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| Fish nutrition - An Overview
by the
A few notes about the Fish Nutrition Research Lab by Dominique Bureau ( dbureau@uoguelph.ca ) The Fish Nutrition Research Laboratory is a joint venture between the University of Guelph and Ontario Ministry of Natural Resources. It is a research facility in existence since 1969. It conducts basic and applied research in the field of fish nutrition and feeding, with particular emphasis on nutrient utilization and requirements, bioenergetics, digestibility, feed formulation, feeding systems and waste management. The studies carry out in the lab are mostly with salmonids and supported by the Ontario Ministry of Natural Resources and the Ontario Ministry of Agriculture, Food and Rural Affairs, the Department of Fisheries and Oceans, AquaNet, NSERC, and the industry. Visitors welcome. In culturing fish in captivity, nothing is more important than sound nutrition and adequate feeding. If the feed is not consumed by the fish or if the fish are unable to utilize the feed because of some nutrient deficiency, then there will be no growth. An undernourished animal cannot maintain its health and be productive, regardless of the quality of its environment. The production of nutritionally balanced diets for fish requires efforts in research, quality control, and biological evaluation. Faulty nutrition obviously impairs fish productivity and results in a deterioration of health until recognisable diseases ensues. The borderlines between reduced growth and diminished health, on the one hand, and overt disease, on the other, are very difficult to define. There is no doubt that as our knowledge advances, the nature of the departures from normality will be more easily explained and corrected. However, the problem of recognizing a deterioration of performance in its initial stages and taking corrective action will remain an essential part of the skill of the fish culturist.
Protein is required in the diet to provide indispensable amino acids and nitrogen for synthesis of non-indispensable amino acids. Protein in body tissues incorporate about 23 amino acids and among these, 10 amino acids must be supplied in the diet since fish cannot synthesise them. Amino acids are need for maintenance, growth, reproduction and repletion of tissues. A large proportion of the amino acid consumed by a fish are catabolized for energy and fish are well-adapted to using an excess protein this way. Catabolism of protein leads to the release of ammonia. Protein is the most important component of the diet of fish because protein intake generally determines growth (protein growth has, in general, priority), has a high cost per unit and high levels are required per unit of feeds. First observations on fish protein and amino acid requirements came from studies on natural diet of different fish. Natural diet (plankton, invertebrates, fish) is generally rich in protein and has a good amino acid balance. All dietary proteins are not identical in their nutritive value. The nutritional value of a protein source is a function of its digestibility and amino acid makeup. A deficiency of indispensable amino acid creates poor utilization of dietary protein and hence growth retardation, poor live weight gain, and feed efficiency. In sever cases, deficiency reduces the ability to resist diseases and lowers the effectiveness of the immune response mechanism. For example, experiments have shown that tryptophan-deficient fish become scoliotic, showing curvature of the spine, and methionine deficiency produces lens cataracts. Salmonid diets generally contain 35-45% digestible protein (DP), or 40-50% crude protein. However, amino acids or protein must be supplied in relation to digestible energy (DE). The recommended ratio of protein to energy in the salmonid diet is 20-26 g DP/MJ DE (92-102 g protein per Mcal). Increasing these proportions increases ammonia excretion; the requirement for dissolved oxygen is also increased because the efficiency with which the energy is used is decreased. Why do fish have such high requirements for protein? The main factors explain this phenomenon: 1) The protein requirement in terms of dietary concentration (% of diet) is high but the absolute requirement isn’t (g/kg body weight gain). This is due to the fact that fish have a lower absolute energy requirement than mammals. This results in similar g body weight gain/g protein ingested as mammal but better feed efficiency (gain:feed). 2) Protein (amino acids) is used as a major energy source. Some economy can be made here if other dietary fuel are present in adequate amounts, e.g. increasing the lipid (fat) content of diet can help reduce dietary protein (amino acid) catabolism and requirement. This is referred to as protein-sparing effect of lipids. Protein to useful energy ratio is the factor that should be considered, not % protein of the diet per se. 10 Indispensable amino acids
Phenylalanine (Phe)
Histidine (His) Isoleucine (Iso) Leucine (Leu) Table 1. Indispensable amino acid requirements of different species of teleost (g / 100 g protein)
Table 2. Amino acid composition of common protein sources (g/ 100 g protein).
Lipids (fats) encompass a large variety of compounds. Lipids have many roles: energy supply, structure, precursors to many reactive substances, etc. In the diet or carcass of fish, lipids are most commonly found as triglycerides, phospholipids and, sometimes, wax esters. Triglycerides are composed of a glycerol molecule to which three fatty acids are attached. Phospholipids are also composed of a glycerol molecule but with only two fatty acids. Instead of a third fatty acid a phosphoric acid and another type of molecule (choline, inositol, etc.) are attached. Wax esters are made of a fatty acid and a long chain alcohol and are a common form of lipid storage in certain species zooplankton . The main role of triglycerides is in the storage of lipids (fatty acids). Phospholipids are responsible for the structure of cell membranes (lipid bi-layer). Fatty acids are the main active components of dietary lipids. Fish are unable to synthesize fatty acids with unsaturation in the n-3 or n-6 positions yet these types of fatty acids are essential for many functions. These two types of fatty acids are, therefore, essential for the animal and must be supplied in the diet. Deficiency in essential fatty acid result in general, in reduction of growth and a number of deficiency signs, including depigmentation, fin erosion, cardiac myopathy, fatty infiltration of liver, and "shock syndrome" (loss of consciousness for a few seconds following an acute stress). Salmonids require about 0.5 to 1% long chain polyunsaturated n-3 fatty acids (EPA (20:5 n-3) and DHA (22:6 n-3)) in their diet. This amount is easily covered by ingredients of marine origins, such as fish meal and fish oil, which are always present in significant amounts in salmonid feeds. Carbohydrates represent a very large variety of molecules. The carbohydrate most commonly found in fish feed is starch, a polymer of glucose. Salmonid and many other fish have a poor ability to utilize carbohydrates. Raw starch in grain and other plant products is generally poorly digested by fish. Cooking of the starch during pelleting or extrusion, however, greatly improves its digestibility for fish. However, even if the starch is digestible, fish only appear to be able to utilize a small amount effectively. Carbohydrates only represent a minor source of energy for fish. A certain amount of starch or other carbohydrates (e.g. lactose, hemicellulose) is, nevertheless, required to achieved proper physical characteristic of the feed. The vitamins are generally defined as dietary essential organic compounds, required only in minute amounts, and which play a catalytic role and but no major structural role. So far, 4 fat-soluble and 11 water-soluble vitamins or vitamin-like compounds have been shown to be essential to fish. Requirement is generally measured in young fast growing fish. However, requirements may depend on the intake of other nutrients, size of the fish, and environmental stress. The recommended levels and the deficiency signs are summarized in Tables 3 and 4. Many symptoms of vitamin deficiency are non-specific. It is also tedious and expensive to analyze diets for vitamins. Therefore, diagnostic of vitamin deficiencies is often difficult. Nutritional disorders caused by vitamin deficiencies can impair utilization of other nutrients, impair the health of fish, and finally lead to disease or deformities. Nutritional deficiencies signs usually develop gradually, not spontaneously. However, the culturist may obtain clues of deficiency indirectly through low feed intake and poor live weight and feed efficiency. Table 3. Vitamin requirement of salmonids.
Table 4. Deficiency signs associated with various nutrients.
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