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, digesti on and assimilation of the MD. This paper reviews recent advances in MD dev elopment on the gilthead seabream with special emphasis on studies that iso lated, 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 th e fish were exposed to the visual and chemical stimuli of various concentra tions of Artemia nauplii. The free amino acids (FAA) alanine, glycine and a rginine and the compound betaine were identified from the Artemia rearing m edium 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 la rval seabream. Moreover, dietary PC appears to have in parallel and/or in t andem a postprandial enhancing effect on lipoprotein synthesis, resulting i n improved transport of dietary lipids from the mucosa of the digestive tra ct 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 gu t zymogens. Seabream larvae ingesting MD supplemented with porcine pancreat ic extract (0.05% DW diet) showed a 30% increase in assimilation and demons trated significantly (P < 0.05) improved growth. Older seabream larvae show ed 6.75 times more radioactivity in tissue lipids when fed C-14-triacylglyc erol (TAG)-labeled MD supplemented with porcine lipase, while younger larva e 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 s uggested that mobilizing the native endocrine factors associated with the f eeding and digestive processes could improve MD performance in gilthead sea bream and other species by maximizing its utilization. (C) 2001 Elsevier Sc ience B.V. All rights reserved.