The effects of oxygen variability during early development on the physiology of Atlantic salmon (Salmo salar)

Variable oxygen availability can challenge the performance and survival of anadromous salmon species from early development in freshwater redds to adult life in the open ocean. Salmon are also a highly valued aquaculture species, and may face similar hypoxic or hyperoxic challenges in hatcheries and sea cage rearing. This thesis investigates the physiological responses of Atlantic salmon (Salmo salar) early life stages to oxygen variability (hypoxia and hyperoxia) that is typically experienced in natural and aquaculture systems. In particular, the thesis focuses on how oxygen variability during incubation affects developmental trajectories that may cause long-term impacts. Salmon incubating in natural under-gravel redds and aquaculture incubation systems can experience oxygen levels from < 20% to 180% dissolved oxygen (DO; % air saturation). While hypoxia is known to compromise the growth and development of salmon and stimulate physiological processes to improve oxygen uptake rate (¬¿œÄvÑO\\(_2\\)) or reduce metabolism, it is unclear whether hyperoxia alleviates respiratory stress and leads to improved performance. Across two experiments, we investigated how various hypoxia and hyperoxia levels affected growth, aerobic metabolism and hypoxia tolerance of salmon from fertilisation until yolk-sac absorption. Rearing in hyperoxia had no effect on ¬¿œÄvÑO\\(_2\\) or O\\(_{2crit}\\), and a negligible effect on growth. On the other hand, salmon incubated in moderate (50% DO) or cyclical hypoxia (100-25% DO daily) grew and developed slower associated with reduced ¬¿œÄvÑO\\(_2\\) and critical oxygen level (O\\(_{2crit}\\)) prior to the eyed-egg stage. Severe hypoxia (~27% DO) caused nearcomplete mortality and deformities. Thus, during development, salmon appear most sensitive to hypoxia prior to hatching and respond to oxygen limitation by reducing their oxygen demand. There was no evidence to suggest that embryos or alevins compensated for hypoxia via mechanisms to increase oxygen supply. Hypoxia exposure during incubation can permanently affect physiology by altering the developmental trajectory, thereby impacting later life performance. Hypoxia imposes limitations on the aerobic metabolic scope that may be compensated for by physiological modifications that increase the maximum attainable ¬¿œÄvÑO\\(_2\\). We tested how moderate or cyclical hypoxia from fertilisation until the fry stage affected subsequent aerobic performance, hypoxia tolerance and haematology of juveniles reared in normoxia. In addition, aerobic performance was measured following a re-acclimation period of up to 44 days in hypoxia. Hypoxia exposure during incubation had no effect on aerobic performance, hypoxia tolerance or haematology, even following re-acclimation to hypoxia. Overall, acute hypoxia (<13 h) reduced aerobic scope, however acclimation to hypoxia (up to 44 days) increased blood-oxygen carrying capacity and reduced the limitation that acute hypoxia had on aerobic scope. The results of this thesis demonstrate that the effects of oxygen limitation on salmon during hypoxia incubation are most severe between fertilisation and hatching. However, hypoxia incubation did not appear to impact the developmental trajectory of salmon, as there was negligible impact on later life aerobic performance and hypoxia acclimation capacity. I conclude that there may be negligible evolutionary advantages to anadromous salmon modifying their long-term physiological phenotype based on the oxygen levels encountered during incubation in natural redds. However, the considerable hypoxia acclimation capacity of Atlantic salmon can alleviate the limitation that hypoxia has on aerobic performance later in life.