Physiological effects of dissolved oxygen are stage-specific in incubating Atlantic salmon (Salmo salar)

Oxygen availability is highly variable during salmonid incubation in natural redds and also in aquaculture incubation systems. Hypoxia generally decreases growth and aerobic metabolism prior to hatching, in parallel with eliciting physiological modifications that enhance oxygen delivery. However, it is less-well known whether developmental hyperoxia can drive the opposite effect. Moreover, there is insufficient understanding of stage-specific developmental windows during which ambient oxygen availability may be of greater or lesser impact to incubating embryos. Here, we tested the effects of hypoxia (50% dissolved oxygen: DO, % air saturation) and hyperoxia (150% DO) on the growth, routine aerobic metabolism (ṀO_2rout) and hypoxia tolerance (O_2crit) of Atlantic salmon (Salmo salar) during seven developmental windows throughout incubation. Embryos exposed to hyperoxia (150% DO) did not differ from the normoxic group in growth, ṀO_2rout or O_2crit at any developmental window. In contrast, embryos exposed to hypoxia grew slower and had a lower ṀO_2rout, but had higher hypoxia tolerance (lower O_2crit) than normoxic and hyperoxic counterparts. Interestingly, these differences were only apparent when the embryos were measured prior to hatching. Larvae (alevins) incubated in hypoxia following hatching grew similarly to normoxia-incubated alevins. Our results provide evidence that Atlantic salmon embryos are most sensitive to hypoxia prior to hatching, probably due to increasing (absolute) oxygen requirements concurrent with restricted oxygen diffusion through the egg. Moreover, the similarities between normoxia- and hyperoxia-incubated salmon demonstrate that embryos are not oxygen-limited under normoxic conditions.