Advances in the development of microdiets for gilthead seabream, Sparus aurata: a review

The performance of microdiets (MDs) for larvae of marine fish is frequently improved when they are co-fed with Artemia. This suggests that nutritional factors in the live food are positively influencing the ingestion, digestion and assimilation of the MD. This paper reviews recent advances in MD development on the gilthead seabream with special emphasis on studies that isolated, identified and tested these live food factors in MD with the aim of improving their performance. MD ingestion rates in gilthead seabream larvae increased up to 120% when the fish were exposed to the visual and chemical stimuli of various concentrations of Artemia nauplii. The free amino acids (FAA) alanine, glycine and arginine and the compound betaine were identified from the Artemia rearing medium as metabolites, which stimulated this larval response. Similarly, MD supplemented with phospholipids (PL), particularly phosphatidylcholine (PC), stimulated feeding activity and was consumed up to 45% better in young larval seabream. Moreover, dietary PC appears to have in parallel and/or in tandem a postprandial enhancing effect on lipoprotein synthesis, resulting in improved transport of dietary lipids from the mucosa of the digestive tract to the body tissues. Live food may also contribute exogenous enzymes to the digestion process or provide factors that stimulate larval pancreatic secretions or activate gut zymogens. Seabream larvae ingesting MD supplemented with porcine pancreatic extract (0.05% DW diet) showed a 30% increase in assimilation and demonstrated significantly (P < 0.05) improved growth. Older seabream larvae showed 6.75 times more radioactivity in tissue lipids when fed C-14-triacylglycerol (TAG)-labeled MD supplemented with porcine lipase, while younger larvae demonstrated no improved assimilation. Factors in live Artemia may influence digestion by stimulating an endocrine response. This was shown when Artemia consumed by seabream larvae elicited a 300% increase in the level of the digestive hormone bombesin compared to levels in larvae given only a MD. On the other hand, liposomes containing the FAA methionine ingested by halibut juveniles elicited higher levels of the digestive hormone cholecystokinin (CCK) compared to juveniles ingesting liposomes containing physiological saline or fish extract. These studies suggested that mobilizing the native endocrine factors associated with the feeding and digestive processes could improve MD performance in gilthead seabream and other species by maximizing its utilization. (C) 2001 Elsevier Science B.V. All rights reserved.