia River. Large-scale and bottom-up indicators such as the status of the Pacific Decadal Oscillation and prey abundance describe much of the variation in eulachon abundance. Time series analysis also indicates eulachon abundance ...
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