Digestive enzymes in fish larvae and juveniles - implications and applications to formulated diets

Age-dependent changes in digestive tract anatomy/physiology during larval-juvenile 'metamorphosis' of fish have been examined with respect to diet, biochemical characteristics and nutrient utilization. In fish larvae, basal capacity and rates of the intestine to hydrolyze and transport specific nutrients is qualitatively and quantitatively set in genetic memory, to correspond to a natural diet. With this premise in mind, we should be able to produce artificial zooplankton which would satisfy the needs of all larval fish. At first feeding, the digestive tract in most fish species contains the enzymes related to metabolism (digestion, absorption and assimilation) of molecules such as proteins, lipids and glycogen. The enzymes' activity has been observed to be relatively low compared with adult fish levels. Each enzyme develops independently during ontogenesis, with variation related to fish species and temperature. It has been suggested that the live food (i.e. zooplankton) organisms consumed by the larvae assist the digestion process by 'donating' their digestive enzymes, either by autolysis or as zymogens that activate the larval endogenous digestive enzymes. Live food organisms also contain gut neuropeptides and nutritional 'growth' factors which enhance digestion. These substances are frequently omitted in formulated diets. Moreover, particulate diets for larvae contain proteins and other ingredients that are difficult to digest, (especially since formulated diets are 60-90% dry matter while zooplankton is only 10%). Based on this hypothesis, the inclusion of different digestive enzymes, especially proteases, in the diets for fish larvae resulted in significantly improved nutrient utilisation and performance of the larvae; however, these were still not at the level of live food-fed larvae. The next step, inclusion of pre-hydrolysed proteins in the diets, gave mixed results depending on percentage of hydrolysate and larval age. The effect of inclusion of digestive system neuropeptides in formulated diets was investigated with different fish species in recent years. The results suggest that inclusion of bombesin may increase assimilation of diets and larval growth. However. other trials with juvenile fish have shown no effect of these additions of the neuropeptide. Future research needs to be focused upon deriving a greater knowledge of ontogenic development of the larval and juvenile digestive systems and the application of this knowledge to diet formulation. (C) 2001 Elsevier Science B.V. All rights reserved.