Aerobic scope is sustained through a heatwave in juvenile Atlantic salmon (Salmo salar).

Aquatic ectotherms are vulnerable to heatwave-induced physiological stress, which arises from increased energy demands and reduced dissolved oxygen content in warmer waters. Understanding thermal physiology is critical for predicting how commercially and ecologically important populations could be affected by the increasing risk of rising temperatures. Heatwave risk assessments often examine extremities of time scales: immediate impacts or long-term consequences. However, little is known about how consistently increasing mid-term thermal stress shapes aerobic performance in commercially important species such as Atlantic salmon (Salmo salar), which may face heat stress in rivers, especially at juvenile life stages. By measuring how salmon juveniles manage their aerobic capacity at 16, 19 and 22°C using intermittent flow respirometry, we test if their thermal performance curve declines at temperatures commonly occurring during heatwaves. Whole-animal metabolism was measured from control individuals kept at 16°C before and after the heatwave, and after 4-5 days exposure at 19 and 22°C during the heatwave. We show standard metabolic rate increases with temperature, but maximum metabolic rate and aerobic scope do not change between these temperatures. These findings suggest that juvenile Atlantic salmon may have limited capacity to increase aerobic performance during moderate heatwaves, leaving them vulnerable to cumulative effects of oxygen limitation to vital functions such as growth and stress responses. As climate change intensifies, incorporating thermal performance curves into conservation strategies can be used for predicting population resilience and informing effective management.