The maternal effect of differences in egg size influence metabolic rate and hypoxia induced hatching in Atlantic salmon eggs: implications for respiratory gas exchange across the egg capsule

The maternal effect of fish egg size has profound implications for oxygen transfer across the egg shell surface, and therefore metabolism, especially under adverse environmental conditions like hypoxia. We found that metabolic rate (MO₂) of Atlantic salmon alevins was higher than of eggs in normoxia and hypoxia. Equally, the MO₂ of smaller eggs from maiden spawners was lower than that of larger eggs from repeat spawners. Critical partial pressure of oxygen (PO₂) for hatching was lower in eggs from repeat spawners. Generally, the PO₂ within the egg capsule was lower for a given ambient PO₂ and decreased further with hypoxia, and the internal PO₂ was higher, the bigger the volume of the egg. Therefore, we conclude that the egg capsule poses a major barrier to oxygen exchange for the mature embryo that is more severe in eggs from maiden spawners than in eggs from repeat spawners. This was corroborated by a more advantageous egg surface area to MO₂ ratio in eggs from repeat spawners. These findings challenge the "bigger is worse during incubation" hypothesis.