FISH NUTRITION IN AQUACULTURE

SANGIPRAN BAISHYA¹, BIPUL PHUKAN², ANKUR RAJBONGSHI³ AND RANJIT BORDOLOI⁴

^1,2,4 College of Fisheries, Assam Agricultural University, Raha, Nagaon — 782103, Assam.

³ NAIP Cell, Assam Agricultural University, Jorhat — 13, Assam.

Email: ¹ sangipran@gmail.com

Introduction

The aquaculture industry as such has grown at a significant rate over the last few decades. During this period it has begun to be transformed from an art to a science, however even to this date this transition remains far from complete. As in other forms of animal husbandry, feeds and feeding are crucial elements in the culture of aquatic animals. Feed cost is considered to be the highest recurrent cost in aquaculture, often ranging from 30% to 60%, depending on the intensity of the operation. Any reduction in feed costs either through diet development, improved husbandry or other direct or indirect means is therefore crucial to the development and well being of the industry. Importance has been given to simple feed formulations, utilization of non — conventional feedstuffs and feed processing. In this regard the aspects related to feeds and feeding which are increasingly becoming important to the aquaculture industry, in particular feeds and feeding in relation to the environment and the aquafeed industry.

For many years water quality has been the most important limitation to fish production. Advances in life support technology have been substantial in recent years, and nutrition is increasingly regarded a key limitation to increased production efficiency as well as the growth and propagation of "new" species. Feeding of artificial diet balanced in all nutrients has assumed foremost importance in aquaculture industry. Artificial feeding is an essential practice in an aquaculture operation accounting for over 60% in total input cost.

Nutritional composition of fish diets

Generally, fish diets tend to be very high in protein. Foods for fry and fingerlings frequently exceed 50% crude protein. As growth rate decreases and fish age, protein levels in diets are decreased accordingly. Protein levels on grow-out diets often approach or exceed 40% crude protein, while maintenance diets may contain as little as 25-35%. In addition to decreasing the protein content of the food as fish grow, the particle size must also be changed. Many fish require live food when they are hatched because their mouth parts are so small. Some fish are large enough to place on a fry diet immediately without having to bother with the expense and labour needed for live foods.

Fish meal should be a major protein source in fish diets. There are essential amino and fatty acids that are present in fish meal but not present in tissue from terrestrial plants or animals. Low cost formulations in which fish meal has been eliminated and replaced by less expensive proteins from terrestrial sources (ie soybeans) are not recommended for fish. Fish meal and fishery by-products have high lipid content and therefore rancidity can be a problem if foods are not properly stored. Feed storage is discussed briefly below.

In addition to the concern for essential amino acids that may be present in fish meal, fish require long chain fatty acids (C20 and C22) that are not found in tissue from terrestrial organisms. Fish meal, shrimp meal and various types of fishery by-products are the source for these essential fatty acids. In addition, crustacean by-products serve as a source of carotenoid pigments that are excellent for colour enhancement. There is a high oil content associated with carotenoid pigments, so vitamin E supplementation is recommended when these are used.

Most fish require dietary ascorbic acid (vitamin C). This becomes very important if fish are reared in a poorly lit area where algae cannot grow, or if they are so crowded that they cannot consume any natural food items that might be in the water. Ascorbic acid added to fish foods should be phoshorylated to stabilize the vitamin and increase storage time. In addition, vitamins A, D, E and B complex should be added to fish foods. The concentration of vitamin E is often inadequate, especially in diets that are high in fat. If fish are housed in natural systems with algae and phytoplankton, and stocking rates are not too great, then vitamin supplementation seems to be less important, presumably because of the availability of natural food items.

Feed storage

Because fish feeds usually contain relatively high amounts of fish meal and/or fish oil, they are very susceptible to rancidity. In addition, ascorbic acid is highly volatile, but critical to normal growth and development of most species of fish. For these reasons, fish feeds should be purchased frequently, ideally at least once a month and more frequently if possible. Feeds should be stored in a cool, dry place and should never be kept on hand for more than three months. Refrigeration of dry feeds is not recommended because of the high moisture content of that environment. Freezing is an acceptable way of extending the shelf life.

Types of feed

Commercially milled fish foods are usually sold as dry or semi-moist pellets or as flakes. Pellets are typically the most complete diets. They are cooked, and, if marketed as a complete ration, the nutrition in each particle should be uniform. Disadvantages include the potential for rapid sinking unless the pellet is extruded. In addition, the pellet size is very important. It may be impossible to manufacture a particle small enough for some fish, especially juveniles of many species. For larger animals, a very small pellet may be unacceptable. Semi-moist diets are soft and compact. Many of these are expensive, but they tend to be high quality diets and may be an excellent choice for some species.

Flakes have been used extensively in the ornamental fish industry for many years and have the advantage of being soft enough for very small fish to consume. They also sink very slowly. Unfortunately, the volume required to meet the nutritional needs of the animals may be deceptively high.

Technology associated with rearing of live foods is improving rapidly. This is having a positive impact on larval rearing, a frequent bottleneck for commercialization of "new" species. Rotifers are the smallest live food that is routinely used for larval rearing. Newly hatched brine shrimp are larger, but still quite small, and are commonly used in fish hatcheries. Cultured live foods can provide a source of high quality nutrition, but care must be taken to avoid perpetuation of infectious disease. Use of wild caught food items is also risky because of the potential for disease introduction.

Nutritional diseases

Nutritional disease is often a diagnosis of exclusion. Other explanations for the problem are ruled out and then the feeding program is critically evaluated. Several examples of nutritional disease merit mention. These include starvation, scoliosis and nutritional anemiaa. Each is discussed briefly below.

Starvation is usually the result of poor husbandry and, in many cases, is a sequel to environmental problems. A poorly designed or maintained system is likely to develop water quality problems with related morbidity or mortality among the fish. In an effort to correct the water quality problems aquarists may cut back on feed to the point where the animals are in a negative caloric balance and begin to lose weight. If the problem becomes chronic, starvation can result.

The classical cause of scoliosis, or "broken back disease" in fish is ascorbic acid deficiency. Improvements in feed manufacture, including phosphorylation of vitamin C, and feed storage, have decreased the incidence of nutritionally derived scoliosis. Still, ascorbic acid deficiency must be considered as a possible cause of scoliosis and a thorough review of feeding practices is warranted when evaluating such cases.

Nutritional anaemia is caused by folic acid deficiency. The diagnosis is often based initially on history, with multiple units developing similar signs at the same time. When suspected, a sample of feed should be frozen for later analysis, but all affected ponds should have the feed changed immediately to a fresh lot. The problem is caused by bacterial contamination of feed, so it is not related to particular brands or formulations.

Conclusion

Feed preparation must be done on logical approach for simple formulations that should be location specific and resource oriented using a large proportion of alternative protein sources with due consideration for less expensive feeds to support sustainable and economically sound aquaculture. Establishment of regional feed centres should be given due priority to understand and identify farmers' feed related problems for their development that may go a long way to village level production of improved farm made feeds through small feed mills, particularly by small farmers who account for more than 80% in India.

References

De Silva, S.S. and Anderson, T.A. 1998. In: Fish Nutrition in Aquaculture, London, UK, 287 pp.

Lovell, T. 1988. Nutrition and Feeding of Fish. Van Nostrand Reinhold, New York, NY 260 pp.

Roberts, R.J. 1989. Nutritional pathology of teleosts, In: Fish Pathology, R.J. Roberts (Ed), Balliere-Tindall, London Pp. 337-362.

Winfree, R.A. 1992. Nutrition and feeding of tropical fish, IN: Aquariology: The Science of Fish Health Management. J.B. Gratzek (ED).Tetra Press, Morris Plains, NJ Pp. 197-206.