Substitution of live food by formulated diets in marine fish larvae

Until recently, it was considered impossible to feed newly hatched marine fish species with a compound diet. Substituting a compound diet for live prey was performed several weeks after hatching, depending on the species. Compound diets were well ingested at the early stage but larvae died with a gut full of food, suggesting that larvae were unable to digest the compound diet. The hypothesis was that younger larvae have insufficient digestive enzymes to thrive on compound diets, and that exogenous enzymes, provided from live prey, are necessary for early stages. The organogenesis of marine fish larvae is not completely achieved at hatching and histological studies have revealed that the anatomy of the digestive tract undergoes developmental changes over some weeks. Nevertheless, biochemical studies over 20 years have shown that most of the digestive enzymes are present in young larvae. Recent studies have provided better understanding of digestion mechanisms in larvae and have led to proposed dietary compositions meeting larvae nutritional requirements. Pancreatic digestive enzymes are detected before mouth opening. Their synthesis is not induced by diet ingestion, but secretory mechanisms in the pancreas, and so enzymatic action, become efficient chronologically after those of synthesis. Inadequate diets can delay the onset of secretion mechanisms. The ratio of secreted enzymes to total enzymes indicates the nutritional value of the diet ingested by the larvae. At the intestinal level, cytosolic enzymes, which are peptidases, exhibit high activity in the early stages, suggesting a high capacity in larvae to digest protein hydrolysate. Indeed, larvae growth and survival is improved by the incorporation of a moderate concentration of peptide or hydrolysate in the diet. Peptidase activity abruptly decreases around day 25 in sea bass, concurrent with an increase in enzymes of the brush border membranes. This corresponds to a normal maturation process of enterocytes. Compound diets can slightly delay the onset of this maturation process, and inadequate diet can prevent it, leading to near death of the larvae. A proper onset of the maturation process has been associated with high larvae survival. The early developmental stage larvae exhibit high hydrolytic capacity, related to their weight. Enzyme activity pattern is age-dependent, but can be modulated by diet composition. Thus, larvae have the ability to digest and thrive on compound diet, if this diet is well adapted. Larvae have different specificities in digestion and nutritional requirements when compared to juveniles. Taking these specificities into consideration, recent research has led to the formulation of a compound diet that was well adapted for larvae from mouth opening, and could totally replace live prey.