Comparison of growth, digestive system maturation and skeletal development in sea bass larvae reared in an intensive or a mesocosm system

Abstract

The quality of development in intensive or mesocosm hatchery-reared Dicentrarchus labrax larvae was investigated using physiological indicators assessing ontogeny. Larvae were reared in intensive (120 L tanks) and in mesocosm systems (20 m(3) enclosures) with the same feeding sequence, excluding the wild zooplankton naturally available in mesocosms. Faster growth was recorded since early development [16 day after hatching (DAH)] in the mesocosm. Maturation of the digestive system also occurred earlier as indicated by the higher amylase secretion ratios, the intestinal maturation index (alkaline phosphatase/leucine-alanine peptidase and aminopeptidase-N/leucine-alanine peptidase ratios) and the more developed intestinal epithelium at 23 DAH. Nevertheless, the delay in digestive maturation in the intensive system seemed retrieved within few days. In both the groups, the number of vertebrae ranged between 24 and 26, with the dominant class being 25 vertebrae. However, the distributions differed between treatments for meristic characteristics, ossification stages and incidence of malformation types. Loss of a vertebra was more frequent in the intensive system, while the appearance of an additional vertebra was more frequent in the mesocosm. Ossification at 37 DAH was also more advanced in the mesocosm in addition to a lesser rate and severity of skeletal malformations. It is suggested that the early nutritional contribution of mesocosm wild zooplankton, yet at densities of 0.2-0.7 prey mL(-1), had key effects on larvae development since the early stages.