On-growing Atlantic salmon (Salmo salar L.) acclimate behaviourally and physiologically to chronic hypoxia, but exhibit reduced feed intake, growth and lipid retention

The present study examines the effects of long-term (∼3 months) exposure to mild (60 % dissolved oxygen, DO) and moderately low (50 % DO) hypoxic conditions on growth performance, feed intake, nutrient utilization, and physiological acclimation of >1 kg on-growing seawater-phase Atlantic salmon. The experimental conditions were designed to study the behavioral and physiological mechanisms of acclimation to long-term hypoxia in Atlantic salmon. Fish exposed to reduced oxygen levels exhibited physiological acclimation to hypoxia by decreasing feed intake. Feed intake was significantly lower in the groups maintained at 60 % and 50 % DO, averaging at 72 % and 60 %, respectively, of the intake observed in the normoxic control group (90 % DO). This reduction in feed intake was acompanied by a significantly lower growth performance in the low DO groups, with final biomass in the tanks in the mild and moderately low DO groups reaching approximately 82 % and 74 %, respectively, of that recorded in the normoxic control group. Despite the reduced feed intake, feed efficiency remained unaffected, and nutrient retention was only marginally influenced, with the exception of a slight reduction in lipid retention in the hypoxia-exposed fish. Physiological responses to hypoxic conditions included elevated hematocrit (Hct) and increased ventilation frequency, indicating acclimation by enhancing branchial oxygen uptake and improving the blood oxygen transport capacity. The hepatosomatic index (HSI) was significantly lower in fish exposed to hypoxia, suggesting an effect of DO levels on hepatic energy storage and/or mobilisation. A significant incidence of sexual maturation was observed across all treatments, particularly among males, although with no correlation between oxygen levels and maturation rates. This suggests that reproductive investment was likely initiated prior to the onset of the trial and was not affected by subsequent hypoxic constraints. Overall, the findings confirms that Atlantic salmon possess the plasticity necessary to acclimate to prolonged exposure to suboptimal DO levels through behavioral and metabolic adjustments aimed at optimising oxygen utilization. However, the associated reduction in feed intake and growth performance underscores the importance of maintaining optimal oxygen conditions in aquaculture settings to support high productivity. Understanding these mechanisms of acclimation can inform management strategies to mitigate performance losses during periods of low DO in commercial operations. • Atlantic salmon exposed to long-term hypoxia reduced feed intake and growth. • Feed efficiency was not affected by hypoxia. • Nutrient efficiency was not affected, apart from a small decline in lipid retention. • Fish acclimated to hypoxia with increased hematocrit and ventilation frequency.