Nutrition and related ontogenetic aspects in larvae of the African catfish, Clarias gariepinus

The absence of adequate techniques for rearing fish larvae constitutes a bottleneck for sustainable aquacultural growth. Important constraints are the tiny size of the larvae, the dependance on live food organisms and the developmental stage of the fish larvae. The development of dependable rearing techniques requires knowledge of the critical aspects in nutrition in relation to the development of the digestive and metabolic system. In the present study, this relation between fish nutrition and development was investigated using the African catfish Clarias gariepinus (Burchell) as research model. The study was subdivided into three phases, dealing respectively with husbandry conditions, developmental aspects and with the lipid metabolism. During the first phase, in a comparative study with different dry diets, the dependance of African catfish larvae on live food organisms (e.g., Artemia ) was demonstrated. A technique to estimate quantitative food rations was developed and used to elucidate the quantitative relations between food ration, water temperature and growth. African catfish larvae can be successfully weaned to a dry diet without growth loss provided that they receive live food organisms during the first four days of exogenous feeding. Durint the second phase of the project, the morpho-functional development of the digestive system and the activities of metabolic enzymes were investigated. The results indicated that at the onset of exogenous feeding, African catfish larvae can eat, digest, absorb and metabolize the absorbed nutrients. The changes in feeding habits were correlated with the development of stomach functions. Together with this functional development of the stomach, several other ontogenetic changes in the larval physiology occurr. This combined change of ontogenetic events occurs at a body size of approximately 10-20mg. Therefore this size is suggested as a marker for the end of the larval period in C. gariepinus . The different studies on the lipid metabolism, carried out during the third phase, showed that the eggs of African catfish contain moderate amounts of lipid of a high polar nature (70- 75% of the lipid consist of phosphatidylcholine, PC). PC is an important energy source during the early life stages of this species. African catfish is capable to biosynthesize important n -3 highly unsaturated fatty acids (HUFA) from the yolk sac period onwards. Dietary enrichment with n -3 HUFA does affect liver ultrastructure. It does not influence growth and survival. Consequences of these results for defining an optimal feeding and rearing strategy for larval fish are discussed.