The Role Of Modern Concepts Of Fish Digestion In Aquaculture

V.V. Kuz'mina
Institute of Biology of Inland Waters, Russian Academy of Sciences, Borok, Yaroslavl, Russia

Abstract

New technologies used in aquaculture, assume an employment of a wide spectrum of the approaches to work out a problem of artificial forage formula, and also search of optimal combinations of artificial forages and components of natural food. The expediency of the use of the latter should be based on modern concepts of feeding fish physiology. According to modern theories, there exist five major types of digestion in fish: cavital, intracellular, membrane, symbiont and induced autolysis. The characteristics of the digestive hydrolases effecting these processes vary between fish of different ecological groups. The alterations in enzyme activity with age are mainly conditioned by changes in fish feeding spectrum, while the seasonal dynamics of enzyme activity depend on the type of fish feeding and the position of the enzyme in the chain of hydrolases. The differences in seasonal dynamics between the digestive hydrolases are conditioned by differences in the kinetic characteristics of the enzymes (t 0-function. ?act, Vmax, ?m) and in the lipid fatty acid content of the intestinal mucosa.
Finding of the mechanism of induced autolysis, realizing processes of victim autodegradation by various enzymes of victim tissues (mainly lysosomal hydrolases), and appreciation of its role in digestion of fish have allowed to explain high efficiency of initial stages of food assimilation in fish from natural populations. So, ensyme activity of a victim depending on a type of fish feeding can account to 50 % up to 800 % of activity of the same hydrolases of assimilative organisms. In some cases, for example in a pike, the activity of a victim can make up to 94 % of total acid proteinase activity of stomach mucosa and victim tissues.
Besides the possibility of participation of microflora in degradation of various components of fish food (major, minor, specific) was proved. The possibility of induced autolysis mechanism realization in fish from natural ecosystems, and also symbiont digestion realized by enzymes of enteral microflora significantly increases efficiency of the initial stages of food assimilation. In spite of the fact that now correct estimation of the contribution of microflora enzymes to processes of digestion is absent, the elaboration of forage optimal formula for different species of aquaculture fish should be based on the basic postulates of the theory of an adequate feeding taking into account endoecology feature of a digestive tract in different species of fish.
Thus, it is necessary to estimate not only structure of major and minor components of food objects of wild forms of fish-breeding objects, but also ratio of the same hydrolase activity of fish alimentary tract as well as potential victim and enteral microflora. The special attention should be given to adequacy not only protein and lipid content, but also ratio of mono-, oligo- and polymers in structure of these components in different fish species and age groups forages, as well as the structure of microbial communities of enteral medium creating a secondary flow of nutrients. Besides it is important to take into consideration the presence of the factors in forages lowering their food value (conserve substances, bacterial tiaminase, high concentration of heavy metals etc.) due to decrease of digestive hydrolase activity and suppresse of epithelial transport systems.
This work was carried out at partial support of Russian Fund of Basic Research (grant N 01-04-49120).