The effects of constant and cyclical hypoxia on the survival, growth and metabolic physiology of incubating Atlantic salmon (Salmo salar)

Salmonids reared in aquaculture incubation systems are susceptible to periods of cyclical or fluctuating hypoxia caused by embryo crowding, water flow rates and management protocols. Hypoxia during early development can reduce salmon growth, development and survival, and delay hatching, potentially impacting future performance and survival in subsequent phases of life. However, salmon embryos can also limit the effects of hypoxia by metabolic depression, premature hatching or physiological modifications to improve oxygen delivery. Here, we investigated the effects of constant and cyclical hypoxia on the growth, development, hatching, survival, oxygen uptake rate (ṀO2) and hypoxia tolerance (O2crit) of Atlantic salmon (Salmo salar) from fertilisation, through hatching, until 113 days post-fertilisation (DPF). We incubated salmon in either normoxia (100% dissolved oxygen [DO; as percent air saturation]), moderate hypoxia (∼63% DO), severe hypoxia (∼27% DO) or cyclical hypoxia (∼100-25% DO daily). At the eyed-egg stage routine oxygen uptake (ṀO2rout) was reduced by ~20% in salmon from the cyclical and moderate hypoxia treatments, although reduced ṀO2rout was not associated with a reduction in O2crit. Moderate hypoxia did not affect growth, development, survival or hatching. However, at 113 DPF cyclical hypoxia-incubated salmon were ∼12% smaller and developmentally delayed by ∼5 days compared to normoxia-incubated salmon as a result of reduced growth and development prior to the eyed-egg stage (∼39 DPF). Survival was unaffected by moderate or cyclical hypoxia, but hatching was delayed by ∼2-3 days in cyclical hypoxia incubated salmon. Severe hypoxia produced deformed embryos, greatly decreased growth and development and resulted in 99.3% mortality by 113 DPF. This study demonstrates that Atlantic salmon have a considerable capacity to survive daily bouts of extreme hypoxia, however, the consequent reductions in growth, development and hatching may still have implications in aquaculture systems