Background: Rising ocean temperatures and concurrent hypoxic events pose a threat to Tasmanian Atlantic salmon (Salmo salar) aquaculture. These conditions have been increasing in severity and frequency over the past decades, pushing this species to its physiological limit. Increasing environmental temperatures increases the oxygen demand of fish (metabolic rate), while hypoxia reduces oxygen availability. Together, these stressors can reduce the oxygen available in the organism (aerobic scope, AS) for aerobic activities such as digestion, leading to poor growth and physiological performance. Digestion can occupy a significant amount of the AS and competes with other fundamental aerobic demands. Consequently, decreased feed intake, growth and mortality have been observed at elevated temperatures and during periods of hypoxia. Therefore, the loss of appetite at high temperatures and low aquatic oxygen levels could be a protective mechanism to preserve the available AS.
Aims: To investigate the impact of environmental stress – chronic high temperature and acute hypoxia - on Atlantic salmon's aerobic capacity and digestion.
Methods: AS and SDA were measured by intermittent flow respirometry at 15°C and 21°C, at normoxia (above 75% air saturation) and hypoxia (1.5h exposure; ~50% air saturation).
Results and conclusion: There were no differences in the AS between temperatures under normoxia. In contrast, hypoxia significantly reduced the AS at both temperatures. As a result, digestion occupied a significantly higher proportion of the AS under hypoxia at both temperatures than under normoxic conditions. Thus, acute hypoxia exposure compromised Atlantic salmon digestion at both optimal and high temperature by limiting the AS and increasing the relative (proportional) energy costs associated with digestion.