Abstract: Low water oxygen content (hypoxia) is a common feature of many freshwater and marine environments. However, we have a poor understanding of the degree to which diminished cardiac function contributes to the reduction in fish swimming performance concomitant with acute exposure to hypoxia, or how fish cardiorespiratory physiology is altered by, or adapts to, chronic hypoxia. Thus, we acclimated adult Atlantic cod (Gadus morhua) to either ~8–9kPa O₂ (40–45% air saturation) or ~21kPaO₂ (100% air saturation; normoxia) for 6–12weeks at 10°C, and subsequently measured metabolic variables [routine oxygen consumption (MO₂), maximum MO₂, metabolic scope] and cardiac function (cardiac output, Q; heart rate, fₕ; and stroke volume, VS) in these fish during critical swimming speed (Ucrit) tests performed at both levels of water oxygenation. Although surgery (flow probe implantation) reduced the Ucrit of normoxia-acclimated cod by 14% (from 1.74 to 1.50BLs–1) under normoxic conditions, exposure to acute hypoxia lowered the Ucrit of both groups (surgery and non-surgery) by ~30% (to 1.23 and 1.02BLs–1, respectively). This reduction in swimming performance was associated with large decreases in maximum MO₂ and metabolic scope (≥50%), and maximum fₕ and Q (by 16 and 22%), but not VS. Long-term acclimation to hypoxia resulted in a significant elevation in normoxic metabolic rate as compared with normoxia-acclimated fish (by 27%), but did not influence normoxic or hypoxic values for Ucrit, maximum MO₂ or metabolic scope. This was surprising given that resting and maximum values for Q were significantly lower in hypoxia-acclimated cod at both levels of oxygenation, because of lower values for VS. However, hypoxia-acclimated cod were able to consume more oxygen for a given cardiac output. These results provide important insights into how fish cardiorespiratory physiology is impacted by short-term and prolonged exposure to hypoxia, and further highlight the tremendous capacity of the fish cardiorespiratory system to deal with environmental challenges.
