Previous Page Table of Contents Next Page


2.1 Feedstuffs for aquaculture
2.2 Composition of feedstuffs
2.3 Feed quality

The survey of feed ingredients in each country covered all materials of proven value as fish feeds currently used in traditional aquaculture as well as those materials heretofore not utilized as such in that country but, by virtue of their nutrient composition and established safety for use on livestock, are considered suitable for incorporation into formulated feeds for fish. The latter consist largely of what are commonly referred to as "conventional" feedstuffs, such as fish meal and animal rendering by-products, feed grains, and by-products from the milling of cereals and oil seeds as well as the so-called "non-conventional" feedstuffs such as processed waste from the food industry, and protein-rich single-celled micro-organisms from the biotechnology industry.

The survey was carried out at both local and national levels. Essential information on quality and availability was obtained for each ingredient. A questionnaire prepared specifically for this purpose was used (See examples in Annexes).

2.1 Feedstuffs for aquaculture

2.1.1 Legumes
2.1.2 Miscellaneous fodder plants
2.1.3 Roots and tubers
2.1.4 Cereals and cereal by-products
2.1.5 Oilcakes and oil meals
2.1.6 Feeds of animal origin
2.1.7 Miscellaneous feedstuffs

Feeds traditionally used for feeding monogastric farm animals are believed to be suitable for feeding fish (Ling, S.W., 1967; Fowler, L.G. and J.L. Banks, 1976, 1976a; Hastings, W., 1975; Randall Robinette, H., 1977). These materials include the feed grains, oilcake and meals, animal by-products (including fish meal), and an assortment of industrial and agricultural waste products. Most of these, however, have not been properly evaluate as fish feed. Judging from those that have been, their feed value depends invariably on digestible nutrient content. Although fish, being poikilotherms, are less capable of utilizing carbohydrates at low temperatures, they utilize fats and proteins at least just as well as land mammals. Fish, the herbivorous species excepted, are not capable of utilizing cellulose.

The feedstuffs described below represent the broad spectrum of feed raw materials known to be available among the twelve countries covered in the survey. Some feedstuffs are indigenous to a few countries and are not available even as imported items elsewhere. Others such as rice bran are widely available. Because the quality of the same feed material is expected to vary from country to country, indeed among different production sources within a country, there is no single set of compositional analysis that can be generally applied in the feed formulation process. The data reported in the Table of Composition of Feedstuffs in this section were drawn from major published sources (Göhl, B., 1980; Sen, K.C., 1978; Devandra, C., 1979; Oyenuga, V.A., 1968; McDowell, et al., 1974) and represent average values for feedstuffs from the respective regions. Only in one instance (in Orissa, India) was it possible to carry out actual chemical analysis on a variety of indigenously available feedstuffs. The results of those analyses are included in the table.

Selection was based on known value or potential value of individual feed ingredient for aquaculture production. In classifying each ingredient, the simple, and more practical, system of the FAO's Animal Production and Health Division (Göhl, B., 1980), rather than the complicated INFIC 1/ nomenclature for feedstuffs, is employed. The list is not exhaustive and does not include live foods and animal waste materials such as manure used primarily for increasing natural productivity in ponds. Certain food materials normally reserved for human consumption have also been included because of occasional availability of substantial quantities rendered unsafe for that purpose but usable as feed for fish and livestock.

1/ International Network of Feed Information Centres. See Harris, L., 1980

Few comparative feeding trials have been conducted to evaluate the relative feed values of common feedstuffs for fish. Studies reported in the literature deal primarily with high protein feed materials (Fowler, L.G. and J.L. Banks, 1976; Jackson, A.J., B.S. Copper and A.J. Matty, 1982; Tiews, K., J. Gropp and H. Koops, 1976) although reports from Auburn University in the USA have also focused on relative energy values of various feedstuffs (Lovell, R.T., 1977).

2.1.1 Legumes

Legumes are potentially a valuable feed resource for aquaculture in the tropics because of their widespread distribution in those areas. As feeds they are rich in protein and minerals. Despite the many varieties that have been identified for use as fodder for livestock, only a few have been utilized commercially for feed manufacture. The raw seeds of many legumes contain anti-nutritive factors (Liener, I.E., 1978). This problem can usually be overcome by processing. Most varieties have not been evaluated as feed for fish.

B.31 Red gram or dahl (Cajunus cajan L. Huth)

Red gram is grown primarily as food for humans. It is one of the most common legumes of the tropics and sub-tropics. The upper (leafy) part of the plant has a high protein content and is usually fed to cattle. The seeds are often used at up to 30 percent in poultry rations. Red gram bran, a by-product of gram flour production, has almost the same protein content as the seed and is used for the same purpose. Although not evaluated as feeds for fish, both seeds and bran are judged safe for incorporation into fish feeds at moderate levels.

B.41 Chick pea, Bengal gram (Cicer arietinum L.)

The chick pea is native of India but is also cultivated in other parts of the tropics. Although grown mainly for human consumption, the seeds of the chick pea are frequently used as feed for poultry and livestock. Although not evaluated as a feed for fish, it is presumed to be safe for inclusion as a major constituent in compound fish feed. The leaves of the plant are also suitable for feeding to fish.

B.65 Lablab or Egyptian bean (Dolichos lablab L.)

A perennial plant native of India, the lablab is grown in many parts of the tropics. The seeds of the lablab, which are fairly rich in protein, have been fed as concentrates to livestock, but have not been evaluated in fish. This plant is not to be confused with lab-lab, an algal complex fed to milkfish in the Philippines.

B.79 Lentil or red dahl (Lens culinaris, Medik)

The red dahl is native of Asia but is also grown in Africa. The plant is cultivated for its seeds which are used primarily for human consumption. During the milling of the seed, a bran rich in protein is produced. The material has a high feed value and, although not evaluated in fish, probably can be used at fairly high levels in compound fish feed.

B.81 Ipil-ipil or kathin (Leucaena leucocephala, Lam de Wit)

The ipil-ipil is one of the two legumes whose leaves and stems have been used commercially on a regular basis in animal feed manufacture in many parts of Asia. Ipil-ipil leaf meal is fairly rich in protein but may not be fed at excessive levels to farm animals due to the presence of the toxic amino acid mimosine. Ipil-ipil has been used at low concentrations (5-10 percent) in compound fish feed in Thailand and the Philippines without noticeable adverse effects. One study conducted on tilapia showed very poor growth when Leucaena constituted 25 percent or more of the dietary protein (Jackson, A.J., B.S. Copper and A.J. Mathy, 1982).

B.84 Lupin (Lupinus spp.)

The lupins are grown in the cooler parts of the tropics. Both the leaves and seeds are rich in protein, the latter fed at up to 20 percent in rations for monogastric animals. Although not evaluated as a feed for fish, it is believed that the seeds of the lupin can be fed at moderate amounts to fish without harm.

B.88 Lucerne or alfalfa (Medicago sativa L.)

The alfalfa is the better known of the two legumes whose leaves and stems are exploited commercially as a principal ingredient in compound feed manufacture. Alfalfa meal, unlike ipil-ipil, does not appear to contain anti-nutritive factors that limit its use in compound feeds. The protein content of alfalfa is comparable to that of ipil-ipil. Alfalfa is known to be safe for incorporation into compound fish feed.

B.106 Velvet mesquite (Prosopis veleutina, Woot.)

A shrub native of Mexico, the mesquite produces an abundant crop of pods which are sugary and contain seeds rich in protein. The seeds are encased in a hard shell which must be crushed to enhance digestibility.

B.110 Saman or cow tamarind (Samanea saman, Jacq. Merr)

The samman is a tree that grows in the tropics. Its leaves are fairly rich in protein but also high in tannins. Its seeds are also used as concentrates for livestock. Both leaves and seeds have not been properly evaluated in fish although preliminary studies in Venezuela have shown that the ground pods fed at moderate levels have no adverse effect on the cachama (a native species belonging to the Characidae family) 1/.

1/ Studies conducted at the Warmwater Fish Culture Station at Guanapito

B.111-113 Sesbania (Sesbania grandiflora, L. Poir.)

These small fast-growing shrubs are very common in the tropical lowlands of Asia. Its leaves are rich in digestible protein and its seeds are often fed as concentrates to poultry. Although not evaluated in fish, ready acceptance of both leaves and seeds by poultry indicates its safeness.

B.130 Mung bean or black gram (Phaseolus mungo L.)

The mung bean is a native of central Asia, grown primarily for the bean which serves as food for humans. Both leaves and seeds are fairly digestible for monogastric animals. Although not evaluated as feed for fish, the absence of glucosides in the seeds makes them safe for use at moderate levels.

B.132 Red bean or rice bean (Vigna umbellata, Thumb. Ohwi & Ohashi)

Grown primarily for its seeds which are used for human consumption, the red bean is found throughout tropical Asia. It has not been evaluated as a feed for fish, but only the seeds appear suitable for use in compound feed.

B.133 Horse gram (Dolichos biflorus L.)

Also a herb found in tropical Asia, the seeds of the horse gram are used as concentrate feed for cattle. Although not evaluated in fish, because of their low fibre content, these seeds appear suitable for use at moderate levels in compound fish feeds.

B.134 Cow pea or black-eye pea (Vigna unguiculata L.)

The cow pea is grown primarily for human consumption. Its seed is usually too expensive for use as feed for livestock. Its leaves, however, are equally good and appear quite suitable for use in fish feeds, although no evaluation has yet been made.

2.1.2 Miscellaneous fodder plants

Although there are many varieties of fodder plants which can be fed to fish, their exploitation for this purpose, for the most part, has been hindered by low digestibility of nutrients and difficulty in obtaining adequate and regular supplies. Two exceptions in this category are the sago and the kangkong which are relatively abundant in the tropics and which have high feed value for animals because of their high digestibility.

C.62 Kangkong or swamp cabbage (Ipomoea aquatica, Forsk)

Mainly cultivated for food, the kangkong has also been used as stock feed in South East Asia. It has a fairly high feed value, with a higher protein content than alfalfa. Kangkong has been fed successfully to carps in Sri Lanka as a principal component of compound feed.

C.73 Sago palm (Metroxylon sagu, Roth)

The sago palm, which often proliferates in freshwater swamps, is valued for the starch contained in its trunk. The starch, which is normally extracted in small village mills, has a variety of industrial uses apart from its value as food. The unrefined starch, or meal, has been fed to livestock and is considered equally safe for fish. Small quantities incorporated into compound feeds improve pellet quality because of the easy gelatinization of sago starch under the conditions of pelleted feed manufacture.

2.1.3 Roots and tubers

For the most part, roots and tubers are grown as food for human consumption in the tropics. However, because of their high starch content, some varieties are grown as industrial raw material. Notable among these is cassava or tapioca. Roots and tubers are also suitable for feeding to fish, although their use for this purpose appears very limited due to their relatively high cost. Also, because the feed value of roots and tubers is in their carbohydrate content, they cannot be used at too high levels in feed for fish, which have a lower tolerance for carbohydrates than farm animals.

E.13 Sweet potato (Ipomoea batatas, L. Lam.)

Sweet potato is mainly grown as food for human consumption in the tropics. The tubers consist primarily of carbohydrates with a high content of sucrose. Protein content, although low, is almost twice that of cassava. The leaves of the sweet potato are highly digestible, free from toxins, and fairly rich in protein. The starch in the tuber provides good binding for dry and moist-type pellets if heat processing is employed.

E.14 Cassava, tapioca or yucca (Mannihot esculenta, Crantz)

Fresh cassava is seldom used as feed for fish. Most varieties are toxic unless properly processed to prevent the release of hydrocyanic acid by an enzyme present in the tuber. Heat applied during processing destroys the enzyme.

Cassava has low feed value in diets for fish but its presence in small quantities improves the water stability of pelleted feed. Fresh cassava has been included in the diet of milkfish in Indonesia (Hastings, W., 1975).

2.1.4 Cereals and cereal by-products

Grown primarily for human consumption, cereal grains are no less important as feed for efficient animal production. Present-day high energy diets for monogastric farm animals often contain up to 80 percent cereal grains and their by-products. The use of grains is somewhat restricted in fish diets because of generally lower tolerance for dietary carbohydrates by fish. This effect appears to be more pronounced among cold-water species such as the trout than among warm-water species.

While cereal grains are considered mainly as a source of dietary energy, their byproducts represent a fairly rich source of protein and polyunsaturated fatty acids (PUFA).

Cereal grains sometimes constitute a significant percentage of fish feed processed into pellets. Starch present in cereals act as good, natural binders when gelatinized under normal pelleting conditions, giving products that have high water stability. Consequently, cereal grains are indispensable in the manufacture of floating-type pelleted feed.

F.1 Millet (in the tropics, mainly Eleusine coracana, Paspalum scrobiculatum and Pennisetum americanum)

Grown primarily for human consumption, millet is only occasionally available as feed for livestock. Its use as a feed for fish has not been documented. Judging from favourable growth response of monogastric farm animals fed the cereal, millet should, at least, be comparable to maize in feeding value. The seed is hard and proper grinding is necessary before use in fish diets.

F.7 Rice (Oryza sativa L.)

Rice is seldom used for animal feeding because of its high cost, although damaged grain and portions considered unfit for human consumption, viz., sweepings from warehouses and mills, are available for that purpose. In a few countries, notably Thailand, where it is the principal grain crop, rice of low commercial value such as broken rice is also used as livestock feed. On the other hand, by-products from mills are more generally available and constitute the most important feed resource in all rice-producing countries. The by-products include: rice bran, rice polishings and rice mill feed. Rice and its mill by-products are important feed constituents in fish feeds in each of the countries covered in this survey.

Broken rice and cargo rice

Rough rice is very seldom fed to fish. Instead, broken rice (polished) and cargo rice (unpolished) are used in the preparation of traditional type fish feeds as well as in compound diet formulations. The rice is first boiled in water before mixing with other ingredients to make the ration. Rice is an excellent energy source with a low but fairly good quality protein content. No digestibility study has been made with fish but for mono-gastric farm animals, uncooked rice is more digestible than maize. Cargo rice, especially, is rich in the B-vitamins.

Rice bran

Freshly produced rice bran has a high oil content (14-18 percent). The oil is sometimes recovered by solvent extraction because of its high commercial value, especially in countries where demand for cooking oil exceeds supply. The oil present in the bran is rich in polyunsaturated fatty acids which undergo rapid oxidation under normal storage conditions. Rice bran that has turned rancid has markedly reduced feed value. Rice bran has a higher protein content than the grain. It is also fairly high in fibre, thus limiting its use in fish feeds. Often, rice bran is mixed with rice polishings at the mill and sold as rice bran. Adulteration with hulls lowers its feeding value considerably. Good quality pellets can be made from feeds containing unextracted rice bran.

Oil extracted rice bran is almost totally devoid of oil. It is the type of rice bran favoured by feed mills because of its longer shelf-life. Its protein content is somewhat higher than fresh rice bran while its energy content is lower. Pelleted feed containing a high proportion of de-oiled rice bran are not very water-stable. This is because of the water absorption characteristics of fibre, which will be fairly high in such feed.

Where available, rice bran has been successfully used in compound fish feeds for both warm as well as cold water species.

Rice polishings

Rice polishings are produced during the scouring of rice after the bran has been removed. The material, therefore, contains a higher proportion of starch but is lower in protein and fibre content. It is not produced in as large quantities as rice bran, and, in most mills, is often mixed with the latter to give rice pollard. This, however, is usually sold as a grade of rice bran in most countries.

F.9 Sorghum (Sorghum bicolor and S. vulgare)

Sorghum is grown in warm areas where rainfall is inadequate for maize cropping. Its feed value is comparable to maize for most species of livestock. In Mexico where maize is normally reserved for human consumption, sorghum is used as a substitute in compound feed for trout and carp.

Sorghum contains tannic acid which decreases the availability of methionine in the diet. The amount of tannic acid in sorghum depends not only on the variety of sorghum but also on the conditions under which the crop is cultivated.

Sorghum is available in the majority of the countries covered in this report.

F.10 Wheat (Tritricum aestirum L.)

Although primarily a crop of temperate countries, wheat is also grown in parts of the tropics where there is a long period of relatively cool weather. Where it is not grown, it is imported for flour production. Wheat products are, therefore, available in practically every country.

Wheat, often used as an ingredient of compound fish feed in temperate countries is seldom used in the tropics because of its high cost. Damaged grain is, however, frequently available and when fed to fish is at least equivalent to maize in feed value.

Milling of wheat produces three major by-products, two of which are almost exclusively used as feed for livestock. These are: wheat bran and wheat middlings (or fine bran). The third, wheat germ, has higher commercial value as a food item for humans. Occasionally, damaged flour is also available as feed.

Wheat bran

Wheat bran is the primary coat of the wheat grain. It has a fairly high protein content. Because it is also high in fibre, it has a laxative effect when fed at excessive levels to animals. Such an effect, however, will be more difficult to determine in fish. Nevertheless, wheat bran has been successfully fed at fairly high levels to various species of fish without adverse effects on growth. Too much wheat bran in a formulation results in pellets with poor water stability due to the water absorption characteristics of fibre.

Wheat pollard

Wheat pollard is also known as fine bran. The material is less fibrous than wheat bran and has a higher feed value. Wheat pollard also contains a higher proportion of gluten which is a good natural binder for pelleted feed production.

F.11 Maize (Zea mays L.)

Maize is grown throughout the tropics and is the principal cereal grain employed in the feeding of livestock. Maize has also been fed successfully to fish. Whole grain has been used as a supplementary feed for tilapia and carp. However, maize is usually used in finely ground form as an energy component in compound feeds. While there is little published data on the digestibility of starchy components of cereal grains in general, studies on maize have indicated enhanced utilization after cooking. Normal pelleting processes only partially gelatinize the starch granules whereas total starch gelatinization occurs when the cereal is subject to high temperatures under moist conditions as in extrusion cooking. For channel catfish, such processing methods greatly increase the digestible energy value of maize, indicating the relatively low digestibility of raw starch for that species (Lovell, R.T., 1977).

Wet milling of maize is an industrial process for production of valuable components of the maize kernel. The main products of the process are: corn oil and starch (from which dextrins and glucose are obtained after further processing). The by-products include: maize gluten meal, maize bran and maize germ meal. Often all three are mixed together and sold as maize gluten feed. The protein content of maize gluten meal is comparable to that of soyabean meal. However, maize protein has a very low lysine content, and maize gluten meal or maize gluten feed should be used along with other lysine-rich protein sources (animal protein, or soyabean meal).

2.1.5 Oilcakes and oil meals

Oilcakes are by-products of the vegetable oil extraction industry. Although many varieties of seeds and fruits are cultivated primarily for their oil content, the protein-rich residues left after oil removal represent an immense resource upon which the world's production of animal protein for human consumption largely depends.

In the tropics and sub-tropics, soyabean, groundnut, and sesame (gingelly) are the principal oil seeds yielding protein-rich oilcake or meal after oil removal. Among oil-bearing fruits, the coconut and the oil palm nut are the most important sources.

Oil extraction from seed or fruit is carried out by two methods: by pressing, or with chemical solvents. The product obtained by pressing is termed oilcake and that by solvent extraction, oil meal.

Oilcake production is carried out by one of two mechanical processes. The first process involves the use of hydraulic presses that press out the oil from oil seed, or previously shredded dried fruit kernels (e.g., copra), wrapped in cloth. The resulting residue is in slab form. The presses used in this process are of relatively simple design and can be fabricated quite easily. They are, however, not very efficient. The second process involves the passage of seeds or fruit kernels along a screw auger confined in a steam-heated conically shaped jacket with the oilcake emerging at the smaller end. This process, known as the screw expeller process, is more efficient in removing oil. However, because of the high temperatures associated with the pressure applied, oilcakes produced by this method may come out burnt with subsequent lowering of feed value (especially with regard to protein digestibility in general and lysine availability in particular). Because of the high value of edible oils relative to oilcake residues in developing countries, the screw expeller process is favoured over the less efficient hydraulic press process for mechanical expression of oil from seeds and fruit. However, in South Asia as well as in South East Asia, with the possible exception of Thailand, hydraulic-press extraction continues to be the principal method employed in the vegetable oil industry.

Oil extraction involving chemical solvents is of fairly recent origin. Solvent extraction plants are technically more sophisticated than the screw expeller mills and their operation is economical only for processing large quantities of oil seeds or fruits. These plants are found in most of the countries that came under this survey, although they are more prominent in some than in others. Where they are located, these plants are usually used for extraction of oil from soyabeans that are either grown domestically, or imported. The process is now extensively used for secondary recovery of oil in the palm oil industry in Malaysia as well.

The oil meal that is produced as a by-product of the solvent extraction process has a very low fat content; often less than 1 percent. Temperatures involved in the solvent extraction process are generally low compared to the expeller process. Because of this and because of the need to de-activate antinutritive factors generally present in some oil seeds, toasting of the extracted residues follows solvent recovery. Due to the relatively mild treatment given to oil meals, these products have a comparatively higher feed value than expeller cakes. The lower residual oil content in an oil meal also gives it a higher protein content compared to a similar product obtained by the screw expeller process.

G.3 Groundnut (Arachis hypogaea L.)

Groundnut oilcake is a safe feed for fish. It has been demonstrated in India that diets consisting of 50 percent of the material as the principal protein source can be used in complete feeding of carps. Its high polyunsaturated fatty acid (PUFA) content also makes further addition of fats to such diets unnecessary. Because groundnut protein is especially low in methionine, the oilcake should be used with methionine-rich protein supplements or with synthetic methionine to achieve a proper balance of essential amino acids in the diet.

Quality in groundnut oilcake depends on whether the material is made from decorticated nuts. Removal of the fibrous hulls yields a better quality product with higher protein content. This will also depend on the amount of residual oil in the cake. In a few countries there are two varieties of groundnut oilcake: the mill produced and the 'country' produced. The former is an industrial product with a generally lower residual oil content (less than 7 percent), whereas the latter is produced on often a very small scale in villages and may contain as much as 13 percent oil.

The principal (and often very serious) contaminant of groundnut cake is aflatoxin. Aflatoxin is a group of highly toxic substances produced by the mould Aspergillus flavus. The most prominent of these are Aflatoxins B1, B2 and G1. The toxins are produced only when the mould exists as a pure culture. Aflatoxin is a hepatotoxin and mortality among afflicted animals and fish invariably results from severe liver damage. Small doses over an extended period of time produces cancer of the liver in humans. The half lethal-dosage, or LD50 1/ for trout weighing 100 g is 0.5 ppm in the diet. Improper post-harvest handling of nuts is usually blamed for the presence of aflatoxin in groundnut although pre-harvest infestation has also been documented. Aflatoxin is not destroyed by heat.

1/ LD50, the dosage at which mortality among animals consuming the contaminated feed is 50 percent

G.12 Coconut (Cocos nucifera L.)

Coconut oilcake is widely available as a feed for fish and livestock in countries such as Sri Lanka, Indonesia and the Philippines. On the average, 1 000 nuts yield 60 kg oilcake (called poonac in South India and Sri Lanka). Compared with residues of oil seed extraction, coconut oilcake is low in protein and high in fibre. It is difficult to make a water-stable pelleted feed if the formulation contains too much coconut oilcake, as the material tends to absorb large amounts of water, causing the pellet to disintegrate. Poonac is commonly used as a supplementary feed in the pond culture of carp in Sri Lanka. It has also been included in the diet of milkfish in Indonesia (Hastings, W., 1975).

One of the main problems in its use is the tendency of coconut fat to turn rancid in storage. The effects of feeding rancid coconut oilcake to fish have not been studied, although short-chain free fatty acids in the diet have been shown to suppress growth in at least two species, the trout and channel catfish. However, rancid soyabean oil does not appear to affect the growth of carp.

G.15 African oil palm (Elaceis guineensis Jacq.)

The African oil palm has been successfully transplanted in South East Asia. In Malaysia especially, the oil palm has emerged as the major source of cooking oil. The residue of the palm kernel has a slightly lower protein content than coconut oilcake or meal but its feed value is not very different. Like coconut oil, palm kernel oil is low in PUFA's. Use of palm kernel oilcake in fish diets has not been documented although, like coconut oilcake, it should be used sparingly in diets for fish.

G.16 Soyabean (Glycine max (L) Merr.)

Soyabean is one of the world's most important oil seed crops. A legume, like the groundnut, soyabean has a relatively low oil content. Because of this, and because the seeds can be easily flaked by crushing, oil removal of flaked soyabean is carried out by solvent extraction. The residue is very low in fat (less than 1 percent) and high in protein. Decorticated soyabean meal may contain up to 55 percent protein.

Soyabean meal is a good source of essential amino acids (EAA) and is one of very few plant sources rich in lysine. In recent years, soyabean meal has been increasingly used as a substitute for more expensive fish meal in compound fish feeds. Soyabean meal has a relatively low methionine content which can be corrected by addition of synthetic forms of the amino acid in the feed. Anti-nutritive factors, principally urease and trypsin inhibitor present in the raw bean are usually destroyed by heat during the oil extraction process.

Full-fat soyabean processed under high pressure has recently been successfully used as a complete substitute for fish meal in the diets of both warm-water (catfish) as well as cold-water (trout) species (Brandt, T.M., 1979; Reinitz, G.L. et al., 1978). The high PUFA content in full-fat soyabean makes supplementing with extraneous PUFA's unnecessary.

Considerable work has been carried out on the extent to which processed soyabean products can replace fish meal as a protein source in fish diets. While such replacement can be total for carp without effect on growth (Viola, S. et al., 1981) similar replacement in diets for trout resulted in slower growth and poorer feed conversion (Koops, H. et al., 1976).

G.17 Cotton (Gossypium spp.)

Cottonseed per se is rarely used as feed. The seed is processed by first removing the lint and then cutting the tough hull to release the kernel. These operations are carried out by machines. The kernels are then crushed and the oil removed by mechanical means or by solvent extraction. In developing countries the former process is most common. Although cottonseed cake has a high protein content, it is rather lacking in lysine. For fish, therefore, it should be used in combination with other materials such as soyabean meal and animal protein sources.

An important factor limiting usage of cottonseed cake or meal in fish feed is its gossypol content. Gossypol is a toxic phenolic compound confined to the genus of cotton, Gossypium. It is the yellow pigment which constitutes 20 to 40 percent of the substances inside the glands of the seed kernel. The amount of gossypol is proportional to the number of glands present as it does not occur elsewhere in the seed. Cottonseed usually contains 0.4 to 1.7 percent gossypol. There have been few studies on the toxic effects of gossypol in fish although from animal studies, it has been shown to inhibit digestive enzymes. Studies with trout have shown that the toxin persisted in body tissues 12 months after its feeding to the fish. Much of the retained gossypol was found in the liver, kidneys and spleen (Lovell, R.T., 1981). On the other hand, glandless (gossypol-free) cottonseed meal ha? been used at up to 40 percent in diets for salmon without deleterious effect (Fowler, L.G. and J.L. Banks, 1976).

Binding of gossypol in the oilcake or meal renders the product non-toxic. This can be achieved by treating with water and cooking with steam. The gossypol effect may be counteracted by the addition of ferrous sulphate, although the level of additions required has not been established for fish. The range for land species varies from 1 part ferrous sulphate to 1 part free gossypol for pigs to 4 parts to 1 part for poultry layers. Only free gossypol which can be extracted with aqueous acetone is physiologically harmful.

Although the amino acid profile of cottonseed protein is somewhat similar to that of soyabean, and superior to that of groundnut, the danger posed by high gossypol content makes it unwise to use cottonseed meal beyond 15 percent in fish diets. The rather high fibre content of cottonseed meal also restricts its use.

Cottonseed meal is available in limited quantities in the majority of the countries surveyed, except in Egypt and India, where there are large annual crops of cotton and the feedstuff is the principal protein source for feeds. Cottonseed meal is generally cheaper than soyabean meal.

G.19 Sunflower (Helianthus annuus L.)

Sunflower is a fast-growing cash crop grown primarily for its oil. The leaves and stems of the plant, at two months, are rich in protein and low in fibre and are suitable as feed for fish. There are no known phytotoxins present in the sunflower plant.

Sunflower seed cake made from dehulled seeds has a fairly high protein content. It is also richer in the sulphur-amino acids than soyabean meal although its lysine content is much lower. Sunflower seed meal has been shown to be a good substitute for fish meal in diets for tilapia (Jackson, A.J., B.S. Copper and A.J. Matty, 1982)

G.20 Para rubber (Herea brasiliensis Muell, Arg.)

The seed of the rubber tree has only been exploited recently as a protein source for animal feeding. The release of prussic (hydrocyanic) acid in raw rubber seeds is prevented by wet-heat processing, which destroys the enzyme that produces it from a glucoside present in the seed. The high temperature associated with expeller cake manufacture also destroys the enzymes.

Decorticated rubber seed oilcake has a composition somewhat similar to coconut oilcake. No data on its amino acid composition is available. Use of rubber seed oilcake in fish feeds has not been documented although it appears that its use will be safe at or below the level used in chicken feeds (between 10-15 percent of the diet).

Rubber seed oilcake is available in rubber producing countries such as Brazil, Sri Lanka, India (southern states), and the South East Asian countries.

G.23 Linseed (Linum usitatissimum L.)

Linseed, or flax, is grown for the production of fibre and linseed oil - a drying oil used widely in paints. The oilcake has a protein content comparable to cottonseed oilcake but is considerably richer in methionine. Linseed cake, like rubber-seed oilcake, contains an enzyme that releases prussic acid from a glucoside present in the seed. Normal processing of the seed for oil removal destroys this enzyme. However, it contains other toxins of unknown identity that cause depressed growth in poultry, even when fed at low levels (3 percent of the diet). The toxin may have a more pronounced effect on fish since the requirement for pyridoxine, a B-vitamin very much associated with high-protein diets, appears to be greatly increased in rations containing linseed oilcake.

Linseed oilcake is available in considerable quantities in India where it is safely fed to cattle and in lesser quantities in Thailand. Use of the material in compound fish feed in these countries has not been documented, however.

G.33 Sesame, or gingelly (Sesamum indicum L.)

After the extraction of oil from sesame seeds, a valuable high-protein feed is obtained. The oilcake is highly acceptable to livestock as well as fish. Experiments on the rohu (Labeo rohita) in India have shown that up to 50 percent of gingelly oilcake may be fed in a complete diet with good results.

Although sesame oilcake usually has a higher fibre content than groundnut oilcake, the protein content of both feedstuffs are about the same. Furthermore, sesame oilcake is very rich in methionine (in fact, the richest among oilcakes and meals). Its lysine content, on the other hand, is lower than that of groundnut oilcake. Sesame oilcake and groundnut oilcake used in combination can substitute for the more expensive animal proteins such as meat meal or fish meal. Effects of excessive use of sesame oilcake has not been studied in fish although in conventional livestock, it has been shown to have a laxative effect. Also because of its high phytic acid content, supplemental phosphorus will also be required.

Sesame oilcake is available in fairly large quantities in India, Thailand and Venezuela, and prices are generally lower than soyabean meal but higher than cottonseed meal or groundnut oilcake.

2.1.6 Feeds of animal origin

One distinguishing characteristic of feedstuffs of animal origin is their high protein quality. Animal proteins are rich in essential amino acids (EAA's), especially lysine and methionine that are limiting in most plant proteins. Animal proteins are also rich in trace minerals, vitamins and identified growth factors (UGF). The presence of certain animal proteins, particularly fish meal, even in small amounts, can greatly improve the nutritional value of the entire diet. Their availability among the countries surveyed varies. Nevertheless, they are of great importance as ingredients in fish diets.

H.1 Animal protein supplements (meat meals and raw slaughterhouse waste)

Animal protein supplements, or meat meals, are the principal by-products of animal slaughterhouses. They include meat scraps and trimmings after the fat has been removed during the rendering process, as well as internal organs of slaughtered animals. Their availability as feed depends in part on the food habits of the human population. Animal rendering on a commercial scale is more frequently practised in countries where animal slaughter is stringently regulated and where the internal organs of animals are not valued as human foods.

The quality of meat meal as a protein supplement depends on its production process as well as raw material used. Good quality meat meal is low in rat and mineral content. Meat meal is frequently used as an animal protein source in compound fish feed manufacture although its feed value is generally considered inferior to that of soyabean meal and fish meal (Fowler, L.G. and J.L. Banks, 1976). Fats in meat meal although poor in the PUFA's required by cold water species, are good energy sources for both carp and rainbow trout. (Watanabe, T., 1982).

Meat meal is imported by the majority of the countries covered in this survey to augment local supplies to meet the needs of the feed milling industry. Only Mexico and Venezuela produce meat meal on a commercial scale. In others, raw animal slaughterhouse waste is available instead. Such wastes consist mainly of animal entrails unfit for human consumption. The organs usually involved are livers, kidneys, hearts, lungs and digestive tracts. The entrails of animals have a higher feed value than meat meal; their protein content as well as protein quality are generally higher. However, because of their high water content slaughterhouse wastes are best used in combination with dry feed ingredients to make moist feeds. Heat processing may be necessary and the material used immediately if refrigeration facilities are not available.

Because meat meal is usually an imported item, its cost is usually high. Slaughterhouse waste, on the other hand, is inexpensive when available.

H.2 Blood

Animal blood is obtainable in most areas from abattoirs. In many village abattoirs, it is merely discarded.

Animal blood may be used fresh, or processed into blood meal before using. Although dried blood has a high-protein content and is highly digestible, its amino acid content is not as well balanced as that of muscle tissue.

Fresh blood may be used to enrich commonly available feedstuffs such as rice bran and wheat bran in fish diets. The bran also soaks up the excess moisture in the production of moist pelleted feed. In combination with rumen contents, fresh bovine blood has successfully substituted for fish meal in channel catfish diets (Reece, D.L. and D.E. Wesley, 1975).

Because animal blood is widely available in most countries, often at little or no cost, its use in aquaculture diets can help lower feed cost. Blood meal is superior to meat meal in test-diets for salmon (Fowler, L.G. and J.L. Banks, 1976).

H.4 Bone meals

Bone meals are made by heat processing of animal bones followed by grinding to produce a fine powder. Contrary to common belief, bone meal is not all calcium and phosphorus. Depending on the processing method employed, bone meal can contain up to 36 percent protein. Much of this protein is, however, of low quality since it is mainly collagen, the substance extracted industrially from bones to produce gelatin and glue.

With the development of fish diets containing ever-decreasing content of animal protein such as fish meal and meat meal, bone meal may become more prominant as a dietary source of calcium and phosphorus.

H.11 Hydrolyzed poultry feather meal

Hydrolyzed poultry feather meal is more commonly used as a nitrogen source in ruminant feeding. This is due mainly to its deficiency in most of the EAA's, thus making it unsuitable as a source of animal protein for other livestock. Its protein is predominantly keratin, very rich in the non-EAA cystine (as high as 25 percent). Unless properly hydrolyzed, feather meal is not very digestible for non-ruminants. Fish diets will not be improved by its inclusion unless they already contain other high quality protein sources.

Feather meal is available in those countries that have a highly developed and integrated poultry industry. Among the countries surveyed, only Venezuela, Mexico and Thailand appear to produce it on an industrial scale. Even so, the quantities available are quite limited. Imported feather meal is usually more costly compared to other animal proteins.

H.16 Trash fish

By-catches of the fishing industry have traditionally been used as raw materials for fish meal production or as feed for livestock and fish. The composition of such landings vary with the geographic area where the fish are caught. In tropical waters in the Indo-Pacific region, the threadfin snapper (Lutianus nematophorus) Malaysia, and the silverbelly (Leiognathus splendens) India, Indonesia, Sri Lanka, are among the principal species caught (Disney, 1979; Disney and James, 1980).

Freshly landed trash fish have a high feed value. However, rapid deterioration in quality accompanied by loss of nutrients, particularly protein, occurs with time, unless the fish are frozen. Certain marine species also contain high levels of the enzyme thiaminase which is activated at post mortem. Thiaminase destroys thiamine, a B-vitamin otherwise present in considerable amounts in fish. Trash fish, when fed fresh, with or without prior cooking, are superior to fish meal in terms of protein content (expressed as percentage dry matter) and protein quality. When mixed and properly processed in correct proportions with less expensive but more readily available ingredients, such as rice bran, and with adequate supplementation of vitamins, trash fish make excellent diets for aquaculture. A major problem associated with feeding trash fish is maintenance of water quality. This can, however, be partially overcome by proper choice of the dry components, e.g. rice bran which absorb the excess moisture and appropriate diet preparation methods to produce feeds that remain stable in water until consumed.

Trash fish in the fresh state are used most extensively for aquaculture in Thailand. The other countries with the exception of land-locked Nepal, also possess this resource, but have not utilized it to the same degree in aquaculture. Unlike most other ingredients, supply of trash fish is highly seasonal. When available, trash fish are considerably less expensive than fish meal.

H.17 Fish meal

Although fish meal production in countries such as Thailand and Peru is an industrial activity linked to world demand for this important feed commodity, in other developing countries it remains a way of utilizing surpluses and fish that cannot be sold for human consumption.

There are two principal ways of making fish meal. The first involves direct oven drying of whole fish and/or residues (the latter usually from canning operations). The product is then ground to produce what is known commercially as white fish meal. The second method involves the cooking of whole fish followed by pressing to remove the oil and finally grinding up of the press cake after it has been dried in a steam jacket. This process produces a dark coloured fish meal. Peruvian and Thai fish meal belong to the latter category. There is a notable difference between these two varieties. Whereas Peruvian fish meal is made from the main catch, usually of a single species, anchovy, Thai fish meal is derived from by-catch consisting of many species. The Peruvian product is therefore more consistent with regard to quality. Protein content of Peruvian fish meal is also higher - 65 percent compared to 50-60 percent for Thai fish meal.

Fish meal quality is also very much affected by the drying process. Some backyard fish meal factories in West Malaysia dry the processed fish over fire in concrete tubs. Although the meal is mechanically stirred during this operation, without proper supervision, over-cooking frequently occurs. The fish meal becomes very dark coloured, a sign of scorching of the product. On the east coast of India and in Sri Lanka, fish meal is usually made from sun-dried by-catch consisting mainly of silverbelly (Leiognathus splendens). Often the drying is not complete with the result that the product becomes infested with Salmonella bacteria. This contaminant poses a serious risk for users of sun-dried fish meal.

Another contaminant of fish meal is urea, sometimes added by unscrupulous merchants to give an otherwise inferior product a deceptively high crude protein content. Good quality fish meal should also contain no more than 2 percent salt added as a preservative before processing, and, if kept for long periods before use, should contain an anti-oxidant.

Fish meal remains an important but expensive ingredient in most fish and crustacean diets. It is especially rich in the EAA's lysine and methionine and minerals, and is highly digestible for fish. Its use appears limited only by high cost and availability.

Dry diets containing a high proportion of fish meal are difficult to pelletize.

H.20 Fish oil

Fish oil is a by-product of fish meal production. It is rich in vitamins A and D, and polyunsaturated fatty acids that are required by fish. Although widely used to supply these nutrients in fish diets in Western countries, fish oil is seldom used in commercial fish feed manufacture in developing countries. The cost of fish oil depends in part on food habits of the human population. In some countries, the oil from fish meal production is merely discarded.

H.22 Fish silage

Fish silage has not come into widespread use as a feed for fish. Although developed upon the need for a less expensive method for preserving trash fish, it is also a cheaper alternative to fish meal for use in aquaculture diets. It is used in commercial salmon farming in Denmark and Norway and in experiments in developing countries involving warm-water species, viz the snake-head in Thailand, and the carp in Indonesia.

Fish silage can be prepared in a variety of ways. These include fish/carbohydrate fermentation, and ensiling with mineral and/or organic acids (Raa, J. and Gildberg, A., in press).

When properly prepared, fish silage contains practically all the nutrients of the raw fish. If an extraneous energy source, e.g. rice bran, is added during processing, as is required in naturally fermented silage, the feed value of the added component is also enhanced.

Efforts so far in the use for fish silage in Indo-Pacific region have been for feeding poultry and pigs. A project at the FAO Regional Lead Centre for Aquaculture in Thailand (RLCT), studies the effects of fish-carbohydrate silage on the growth of snake-head. The silage, in semi-moist form, was extruded into pellets before feeding. Preliminary results showed growth comparable with that achieved with fish meal.

Although more costly to use than trash fish, fish silage is less expensive than fish meal. The main advantage it has over trash fish is its comparatively long shelf-life. Another desirable aspect of fish silage as an ingredient for moist pellet production is its good binding properties.

H.23 Shrimp meal

Shrimp meal is made from freezing-plant waste (heads and scales) or from whole shrimps not suitable for human consumption. The material is first dried in the sun or in an oven and then ground. Because shrimp exoskeleton is primarily chitin, an indigestible nitrogen-containing homopolysaccharide, the true protein content of shrimp meal made from freezing-plant wastes is only 50 percent of the value obtained by proximate analysis. This high content of indigestible matter restricts its use to less than 25 percent in aquaculture diets (Randall Robinette, H. and A.S. Dearing, 1978; Meyers, S.P., 1981), Shrimp meal is especially rich in choline, a B-vitamin.

Because crustacea also contain carotenoid pigments, shrimp meal has long been used in trout and salmon diets to improve flesh colouration in the fish. Up to 15 percent in the diet has been used for this purpose. The exoskeleton of shrimp is tough. Unless finely ground, shrimp meal added in large amounts to the diet may result in difficulty in obtaining good quality pellets.

Shrimp meal, where available, is usually far less expensive than fish meal.

H.26 Silkworm pupae and other insect larvae

Available in quantity only in India and Thailand 1/, silkworm pupae are rich in nutrients. When fed fresh they are a good source of protein as well as fats (especially unsaturated fatty acids). The meal, from which the fat has been extracted, has been used with good results as a fish meal substitute in experimental diets for carp (Hickling, C.F., 1962). Due to the high content of chitin in silkworm pupae, its actual protein value is only 75 percent of the value obtained by proximate analysis.

1/ Outside the surveyed countries, China and Japan are major producers

Fly larvae grown on animal wastes have also been successfully used as a protein source in diets for tilapia and channel catfish (Bondari, K. and D.C. Sheppard, 1981).

H.28 Milk by-products

Milk by-products such as milk powder, due to their high cost, are seldom used in preparing fish feed, although the milk protein, casein, is widely used in purified diets for experimental purposes. Nevertheless, quite often milk powder and even cheese become available as surplus or damaged items unsuitable for human consumption. Whey powder, on the other hand, is more commonly available as stock feed. The latter is a residue from the manufacture of cheese. In the Indian subcontinent, a high protein waste product is also obtainable from the refining of ghee.

Whey and dairy-processing wastes have been successfully used at the 10 percent level in diets for rainbow trout (Rumsey, G.L. et al., 1981). Their incorporation into diets also improve pellet quality.

2.1.7 Miscellaneous feedstuffs

Apart from the ingredients described in the preceding sections, there are also feed materials which do not quite fit into the categories mentioned above in this section. Many of these have been loosely termed 'non-conventional' feedstuffs. Although materials such as leaf protein concentrate (LPC) and some single cell proteins (SCP) can be included among them, others such as molasses and yeast have long been among common ingredients used in the formulation of feeds for livestock and fish. The availability of these miscellaneous feedstuffs varies from country to country, but quantities presently involved are small. Nevertheless, these unconventional sources of feed proteins are expected to become increasingly important in the future.

I.2 Leaf protein concentrate, LPC

Leaf protein concentrate is produced by first grinding the plant and then separating the juice by pressing. The juice, which contains the dissolved protein, is then coagulated by heating. The curd obtained is removed and dried.

The machinery required for large-scale production of LPC is expensive; the minimum economical output being 10 tons of leaf protein per hour. Smaller units are being designed for use at the village level.

Dried LPC has a protein content somewhat lower than that of soyabean meal. Its protein quality, on the other hand, is higher than that of the latter but lower than that of fish meal. Problems have been encountered in its use in feeding poultry and pigs. LPC made from the genera Leucaena and mimosa contain toxic mimosine, a cyclic amino acid. Although few studies have been conducted with fish, it is presumed that LPC made from alfalfa and other legumes may be safely used. With other sources, the presence of toxins other than mimosine, viz., haemaglutinins, glucosides, saponins, etc., have to be taken into account. Leaf protein concentrates can be produced from the leaves of tropical legumes such as the Acacia and Desmodium species, and from grasses. Leaf protein concentrate from rye grass used at levels of up to 48 percent of total dietary protein have been successfully tested on carp and trout (Ogino, C., C.B. Cowey and J. Y. Chiou, 1978).

High processing cost of juice extraction is a major constraint in wider use of LPC in compound feeds. However, where high quality protein sources such as fish meal and soyabean meal are not available or are prohibitively expensive, LPC may be an acceptable substitute. The raw juice that is pressed out can be used directly by blending with other dry components, and the entire mixture subjected to heat treatment before extrusion into moist pellets. Heat processing will not only destroy most of the toxins and anti-nutritive factors present in the raw juice, but will also lead to production of pellets with good water stability since the soluble protein in LPC coagulates on heating.

I.7 Cane molasses

Refining of cane sugar produces a by-product known as blackstrap molasses. This highly viscous (specific gravity 80-85°C Brix) material is difficult to handle at temperatures below 30°C. Therefore, blackstrap molasses is normally diluted with water to lower the specific gravity to about 79.5° Brix so that it may be more easily transferred by pumping.

Cane molasses is predominantly sugar of which roughly half is sucrose and the other half reducing sugars. Its principal use in the production of compound feed for poultry and livestock is as a dietary energy source, although it also enhances acceptance of the feed. Its incorporation into pelleted feed has a positive effect on the physical properties of the pellets.

Use of molasses in compound fish feed has not been thoroughly investigated. Presumably, its level of usage should not exceed 5 percent of the dry diet, considering its established laxative effect (when used beyond 5 percent for poultry and 15 percent for pigs).

Because of its relative cheapness and wide availability in the tropics, molasses is a good partial replacement for the more expensive cereal grains as an energy source.

Because molasses ferments rapidly it may also be added in small quantities along with rice bran to trash fish in the production of fish silage.

I.12 Brewer's spent grains

Brewer's spent grains, or brewery waste, as they are often termed, are residues from the initial breakdown of starch to fermentable sugars in beer manufacture. Unless dried, the fresh material, even after pressing, contains a very high water content. Ordinarily, the wet material may be kept for up to two weeks at ambient temperature without spoilage. If longer storage is desired, it should be ensiled or dried, the latter process usually preferred by large breweries.

Fresh brewer's grains are fairly rich in protein and digestible carbohydrates. The material, if mixed with other feedstuffs before feeding, should first be boiled to deactivate enzymes that may otherwise act on carbohydrates to produce alcohol, and then organic acids.

Raw brewery waste is available in considerable quantities in large urban areas of most countries, including those covered in this survey. It is usually discarded by the breweries and, therefore, like fresh animal blood from abattoirs, obtainable at little or no cost. Levels of up to 50 percent in the diet have been successfully fed to tilapia in Africa.

I.14 Brewer's yeast

Brewer's yeast is obtained as a residue after the fermentation process in beer production. The quantities recovered are not as great as brewer's grains, but the material has a higher feed value and contains more than twice as much protein on a dry weight basis. The yeast is usually dried over a steam heated drum. Raw brewer's yeast should also be boiled before use in fish feed to de-activate the enzymes present. Yeast is a rich source of B-vitamins (excepting vitamin B12) but low in calcium. Due to its high cost, it is seldom used at levels beyond 5 percent of the total diet.

I.15 Grain distiller's by-products

Distillery wastes are obtained from the manufacture of alcoholic spirits. Their composition depends upon the type of grain used, viz., maize, sorghum, wheat or rice. Two processes are employed in making alcoholic spirits. The first is known as the British process which involves the two basic steps of grain conversion, followed by separation of spent grains, fermentation of sugars to alcohol and final distillation of the product. This process yields two products: distiller's grains, and distiller's solubles (including yeast). The other process, known as the American process, does not involve separation of spent grains before the fermentation process and yields only one product that includes both the spent grains and yeast.

Distiller's by-products have a lower fibre content than brewery waste. The material, when added to rations, appears to stimulate growth in poultry and livestock. The effect has been attributed to unidentified growth factors believed to be present in the material. Due to its lower availability and higher feed value, distillery wastes are usually more expensive than brewery wastes. The material is used whenever available at low levels in compound fish feed (Reece, D.L. and D.E. Wesley, 1975).

I.17 Fats

Fish have a relatively high requirement for lipids that serve as a source of poly-unsatured fatty acid and as an energy source. Added fats have also been shown to improve the overall digestibility of fish diets. Other effects of addition of fats in manufactured diets are prolongation of pelleting die life and reduction of dustiness during milling operations.

Fats from a variety of sources are available in the tropics. These sources include: animal fats, fish oils, vegetable oils and soapstocks. Certain fats become rancid rapidly unless anti-oxidants are added. The rate and degree of rancidity occurring in untreated fats depend on its PUFA composition. Fish oils and most raw vegetable fats have a high degree of unsaturation and anti-oxidants are normally added during the refining of such fats. The digestibility of hydrogenated (hardened) fats made from both fish oils and beef tallows for carp and trout has been shown to depend on the melting point of the fat. Hydrogenated fat with m.p. exceeding 40°C is poorly digested (Takenchi, T., T. Watanabe and C. Ogino, 1979).

Processed animal fats are not ordinarily available in the tropics. On the other hand, there is an abundance of vegetable oils and soapstocks. Most common are oils from coconut, soyabeans, groundnut and palm nut. Even then, these are usually refined for human consumption, and with the exception of crude palm oil, are too expensive to use in feeds. Whenever available, however, their inclusion enhances the feed value of fish diets significantly.

Soapstocks are by-products of the edible oil refining industry. They constitute the non-fat residues of crude oil plus the alkaline salts of free fatty acids that are removed in the refining process. Soapstocks can be fed safely to fish at up to 5 percent of the total diet without adverse effects.

The following gives the PUFA content of fats from a wide variety of animal and plant sources.



Animal or fish source



Beef tallow


Cod, Atlantic


Herring, Atlantic


Rainbow trout


Common carp




Vegetable source

























Using this information together with lipid content of ingredients provided in Table 1, one could determine if a certain diet is likely to be deficient in polyunsaturated fatty acids.

I.18 Bacterial protein

Bacterial protein is among the single-celled proteins (SCP) that have received increasing attention in recent years. Methanomonas spp. are cultured in an aqueous suspension containing mineral salts and a nitrogen source. A mixture of methane and air is bubbled continuously through the suspension. The harvest of bacteria, usually one every three days, has a high protein content, a good portion of which is nucleo-protein. Although its production may be feasible in countries which have a surplus of methane, the cost of the product is presently high (Anonymous, 1982). Studies on its use in fish feed are not conclusive although there are indications that it is superior to other SCP such as yeast and algae (Atack, T.H., K. Jauncey and A.J. Matty, 1979; Kanshik, S.J. and P. Luquet, 1980).

I.19 Algae

The cultivation of micro-algae as food or feed is not new. The production of algal blooms and benthic algal complexes through fertilization and water management have been practised in Asia for long. Certain species of Chlorella, Scenedesmus and Spirulina have been established as excellent feed for larvae of many cultured species of fish (Rumsey, G.L. et al., 1981). Large-scale mono culture of these SCP's, however, involves technology that is still at a developmental stage. Moderate scale production at the rural level employing animal (particularly pig) manure has been shown to be feasible. The harvested product, when dried, is non-toxic and can be used to supply practically all the protein required without ill effect, although supplementing lysine (for Chlorella) or methionine (for Scenedesmus and Spirulina) may be necessary. Scenedesmus obliquus has been tested quite extensively in carp and found to be a good substitute for fish meal (Meske, Ch. and E. Pfeffer, 1978, 1978a).

I.20 Yeast

The most commonly used and widely available SCP for animal feeding is torula yeast (Candida utilis). This is cultured on substrates comprising a variety of industrial wastes, including: molasses, dried citrus pulp, or sulphite liquor from the wood pulp and paper industries. The harvested yeast is usually dried over steam-heated drum rollers. More recently, paraffin-grown yeasts have been produced in small quantities for evaluation as food for both livestock and humans. Feedstock for production of such yeasts are diesel oil or n-alkanes.

Feed-grade yeast have been shown to be excellent substitutes for fish meal at low levels in diets for both fish and livestock. The amount of substitution depends upon the type of yeast and the manner in which it is produced. In general, yeasts are relatively low in methionine. Proper supplementation with synthetic sources of the amino acid could permit yeast to be the only protein source in the diet. On the whole, petroleum yeasts (Candida lipolytica) have higher feeding value than yeasts made from molasses (C. utilis). Yeasts, like bacterial proteins, are rich in nucleic acids which limit its use as food for humans. Whether this applies also to fish has not yet been determined.

Yeasts from molasses and other industrial wastes are less costly to produce than paraffin-grown yeasts. Nevertheless, due to the high cost of fuel for drying, yeasts as a class of feedstuffs, are still more expensive to use than other protein supplements of vegetable origin. However, this could change in the future.

2.2 Composition of feedstuffs

The composition of the feedstuffs discussed are presented in Table 1. The data have been drawn from sources as noted, including the results of actual analysis at the Regional Aquaculture Lead Centre in India (RLCI). Where values of proximate analysis of particular ingredients from the corresponding countries are not available, published values for comparable materials available in other countries have been substituted. Similarly, where amino acid values for ingredients are not available, computations have been made, based on established values for comparable materials.

Digestible energy values of most feedstuffs have not been established for fish. However, they can be approximated by using the following energy values for the respective nutrients:


Estimated Digestible Energy
(kilo calories/g)

Carbohydrate (non-legumes)


Carbohydrates (legumes)


Proteins (animal)


Proteins (plant)




It will be noted that 'food quotient' values, derived mainly from traditional feeding practices, and frequently employed to compare the value of individual feedstuffs in supplementary feeding of fish, have been omitted in the present consideration. Basically it is an expression of feed conversion ratio. As such, it is not surprising that food quotient values range widely. For instance, the food quotients for maize and groundnut cake are 4 to 6 and 2 to 4, respectively. Even fish meal has a quotient ranging from 1.5 to 3.0.

Food quotient values reflect not only the nutrient composition of individual feedstuffs, but also the contribution, if any, of natural foods present in the culture medium in particular situations. Fish, like all animals, require diets containing nutrients in proper balance. A deficiency in one essential nutrient may affect the absorption and utilization of others, and an excess may also have the same undesirable effects. Supplementary feeding with individual feed ingredients is usually carried out without consideration of the quality of other foods present in the culture medium and can, therefore, be wasteful.

The Feed Code is an alpha-numeric term employed by FAO for generic classification and identification of feedstuffs.

The abbreviations employed are as follows:


Dry matter


Crude protein


Ether extract


Crude fibre


Nitrogen free extract




Total phosphorus

Table 1 Composition of Feedstuffs

2.3 Feed quality

2.3.1 Protein quality
2.3.2 Anti-nutritional and toxic factors

The nutritive value of an aquaculture diet depends not only upon proper balance of nutrients according to requirements, but also on it being free from anti-nutritive factors that might have been present in the ingredients and remain active after processing. In the search for less expensive substitutes for animal protein, certain plant proteins, particularly those belonging to the Leguminosae family, have emerged as the most promising. These new protein sources are especially important because they are widely available in the tropics. Plant proteins, however, are usually low in certain essential amino acids which are necessary for rapid growth in animals, including fish. Leguminous plants are also known to contain natural substances that, unless removed or deactivated, are either toxic to the consuming animal or interfere with its normal digestive processes. Less is known about toxic factors in microbial proteins.

This section looks at the nutritional quality of some plant and single-celled proteins that are presently considered for use as substitutes for animal protein sources in formulation of diets in aquaculture.

2.3.1 Protein quality

It has been determined from human as well as animal studies that chicken eggs are the richest and best source of food protein. Such being the case, egg protein has been used as a standard against which other food proteins are compared. Although the basis for applying this standard to fish feeds has not been established, it is generally recognized that, based on recently completed work involving feeding different protein sources to fish, amino acid content rather than source determines the nutritive value of protein feeds. The egg standard, therefore, can be applied to aquaculture diets for all practical purposes.

There are many ways of expressing protein quality of a feed. One of these is by comparing its essential amino acid content with that of whole egg. This gives a "chemical score" for the feed. In practice, this would first require amino acid analysis of the feed.

Table 2 gives the individual amino acid scores of a variety of plant proteins. The values of some animal proteins, including fish meal, have been included for comparison. The "protein score" as indicated in the last column is a composite score of six essential amino acids which are critical for growth in animals: isoleucine, leucine, lysine, the sulphur-amino acids cystine and methionine, phenylalanine and valine. Although the other amino acids listed, arginine, histidine, threonine and tryptophan, are also classified as essential, their impact on growth is not as great as the others.

With few exceptions, plant proteins are especially deficient in the sulphur amino acids. These amino acids are also limiting in meat meal. The leguminous seeds, quite surprisingly, are relatively rich in lysine, the amino acid that is most often the first to be limiting in compound feed. Meat meal is also limiting in isoleucine, a branch-chain amino acid. This, together with a low content of methionine, makes meat meal a rather poor quality protein source, despite its animal origin.

From the "score" column, it will be noted that the legumes and single-cell proteins range in the 70's, making them almost comparable to fish meal, which has a score of 79. Indeed, fish meal has been successfully replaced by soyabean meal and yeast after proper supplementation with methionine. Of particular interest is the high score of the beans (mung and red) which, although having comparatively low protein content, hold promise as substitutes for fish meal as well. Unlike other plant proteins, sesame and palm kernel proteins are surprisingly rich in the sulphur amino acids. Sesame oilcake, because of its high overall protein content, is especially valuable as a natural source of supplement for these amino acids. When used with soyabean meal, it would help achieve the proper balance of sulphur amino acids and lysine in the diet.

Table 2 Limiting amino acids and chemical score of essential amino acid content of selected feed proteins 1/

1/ Scores based on comparison with whole egg protein of the following amino acid composition (percentage of protein): arginine, 6.7; cystine, 2.2; histidine, 2.7; isolercine, 7.0; leucine, 8.5; lysine, 6.8; methionine, 3.3; phenylalanine, 5.4; threonine, 5.5; tryptophan, 1.9; and valine, 8.2.

2.3.2 Anti-nutritional and toxic factors

The seeds of legumes and other plants contain a wide variety of toxic and potentially toxic substances (Liener, I.E., 1978; American Chemical Society, 1969). These include: trypsin inhibitors, phytohaemagglutinins, goitrogens, cyanogenetic glycosides, anti-vitamin factors, and toxic amino acids. The effects of most of these toxins have not been described in fish,

Protease Inhibitors

The most commonly encountered class of feed toxins of plant origin are the protease inhibitors. Most well known among these is the trypsin inhibitor occurring in raw legume seeds, particularly soyabean. The mode of action of trypsin inhibitor has not been studied in fish. In other laboratory animals, it is believed to lead to pancreatic hypertrophy with concommittant loss of endogenous protein secreted by the pancreas. This protein is largely made up of cystine-rich pancreatic enzymes. Trypsin inhibitors were first described in soyabean, but are also known to be present in groundnut and most other leguminous seeds.

Trypsin inhibitors are heat labile. The degree of deactivation of the toxin depends upon the applied temperature, duration of heating, particle size and moisture conditions. Soyabean trypsin inhibitors are totally deactivated by moist heat at 100°C for 15-20 minutes.

Overnight soaking or germination of the beans, while bringing about improvement in their nutritive value, does not destroy the trypsin inhibitors.


These toxins, which cause red blood cells to agglutinate, are present in almost all legume seeds. The potency of the toxin, however, varies from species to species, with those of Phaseolus vulgaris among the most potent. Phytohaemagglutinins are also heat labile, but care should be taken to ensure that adequate heat (normal boiling temperature) is applied.


Goitrogen effects have only been demonstrated in two legume seeds, the soyabean and the groundnut. The symptom in the rat and chick is enlargement of the thyroid gland. Goitrogens are destroyed by heat.

Cyanogenetic glycosides

Cassava and sorghum are the two feeds most often associated with cyanide poisoning in livestock. Other feeds are linseed meal and certain legume seeds, including the lima bean, the kidney bean, the Bengal gram and the red gram. Cooking in water not only destroys the enzymes responsible for cyanide release, but also volatilizes the HCN released.

Antivitamin factors

Antivitamin D and antivitamin E factors are known to be present in raw soyabeans and raw kidney beans, respectively. These anti-nutritive factors are destroyed by heat.

Toxic amino acids

Ipil ipil (Leucaena leucocephala), a tropical legume exploited as a feed for chicken and livestock, contains a cyclic amino acid, mimosine, which is toxic at high levels. The amino acid is stable to heat and its permanence in the leaf limits use of ipil ipil to less than 20 percent of the diet for most classes of livestock.

Previous Page Top of Page Next Page