Acute but not chronic hyperoxia increases metabolic rate without altering the cardiorespiratory response in Atlantic salmon alevins

Hyperoxia has been shown to affect growth, survival and cellular homeostasis in fish. Previous findings on chronic hypoxia (low environmental O₂) exposure in Atlantic salmon alevins indicate that re-exposure to normoxic conditions after hypoxia exposure (relative hyperoxia) elevates metabolism above normal. Hence, we here investigated whether acute hyperoxia above normoxic conditions also alters O₂) uptake and whether chronic hyperoxia affects O₂) uptake under normoxic and hypoxic conditions. To this end, the effects of acute and chronic hyperoxia exposure on metabolic rate and cardiorespiratory function (heart rate and ventilation rate) in Atlantic salmon (Salmo salar) alevins incubated at 4 °C were investigated, and how it is affected by an increase in ambient temperature (4 °C and 8 °C).

Hyperoxia (15–18 days at 28 kPa) reared alevins display advanced development compared with normoxia incubated animals. While acute hypoxia generally leads to metabolic depression (≈70%, 21 kPa compared with 5 kPa), acute hyperoxia (28 kPa) causes hypermetabolism (≈30% compared with normoxia at 4 °C and ≈20% at 8 °C). Chronic hyperoxic rearing on the other hand did not alter metabolic rate at 4 °C or 8 °C in acute hyperoxia, normoxia or hypoxia. Heart rates and ventilation rates were also unaltered with acute hyperoxia and were unaffected by chronic hyperoxia exposure. It is shown that acute hyperoxia increases O₂) uptake above normoxic conditions but chronic hyperoxia does not result in long term physiological changes. This adds further proof that O₂) uptake is not limited by O₂) transport capacity but by O₂) availability at this developmental stage in salmonids.