Feeding habits and food utilization
The importance of feeding habits and feed efficiency in terms of growth and production was pointed out. Most forms of traditional animal aquaculture rely largely on the production of foods through natural processes, or by fertilization and water management in enclosed areas. To a certain extent, this practice is still followed in extensive and semi-intensive pond farming, but supplementary feeding is resorted to for ensuring adequate availability of food to dense stocks and for enhanced growth and production. In the case of mollusc culture, live foods continue to be the source of nutrition, even though experimental studies are under way to develop inert feeds. In nature, one can observe distinctly different feeding habits among fish and shellfish species, such as those that feed on zoo- and phytoplankters, filamentous algae, macrophytes, benthos, detritus, molluscs and other smaller animal species, etc. Many of them feed on more than one type of food or even on quite a number of them. Fish generally use one or more sensory systems for acquiring feed, such as visual detection, sound and water turbulence and chemical stimuli released by food. Of these, the visual stimuli are best understood and include the properties of size, movement, shape and colour contrast.
Digestion involves the conversion of the three major nutrients (proteins, carbohydrates and lipids) which occur as macromolecules in nature into sizes that pass through the walls of the alimentary canal and are absorbed into the bloodstream. Proteins are converted intoamino acids or polypeptide chains of a few amino acids, carbohydrates into simple sugars and lipids into glycerols and fatty acids. This is made possible through the activity of enzymes. Digestibility ranges from 100 per cent for glucose to as little as 5 per cent for raw starch or 5–15 per cent for plant material containing cellulose. Digestibility of most natural proteins and lipids ranges over 80–90 per cent. Digestibility of a food component is indicative of its bioavailability. Digestibility capacity is species specific and varies with nutrient source and method of treatment of the samples as well as ambient conditions such as temperature (Pfeffer et al., 1991). Indigestible materials are eventually voided as faeces.
The enzyme amylase catalyses the digestion of starch and together with dextrinases produces maltose. Maltase hydrolyses maltose to give the final product of starch digestion, glucose. Most fish have amylase; in plant-eating fish such as tilapia it may be present in all parts of the digestive tract, whereas in carnivorous fish it may be found only in the pancreas, pyloric caeca and intestines.
Cellulase and cellobiase are the enzymes involved in digesting cellulose. Cellulase hydrolyses cellulose to disaccharide cellobiose, which is then acted upon by cellobiase, producing the final breakdown product, glucose. Very few fish have cellulase activity, but the microflora in their intestines may serve as a source of cellulases.
Protein digestion in fish begins in the stomach and is catalysed by pepsin and acid pH ranging from 1 to 4. Pepsin is synthesized in the gastric gland in the inactive form called pepsinogen. Hydrochloric acid, produced by another enzyme-controlled reaction between sodium chloride and carbonic acid, activates the pepsinogen. Pepsin attacks most proteins where the linkages are formed by aromatic and acidic amino acids, such as phenylalanine, tyro-sine, tryptophan and asparatic and glutamic acids.
Trypsin and chymotrypsin are involved in the alkaline digestion of proteins. These enzymes are generally synthesized and stored in the pan-creatic cells as inactive forms, viz. trypsinogen and chymotrypsinogen. These are then transported mainly to the intestines and pyloric caeca, or in some cases to the liver. In the intes-tines, trypsinogen is converted to the active form trypsin by the enzyme enterokinase. Trypsin in turn activates chymotrypsinogen into chymotrypsin. Trypsin is specific for peptide linkages which come from basic amino acids: arginine and histidine. Chymotrypsin attacks linkages with aromatic amino acids: phenylala-nine, tyrosine and tryptophan.
Carboxypeptidases (A and B) hydrolyse the C-terminal peptide of proteins. This is found in the pancreas, pyloric caeca and intestines of fish. Carboxypeptidase A is not active towards proteins with aromatic C-terminal amino acids: phenylalanine, tyrosine and tryptophan, while carboxy-peptidase B acts preferentially on these with lysine and arginine. Amino peptidase hydrolyses the amino terminal peptide of poly-peptidia, releasing one amino acid at a time from the N-terminal end. Most fish also have lipase enzymes that hydrolyse ester linkages in triglyceride and produce glycerol and fatty acids.
The effectiveness of digestive enzymes is influenced by temperature and pH. In general, the reaction rate increases with temperature until the enzymes begin to denature around 50–60°C. However the range of pH within which they function is very limited, often as little as 2 pH units. In the case of channel catfish, which is probably representative of many teleosts, the pH in the stomach ranges between 2 and 4, becoming alkaline (pH 7–9) below the pylorus, decreasing to 8.6 in the upper intestine, and finally nearing neutrality in the hind gut (Page et al., 1976).
Absorption of amino acids, peptides and simple carbohydrates in fish have not been studied much, but presumably they diffusethrough or are transported across the gut epithelium into the bloodstream. Digested food, particularly protein, is not fully available to the fish even after it has been absorbed. Amino acids may be used as absorbed for building new tissue. But if digested food has to be oxidized for energy, deamination (removal of the amino group) which requires an input of energy (a process known as specific dynamic action) would have to occur first. Fish that have not grown due to low temperature or due to low levels of feeding would deaminate most or all of their amino acids. Those reared at high temperatures or having very high metabolic rates due to high activity levels would also do likewise. On the other hand, fish having rapid growth and high protein intake would deaminate a relatively small portion of the digested protein.
The energy for deamination need not come from amino acids, but may be preferentially taken from carbohydrate or lipid, if available. This ‘proteinsparing’ action accounts for the addition of limited amounts of inexpensive carbohydrate in the diet of fish, which helps in reducing feed costs. The calorie-to-protein ratio (kcal : g) can be applied in diets containing adequate energy and protein. Optimal ratios for catfish diets are reported to be between 6.5 and 8.3 kcal of digestible energy per g protein.
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