Metabolic and functional impacts of hypoxia vary with size in Atlantic salmon

The most capricious environmental variable in aquatic habitats, dissolved O₂, is fundamental to the fitness and survival of fish. Using swim tunnel respirometry we test how acute exposure to reduced O₂ levels, similar to those commonly encountered by fish in crowded streams and on commercial aquaculture farms, affect metabolic rate and swimming performance in Atlantic salmon of three size classes: 0.2, 1.0 and 3.5 kg. Exposure to 45-55% dissolved O₂ saturation substantially reduced the aerobic capacity and swimming performance of salmon of all sizes. While hypoxia did not affect standard metabolic rate, it caused a significant decrease in maximum metabolic rate, resulting in reduced absolute and factorial aerobic scope. The most pronounced changes were observed in the smallest fish, where critical swimming speed was reduced from 91 to 70 cm s^-1 and absolute aerobic scope dropped by 62% relative to the same measurement in normoxia. In normoxia, absolute critical swimming speed (U_crit) increased with size, while relative U_crit, measured in body lengths s^-1, was highest in the small fish (3.5) and decreased with larger size (medium = 2.2). Mass specific metabolic rate and cost of transport were inversely related to size, with calculated metabolic scaling exponents of 0.65 for b_SMR and 0.78 for b_MMR. Metabolic O₂ demand increased exponentially with current speed irrespective of fish size (R² = 0.97-0.99). This work demonstrates that moderate hypoxia reduces the capacity for activity and locomotion in Atlantic salmon, with smaller salmon most vulnerable to hypoxic conditions. As warm and hypoxic conditions become more prevalent in aquatic environments worldwide, understanding local O₂ budgets is critical to maximizing the welfare and survival of farmed and wild salmon.