Centennial-scale influence of climate, fishing, and artificial production on salmon population dynamics

Pacific salmon (Oncorhynchus spp.) are common and abundant species with substantial population and life-history diversity. While several studies have documented shifts in salmon abundance and diversity over the last several decades, it has remained a challenge to understand changes over longer periods, such as the last century of major human impacts. Here, I use sockeye salmon (O. nerka) in the Skeena River watershed as a case study to quantify how population-specific abundance and diversity have responded to fishing, artificial production, and climatic change over the last century. In Chapter 2, I demonstrate that contemporary period data are at an all-time low, resulting in an inability to assess the health of one-half of salmon populations returning to British Columbia's mid coast, including over 50% of Skeena sockeye populations. Importantly, 40% of populations assessed as at-risk would have improved in status had Canadian fisheries exploitation been reduced. In Chapter 3, I show that sockeye populations were 56% to 99% larger one century ago, and that fisheries selectivity of larger-bodied populations is the most probable driver of differences in rates of decline between populations. In Chapter 4, I add 1933-1947 baseline data that, when combined with 1913-1923, demonstrate that the total number of wild sockeye now returning to the Skeena is 69% lower than during the historical era. While artificial enhancement has returned aggregate abundances to historical levels, declines across all wild populations has increased the dependency of fisheries on enhanced fish, and decreased the provisioning of salmon to local fisheries and ecosystems. In Chapter 5, I quantify the influence of competition and climate as filtered by habitat on the response diversity of sockeye populations, and model future potential effects of warming temperatures on fish growth. While growth was lower in years with high temperatures and intraspecific competition, population-specific habitat played an important role in modulating the negative effect of temperature. Collectively, this body of work improves our understanding of the magnitude of different responses of populations to a century of change in climate, fishing, and artificial production, which can help realize the potential resilience of salmon in watersheds like the Skeena to future environmental change.