Following the expansion of hydropower in the Swedish north during the 20^th century, wild salmon populations became extinct in most rivers, along with the disappearance of salmon fisheries that had sustained communities for millennia. The most rapid phase of this decline occurred between the 1940s and 1970s. Against this backdrop, this dissertation seeks to examine and explain how societal forces and power relations – both between humans and between humans and salmon – shaped the extinction processes of this period. To historicise these events, the analysis also considers long-term shifts in human-salmon relationships. A central finding of this dissertation is that the extinction of genetically distinct salmon populations can be understood as the culmination of a prolonged historical process within the industrial age, marked by cyclical patterns of capitalist-driven metabolic rifts in salmon reproduction cycles, social marginalisation of locals fishing salmon, and gradual decline of salmon populations. By the 1940s, plans were underway for an accelerating phase of hydropower expansion that would push most wild salmon populations to the brink of extinction. In response, intense debates arose concerning how salmon could be protected. Within this context, fisheries biologists successfully pushed the hydropower industry to finance investments in smolt breeding, aiming to sustain offshore fishery yields despite the obstruction of salmon migration to home rivers. Securing these investments did, at the same time, strengthen the financial resources and professional status of the field of fisheries biology. Ultimately, the expansion of hydropower was propelled by economic and political imperatives rooted in the belief that production and consumption must continuously grow. By theorising that capitalistic extraction has been legitimised by ideas of human superiority over nonhuman beings, this dissertation argues that such ideological frameworks led decision-makers to rationalise population-level extinctions as an unavoidable trade-off for increased industrial extraction and production. This perspective is evident in debates over salmon conservation, where salmon were framed primarily as economic assets and sources of profit, rather than as living beings with intrinsic value. The extinction processes meant that salmon, inherently bound to their home rivers yet prevented from returning, became lost to the world by being lost within it. This loss had, in turn, profound consequences for local environments, as salmon played a crucial role in ecosystem stability. For local communities, the population-level extinctions represented not only the disappearance of a vital sustenance and trade resource but also a deeper cultural loss – an erosion of traditions, identities, and knowledges tied to these fish and their lifecycles. The broader objective of this dissertation is to illuminate the global issue of accelerating biospheric impoverishment caused by human activity and its far-reaching consequences for humans, other life forms, and ecosystems. In this context, contributing with knowledge on fish is particularly relevant, as they are among the most threatened groups of living beings on the planet. The relevance of this project is further underscored by the severe decline of fish populations in Swedish waters and the fact that, despite this crisis, fish have remained a marginal topic in environmental history research on Sweden.