Early life in low oxygen: Salmon survival strategies

Oxygen content in water is much lower and more variable than in air, providing a unique challenge for aquatic organisms to maintain the delivery of oxygen to their tissues to provide the energy for life. Incubating Atlantic salmon embryos and alevins (post-hatch embryos) are unable to move from undesirable conditions, meaning that they must meet the challenge in other ways. 

In the wild, incubating salmon in under-gravel nests face a range of oxygen conditions throughout their incubation. In aquaculture, salmon can also face varying oxygen conditions depending on the rearing system. Variable oxygen conditions can potentially impact development, growth and survival of salmon. It can also cause physiological changes that reduce the impact of low oxygen conditions. In two studies supported by Salmon Enterprises of Tasmania (SALTAS) we investigated how low oxygen conditions effect Atlantic salmon at different stages throughout their development (published in Journal of Comparative Physiology B), and how variable oxygen conditions may have an impact (published in Aquaculture). 

Overall, salmon are extremely well adapted to the challenges of low and variable oxygen conditions. Oxygen levels down to 50% of normal levels do not affect embryo survival but reduce growth and development, especially before hatching. This is most likely due to the egg capsule barrier slowing oxygen diffusion to the embryo. 

Most embryos cannot survive at oxygen levels 25% of normal levels, but survival is unaffected during daily dips to the same level. From our measurements of metabolic rate and hypoxia tolerance, we suggest that the ability for Atlantic salmon to survive such challenging oxygen conditions is due to reductions in metabolic oxygen demand (metabolic depression) in the face of low oxygen conditions. However, more work would be required to confirm this hypothesis. 

Atlantic salmon are physiologically well equipped to handle the typical variations in oxygen conditions that can be experienced throughout incubation, meaning that survival is unaffected even during transient periods of severe low oxygen. However, optimising oxygen levels throughout incubation will help to achieve maximum growth and development. 

CSIRO Aquaculture contact
Dr Andrew Wood  – Experimental Scientist