The primordial inflation of swimbladder normally occurs at a size of 4 to 5 mm in sea bream Sparus auratus and 5 to 6 mm in sea bass Dicentrarchus labrax . However, in intensive larval rearing conditions, lack of inflation is often observed and leads to the production of up to 80% of postlarvae whose swimbladder is not functional. They are, on the average, 20-30% smaller in weight than normal fry. Their survival rate can also be reduced by any kind of stress such as weaning, handling or hypoxic conditions. It is suggested that the selective mortality which affects the abnormal fish results both from a decrease in predatory efficiency and an increase in energetic needs.