Chapter 3
POND FERTILIZATION AND FISH FEEDS

Pond Fertilization

Yu Shigang

Significance of Pond Fertilization

China has a long history of pond fertilization for fish culture. Farmers adopted the method of manuring to rear fry ages ago. For example, “dacao” (green manure) is used in Guangdong and Guangxi provinces and human excrement (night soil) is used in Jiangxi and Hunan provinces to nurture fry into summer fingerlings. In fingerling-rearing ponds, fertilization is aimed at developing natural food organisms and saving artificial feeds.

Phytoplankton, the elementary producers of the pond, carry out photosynthesis, converting the inorganic materials in the water into the organic nourishment needed for their growth and reproduction. Fertilization supplies the phytoplankton with the materials essential for photosynthesis. As the phytoplankton photosynthesize and reproduce, zooplankton, which feed on phytoplankton, flourish. In turn, the fish, which feed on zooplankton, phytoplankton, and benthos, also flourish. Therefore, the importance of pond fertilization lies in the cultivation and propagation of various food organisms for the cultured fish.

The series of interrelations between predators and prey is called a “food chain.” In culturing ponds, fish are the final link: e.g., phytoplankton→silver carp; phytoplankton→zooplankton→bighead; aquatic plants→grass carp; plankton →benthos→black carp. Usually animals use only 5–20 per cent of the energy in both animal and plant feeds. Utilization of energy is related to the length of the food chain: the shorter the food chain, the higher the rate of energy transfer. In other words, the higher the utilization rate of energy, the higher the fish production.

The biota of the ponds is in a constant process of growth and decay. Dead organisms are decomposed from complex organic materials into simple inorganic materials by bacteria. These inorganic materials dissolve in water and are utilized by phytoplankton in photosynthesis. Hence, the materials in the pond are in a constant state of circulation mainly through the food chain (Fig. 3.1). This is called “pond material circulation.”

Varieties of Organic Manure

Organic manures are mainly farm animal excrement. Generally, the term refers to manures containing organic matter. Today, mainly organic manures are applied to fish ponds in China. The following manures are often used: feces and urine of livestock and poultry, night soil, green manure, compost, arnd silkworm dregs. Only through decomposition by microorganisms is the organic manure converted to nutrients that the plants can absorb.

Fig.3.1. Pond material circulation.

Feces and urine of livestock and poultry

Pig manure — Pig manure includes much organic matter and other nutritional elements such as nitrogen, phosphorus, and potassium and is a fine, complete manure (Table 3.1). Pig feces are delicate, containing more nitrogen than other livestock feces (C:N = 14:1), making them more susceptible to rotting. The major portion of pig urine is nitrogen in the form of urea. It decomposes easily.

Table 3.1. Nutritional elements in pig manure.

The excretory amount of a pig is greatly associated with its body weight and food intake. A 50-kg pig discharges around 10 kg/day or 20 per cent of its body weight. A pig excretes 1000 kg of feces and 1200 kg of urine in the growing period of 8 months from pigling to adult. A pig's daily excretory amount is less than a cow's or a horse's; however, pigs are advantageous because of their faster growth, shorter fattening period, and suitability for pen culture. Also, pigs are raised on much larger scale, so it is beneficial to collect their manure.

Cattle manure — The elements of cattle manure are similar to those of pig manure (Table 3.2), but cattle are ruminants and the food stuffs are repeatedly masticated, making the excrement quite delicate. Cattle manure contains less nitrogen than pig manure (C:N = 25:1). Cattle urine contains more nitrogen than pig urine (in the form of hippuric acid, C₆H₅ CONHCH₂ COOH); therefore, cattle excreta decompose slowly. The average daily excreta is 25 kg/cow, in which the ratio of feces to urine is about 3:2. The total annual amount of excrement for each animal is 9000 kg.

Table 3.2. Nutritional elements in cattle manure.

Poultry manure — Poultry manures include the feces of chickens, ducks, and geese, and are rich in both organic and inorganic matter (Table 3.3).

Table 3.3. Nutritional elements in poultry manure.

Poultry manures rot quickly and their nitrogen is mostly in the form of uric acid, which cannot be absorbed directly by plants. Accordingly, poultry manures are more effective after fermentation. The annual amount of excrement per fowl is as follows: chicken, 5.0–5.7 kg; duck, 7.5–10.0 kg; goose, 12.5–15.0 kg. Although the annual excretory amount of each is comparatively small, the quantity of poultry culture is often great; therefore, the total amount of feces is significant.

Night soil

The composition of night soil (human excrement) (Table 3.4) is greatly dependent on the food consumed. Nitrogen is abundant (C:N = 3:1) and 70–80 per cent of it is in the form of urea. This facilitates easy decomposition.

Table 3.4. Nutritional elements in human excreta.

On average, an adult excretes 790 kg/year of waste material. This is equivalent to 22 kg/year of (NH₄)₂SO₄ (Table 3.5).

Table 3.5. Yearly excretion of waste by an adult human.

Night soil to be used as manure must be fermented before application. This is easily done by storing the manure in anaerobic conditions for 2–4 weeks. The decomposition of human waste produces ammonia. Under airtight conditions, the accumulation of ammonia can sterilize human waste. Quicklime (1–2 per cent) and formalin (0.1–0.2 per cent) are effective in killing the harmful pathogens in night soil.

Silkworm dregs

Silkworm dregs are composed of silkworm feces and slough and mulberry residues. They are rich in organic matter: dried dregs are 87 per cent organic matter and 13 per cent nitrogen. They also make good fish feed: 8 kg silkworm dregs can produce 1 kg fish.

Green manure

All wild grasses and cultivated plants, if used as manure, are called green manures (Table 3.6). These manure rot and decompose easily, providing ideal environments for bacteria propagation. Therefore, they are good for application in fish ponds.

Table 3.6. Nutritional elements in green manures (% wet weight).

Compost

Mixed compost consists of green manure and animal waste. Mixing several manures together may produce a fertilizer that is more suitable to plankton reproduction. The ratio of the constituents depends upon the local sources of manure. Experimental data show the following two mixtures to be suitable for plankton reproduction: (1) green grass — cattle feces — human excreta — lime, 8:8:1:0.17; (2) green grass — cattle feces — lime, 1:1:0.02.

Lime is included in the compost mixture to neutralize the organic acids produced during rotting and decomposition. If these acids were allowed to accumulate, they would inhibit the microorganisms responsible for decomposing the organic matter. There are two methods of making compost; heaping and soaking.

Heaping method — The manure heap must be made in aerobic conditions. Spread out a layer of green grass, sprinkle some lime on it, add layer of fecal manure, and repeat the procedure. When the compost reaches 1.5–2 m, cover it with 5–6 cm of mud. The ingredients of the compost will rot and decompose. After 3–4 weeks the compost can be used.

Soaking — Dig a pit near the fish ponds and layer in green grass, lime and fecal manure, respectively and then add enough water to soak the compost ensuring there is no leakage. The compost can be removed for use after 10–20 days of fermentation at a temperature of 20–30°C.

Methods of Organic Manure Application

Application of Dacao

In Guangdong and Guangxi provinces, Dacao is commonly used to fertilize pond water. Dacao consists mostly of composite plants, with some gramineous plants and leguminous plants included. Dacao is applied by heaping it at a corner of the pond and turning the pile once every 2 days. The rotten parts will spread into the water. The roots and stems, which rot slowly, are dredged out of the ponds when the Dacao pile is depleted. The decomposition of green manure in water consumes a great amount of oxygen. Experimental data show that if 1000 kg of grass is applied to a 1-mu pond with a water depth of 1 m, there will be no available oxygen from the 2nd to the 6th day and all the fish will die. The peak of oxygen consumption because of decomposition is on the 2nd and 3rd days. For this reason, it is appropriate to apply green manures frequently and in small amounts, to frequently add fresh water to the pond, or to use aerators in the pond to guarantee sufficient oxygen for the fish.

Application of night soil

In Jiangxi and Hunan provinces, night soil is commonly applied to the fish ponds. Before application, one part night soil is diluted with two parts water. This dilution is then sprayed along the pond dikes once a day. The quantity depends on the fertility of the water and the size of the fish.

Application of livestock manure

The application of livestock manure as a base manure is similar to the method for green manure: heap the manures at a corner of the pond or in small piles in shallow water and with a sunny exposure, allow them to decompose and spread gradually into the water. If the manure is used as an additive, it is added in small quantities every 7–10 days.

Application of mixed compost

After fermentation, the compost is flushed. The liquid is collected and the residue is removed. This liquid is sprayed into the pond around the dikes. In the case of a large pond, the manure may be loaded on a boat, flushed in batches with pond water, and sprayed evenly over the pond. The manure dregs can be used to fertilize crops. Alternative method is to flip the compost and expose the liquid. The appropriate amount of manure liquid can then be ladled out and spread into the ponds.

The nutrients of the compost are quickly absorbed by phytoplankton. They consume less dissolved oxygen because the organic materials are already decomposed.

Effects of Manure Application on Natural Food Organisms

The application of organic manure results in the rapid multiplication of bacteria. Bacteria are added with the fertilizer and use the nutrients to reproduce. Also, organic detritus is rich with bacteria, which are an important food source for the lower aquatic animals and filtering fish.

The initial, predominant species of plankton depends closely on the properties of the manure applied. If organic manure is applied, phytoplankton, (Ochromonas spp, Cryptomonas) and zooplankton (Urotrichia spp.), which are fond of organic materials, will appear first. For inorganic manure, the initial, predominant species will be centric diatoms (Centomonas spp. and Scenedesmus acuminatus). There is a close relationship between the amount of manure applied and the make-up of the plankton community. Large amounts of manure will lead to the presence of some species of green algae (Chlorophyta) and blue algae (Cyanophyta); however, small amounts of manure will lead to the presence of Navicula rostellata and Cyclotella stelligera.

After each manure application, the nutrient content of the water increases, resulting in a planktonic peak. Phytoplankton that are easily digested by silver carp reach a peak after 4 days; those phytoplankton that are not so easily digested attain a climax in 5–10 days. Zooplankton reach a peak in 4–7 days. Protozoans will be the first zooplankton to reach a peak, followed by rotifers, cladocerans, and, finally, copepods. Protozoans multiply by binary fission, increasing the population very rapidly, and, therefore, reaching a peak first. Rotifers usually multiply by parthenogenesis, producing an average of 10–20 eggs during their lifetime. This process is less productive than binary fission and, therefore, rotifers reach a population peak slightly later than protozoans. Cladocerans also reproduce parthenogenically, but the span between hatching and sexual maturity is longer than that of rotifers; therefore, the cladoceran population peaks later. Copepods take longer time than cladocerans to get mature and its population becomes maximized later. The timing of manure application is crucial when preparing a nursery pond. Ideally, the peak in plankton population should coincide with the feeding demand of the fish fry.

Varieties of inorganic manure

Inorganic manures are also referred to as chemical fertilizers. According to composition, chemical fertilizers can be divided into three groups: nitrogenous, phosphoric, and potash fertilizers. The advantages of inorganic fertilizers are their exact composition, their fast effect, the lack of pollution, their beneficial effect on oxygen content (requiring no decomposition), the small amount required, and their convenient utilization. However, when chemical fertilizers are applied in ponds, the first link of the food chain is principally phytoplankton, which are not as nutritious to zooplankton as are bacteria. Therefore, the zooplankton population in ponds treated with inorganic manure often lags far behind that in ponds treated with organic manure. Moreover, in most chemical-fertilizer ponds, the predominant phytoplankton is Chlorophyta, which is not as nutritious as the predominant phytoplankton in ponds treated with organic manure (Chrysophyceae, Bacillatiophyceae, and Cryptophyceae). Another disadvantage is that the effect of inorganic fertilizer is rather short and it is difficult to control the water quality. Taking all these factors into account, therefore, the result of chemical-fertilizer application alone is no better than that of organic-fertilizers application.

Nitrogenous fertilizers

Liquid ammonia — Molecular formula: NH₄OH or NH₃.H₂O. Nitrogen content: 12–16 per cent. Liquid ammonia is an aqueous solution of ammonia, which is an important product of small-scale nitrogenous fertilizer factories and is easily synthesized at a low cost. Aqueous ammonia is readily volatilized and should not be exposed to the air for a long time.

Ammonium sulphate — Molecular formula: (NH₄)₂SO₄. Nitrogen content: 20–21 per cent. Ammonium sulphate is produced from liquid ammonia directly neutralized with diluted sulphuric acid. When pure, it is a water-soluble white crystal: 75 kg of ammonium sulphate will dissolve in 100 L of water at 20°C. It is easily conserved and applied.

Urea — Molecular formula: CO(NH₂)₂. Nitrogen content: 44–46 per cent. Under high heat and pressure, ammonia and carbon dioxide react to form urea. It is a white crystal with a high solubility in water. However, urea does not ionize when dissolved in water and, therefore, cannot be directly absorbed by plants. It can be utilized by plants only after it has been broken down by urease, excreted from urea-decomposing bacteria, and transformed into ammonium carbonate. This process is temperature dependent in normal ponds. At 20°C, total transformation into ammonium carbonate requires 4–5 days; at 30°C, 2 days.

Phosphoric fertilizers

Calcium superphosphate — Main contents: Ca(H₂PO₄)₂.H₂O with 12–18 per cent P₂O₅. Subsidiary contents: CaSO₄.2H₂O, about 50 per cent. Calcium superphosphate is usually a white powder, and apt to absorb moisture. It is corrosive and has an acidic odour because it contains some free acids.

Methods of Inorganic Manure Application

Nitrogen is an essential nutritional element of plants. It is also an essential component of proteins, accelerates the formation of plant chlorophyll, and stimulates photosynthesis. For these reasons, nitrogen content is a decisive factor in phytoplankton production.

Nitrogen is commonly lacking in pond water, so nitrogenous fertilizers should be added. Generally, nitrogenous fertilizer should be used as an additive because of its quick effectiveness. A nitrogenous fertilizer with ammonium must not be mixed with strong alkaline materials e.g. lime; this would result in the volatilization of the ammonium. When using a nitrogenous fertilizer containing ammonium, the toxicity of ammonia must be considered. In aqueous solution, an equilibrium exists between ammonia (NH₃) and ammonium (NH₄⁺).

NH₃ + H₂O ⇌ NH₄⁺ + OH^-

In an acidic state, the equilibrium shifts to the right and the concentration of ammonium ions increases. In an alkaline state, the equilibrium shifts to the left and the ammonia concentration increases. At a water temperature of 25°C, the percentage of nitrogen as NH₃ at various pHs is as follows: ph 6, 0.05 per cent; pH 7, 0.49 per cent; pH 8, 4.7 per cent; pH 9, 32.9 per cent; pH 10, 83.1 per cent; pH 11, 98 per cent.

Ammonia is toxic to fish. It poisons juvenile rainbow trout at 0.3–0.4 mg/L. Chinese carps can tolerate concentrations up to 13 mg/L. Ammonia concentrations below this inhibit growth. The maximum NH₃ concentration permitted for fish farming is 0.1 mg/L. Therefore, the amount per application must be strictly controlled. In addition, ponds pH must be closely monitored to avoid applying NH₃ in strong alkaline water (e.g., just after pond clearing with lime); liquid ammonia is also alkaline itself. The amount of unionized ammonia increases with increasing water temperature. Therefore, special care is needed when nitrogenous fertilizer in the ammonia form is applied in the summer and the autumn.

The application amount of the nitrogenous fertilizer depends on its nitrogen content. In a pond with an area of 1 mu and a water depth of about 1.5 m, 1.5–2 kg N may be applied as base manure. After this, 0.5 kg N/mu is applied 3 or 4 times monthly. In average, 10 kg N are needed for the whole culture period. For example, if the nitrogen content of ammonium sulphate is about 20 per cent to apply 2 kg of nitrogen to a 1-mu pool as base manure, 10 kg of ammonium sulphate is required. The amount of ammonium sulphate required for a culture period can be calculated in the way: 50 kg.

To apply, make a solution and spread it near the dikes. In the case of liquid ammonia, put the container underwater and open the lid to let the liquid ammonia slowly diffuse out. In this way, volatilization can be avoided.

Most water sources lack phosphorus. Phosphoric fertilizer, besides being utilized by phytoplankton, will also accelerate the reproduction of azotobacteria and complement the nitrogenous fertilizer. The application amount can be calculated based on the phosphoric acid content of the fertilizer. A 0.5–1 kg/mu is used as base manure, the amount used for a culture period is about 5 kg. The method of calculation and application is the same as that for nitrogenous fertilizer.

Potassium is also an essential nutritional element of plants. However, it is usually sufficient in the water and there is no particular need to apply potash fertilizer.

Fish Feeds

Significance of Feeding

In addition to fertilizing ponds for the proliferation of natural food organisms, artificial feeds must also be used to meet the demands of various species of fish. Fish feeds are the prime material base of intensive fish culture. Applying artificial feeds in a fish pond can significantly raise per-unit yield. With common carp, for example, output does not exceed 25–30 kg/mu in extensive culture; in intensive culture, however, output will be as high as 200–250 kg/mu. This increase in yield is due to the direct effect of artificial feeding.

Plankton feeder output is also enhanced. The feed is directly consumed by the so-called feed eaters and, in turn, their excreta fertilizers the pond water. This multiplies the natural food organisms of the plankton feeders. In such a culture system, the yield of these species often accounts for one-third of the total fish output.

Requirements for Different Nutrients

The nutrients that fish require are the same as those required by other animals: proteins, carbohydrates, lipids, vitamins, and minerals. The demand for these elements forms the basis for the preparation and selection of artificial fish feeds.

Protein

Just like other animals, fish consume protein and break it down into its component amino acids via the enzymes of the digestive system. The amino acids are then absorbed internally and used for normal growth, mending wears, maintenance and reproduction. Protein is used as a source of energy when fats and carbohydrates are depleted; 1 g protein yields 4 Kcal. The dietary protein requirement of farmed fish is generally 25–40 per cent. Terrestrial animals such as chickens, pigs, or cattle usually require 12–17 per cent protein. Because fish are cold-blooded, they require comparatively low-energy feeds. Different fish have different feeding habits and, therefore, different requirements for protein. Carnivorous fish such as rainbow trout and eel demand feeds with high protein contents; herbivorous fish require less protein.

The nutritional value of the feed depends not only on the quantity but also on the quality of the protein, i.e., its amino acid conformation. Amid acids are the elementary units of proteins. Several amino acids are essential to fish growth and development and, therefore, must be present in the feeds. These amino acids are called essential amino acids. The rest of the amino acids, which may or may not exist in the feed, are called dispensable amino acids. They are needed in only small amounts or can be synthesized internally by the fish. The following 10 amino acids are essential to fish: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, arginine, and histidine. The proportion of these essential amino acids in the feed protein should reflect the amino acid composition of fish protein. Proteins of low nutritional value may be supplemented with the essential amino acids they lack.

Carbohydrates

Carbohydrates are also defined as polyhydroxy aldehydes or ketones. Through digestion, they are decomposed into monosaccharides, which are absorbed and utilized by the fish. Some monosaccharides are oxidized into carbon dioxide and water: 1 g carbohydrate yields 4 Kcal. Monosaccharides are transported to the liver and muscles and stored as glycogen or are converted into fat which serves as a reserve energy source during food shortages.

Cellulose, a carbohydrate, is the major component of plant cell walls. Among cultivated fish, only a few varieties such as tilapia and milkfish can digest cellulose; even then, the digestibility is low. Cyprinids appear to lack the cellulolytic enzyme and, therefore, cannot digest cellulose. A small amount of cellulose in the fish feed is beneficial, however, because it stimulates the digestive movements of the intestines, promoting the digestion and absorption of other nutrients.

Lipids

Lipids are also a source of energy; 1 g lipid yields 9 Kcal. During digestion, lipids are broken down into fatty acids and glycerol components, which are absorbed by the fish. Body fats are synthesized from excess fatty acids and glyceryl and are stored in subcutaneous tissues, muscles, spaces between connective tissues, and the abdominal cavity.

Lipids tend to deteriorate through oxidation, producing substances that are toxic to fish and destroy vitamin E in fish feeds (e.g., aldehydes and ketones). If fish consume too much deteriorated fish meal and silkworm pupae, which contain oxydized lipids, they will suffer from “thin-back” disease, muscular atrophy, and weight loss, and show a high mortality rate.

Vitamins

Vitamins are essential organic compounds required by fish in trace quantities. There are many varieties of vitamins with various physiological functions; however, all of them are essential if fish are to grow and develop normally. Some vitamins participate in the metabolic process: vitamin B₁, carbohydrate metabolism; vitamin B₆, protein metabolism; vitamin C, protein synthesis; vitamin D, calcium and phosphorus metabolism (formation of the skeleton).

It is difficult to determine the exact amounts of the various vitamins required by the fish. In Chinese fish culture, fresh feeds are added to prevent vitamin deficiencies. For example, if pelleted feeds are used in grass carp farming, green grasses will be supplemented to ensure a complete supply of vitamins.

Minerals

Minerals are essential elements in many aspects of fish metabolism and structure. For example, phosphorus and calcium are important components of the skeleton. A deficiency of either substance will result in deformity.

Both fish feeds and pond water serve as a source of minerals. Fish absorb calciferous and phosphorous salts and chlorine and sodium ions through their gill and skin.

Minerals enhance the utilization of carbohydrates, accelerate the growth of certain tissues (e.g., skeleton and muscles), and enhance the fish's appetite; therefore, minerals must be included in the preparation of fish feeds. Additives such as bone powder and table salt in fish feeds fulfill the requirements of the pond fish.

Evaluation of Nutritional Value of Fish Feeds

In feeding, we must be aware of the nutritional requirements of the fish and we must evaluate the nutritional value of the fish feeds. Besides the chemical analysis, nutritional value of fish feeds must be evaluated with other criteria such as digestibility, utilization rate, and food-conversion ratio via feeding trials.

Digestibility

The digestibility represents the percentage of nutrients absorbed by the fish.

Digestibility is affected by many factors and it is possible for the same ration to have various digestibility. For example, if the same ration is fed to different species with different feeding habits and different digestive enzymes, their digestibilities are different. Enzyme activity is associated with temperature and digestibility rate will increase when the temperature rises within the adaptable temperature range. The presence of crude fibre in the diet will reduce the digestive rate of other components. The amount of food consumed by the fish will also affect the food digestibility; a proper food intake will maximize the digestibility.

Food-utilization rate

Food-utilization rate refers mainly to the utilization rate of the crude protein in the diet.

Because the quality of all food proteins is different, the efficiency of synthesizing fish body protein from food protein differs. In this sense, nutritional value not only depends on food-digestion rate but also depends on the utilization rate and quality of the protein.

Of the fodders listed in Table 3.7, Wolffia arrhiza gave the best growth rate for grass carp. The digestible crude protein content of Potamogeton malainus is higher than that of Vallisneria spiralis; however, the growth rate of grass carp fed P. malainus is less than those fed V. spiralis.

Table 3.7. Utilization rate of protein of several fodder (grasses) by grass carp fingerlings.

Food conversion ratio

In production, calculating the food-conversion ratio is a common method to appraise nutritional value. The food conversion ratio is also considered when preparing the food and when predicting fish yield.

The food conversion ratio is affected by many factors, particularly management. An improvement in management level can diminish the food conversion ratio i.e., the same amount of feed could result in a higher fish yield. There are mainly five factors that affect the food conversion ratio.

Feed preparation — Bean cakes must first be thoroughly soaked in water before they are fed to an adult fish or thoroughly soaked and ground up for juvenile fish. The soaking time should be controlled to avoid fermentation and rotting. If this occurs, the cakes will not be eaten by fish; however, the rotten cakes may be used as fertilizer. In general, the ground cake paste, wheat bran, and rice bran are placed on a feeding board and distributed into the pond. Any feed that sink to the bottom of the pond will act as fertilizer. If the brans and cakes were distributed as granulated feeds, food loss might be avoided and food consumption could be reduced.

Feeding regime — Feeds must be evenly spread over the pond and the amount must be controlled according to weather, water quality and fish appetite. Excessive amounts should be avoided to prevent wastage and the consequent high food conversion ratio. Frequent, small feedings are recommended.

Water quality — As far as water quality is concerned, oxygen content is the factor with the greatest influence on the food conversion ratio. The food conversion ratio of cultured common carp doubles if the oxygen content decreases from 3–6 to 0.5–2 mg/L.

Nutritional elements in the food — Fish at different developmental stages or with different feeding habits demand different nutritional elements in their food. To speed up growth and reduce the food conversion ratio, the nutritional elements of the food applied must comply with the physiological needs of the fish.

Species and age — The food conversion ratio of the same feed varies with species (Table 3.8). The food conversion ratio of dry silkworm pupae for common carp and rainbow trout are 1.3–2 and 6, respectively. The food conversion ratio also varies with age: in general, it is lower for the juvenile than for the adult fish.

Food conversion efficiency

Food conversion efficiency is the inverse of the food conversion ratio.

Varieties of Fish Feeds

Plant feeds

Grain — Soybean, wheat, and maize are commonly used grain feeds. Soybean, which is a nutritional food, contains at least 38 per cent crude protein (Table 3.9) and is rich in essential amino acids. Soybeans are usually ground up into “bean milk” for feeding to fry. About 5–7 kg of soybeans can supply the milk required to raise 10,000 fish from fry to summerling. In nurturing fry, some bean milk particles are directly consumed by the fry but most become fertilizer for the proliferation of plankton. For grass carp brooders, wheat and rice sprouts are often supplied. These grains are rich in vitamins, especially vitamin E, which is beneficial to gonad development. For granulated feeds, wheat powder is often used as a binder.

Oil Cakes — Cakes are by-products of oil plants after oil pressing. Bean cakes, peanut cakes, cottonseed cakes, and rapeseed cakes are often used in fish farming. This type of feed is rich in crude protein (30–40 per cent; Table 3.9). If cakes are used to feed fry, they must be broken into pieces, soaked, and ground up into a milk. About 150–200 kg of cake is needed to nurture 10,000 fingerlings with fsa body length greater than 10 cm.

Table 3.8. Food conversion ratio (FCRs) of several common feeds for various species of fish.

Cottonseed cakes are commonly used in carp culture in the USA and the USSR. This is not so common in China. However, China is a cotton-producing country and the use of cottonseed cake as a fish feed shows great potential. There is a little gossypol in cottonseed cakes, which is detrimental to livestock, but harmless to fish.

Wheat bran — Wheat bran is a by-product of the rice- and wheat-processing industry and is rich in vitamin B, apart from crude proteins, fats, and carbohydrates (Table 3.9). It is an important ingredient of compound feeds.

Table 3.9. Nutritional elements of various plant feeds (%).

Green fodders — Green fodders include aquatic plants and terrestrial plants and are mainly used as feed for grass carp and breams and, sometimes, for common carp, crucian carp, and tilapia. The main aquatic plants used are Wolffia arrhiza, Lemna minor, Vallisneria spiralis, Potamogeton malainus, Potamogeton maackianus, Hydrilla verticillata, Eichhornia crassipes, Pistia stratiotes, and Alternanthera philoxeroides. The main terrestrial plants used are Echinochloa crusgalli, Pennisetum alopecuroides, Lolium perenne, Sorghum sudanense, Pennisetum purpurlum of the grass family; Lactuca tenticulata of the composite family; and various leaves and vines from melon and vegetable crops.

Water lettuce (Pistia stratiotes), water hyacinth (Eichhornia crassipes), and alligator weed (Alternanthera philoxeroides) must be minced or fermented before they are given to the fish. To ferment one of these aquatic plants, 100 kg is mixed with 3–4 kg rice bran and 0.5 kg yeast; the container is then sealed and stored for 2 days at 26°C. Alligator weed sometimes contains saponin, which is toxic to fish. If this is processed by adding a little table salt (2–5 per cent concentration), the toxicity is eliminated and the feed becomes palatable. When these aquatic plants are mashed into a paste with a high-speed masher, they are appropriate feeds for fry culture. The fry swallow the mesophyll cells in the paste, which are similar in size to zooplankton and phytoplankton. The material in the paste serves as manure for the reproduction of plankton and other natural foods.

Green fodders contain mostly water and cellulose. However, they also contain the principal nutrients, i.e., fat, protein and carbohydrate, and are rich in vitamins (Table 3.10). Green fodders are the principal feed for grass carp and wuchang fish and serves as supplemental feed for other cultivated fish.

Table 3.10. Composition of some green fodders (%).

Animal feeds

Animal feeds have a higher nutritional value than plant feeds because they give a more complete supply of nutrients and are richer in proteins and essential amino acids (Table 3.11). Among the cultivated fish species in China, most are herbivorous or omnivorous; the only carnivorous species is black carp.

Common animal feeds include fish meal, trash fish, silkworm pupae, fresh-water shellfish (e.g., Viviparus quadratus and Corbicula fluminea), kitchen waste, and earthworms. Viviparus quadratus lives in rivers and lakes with a high fecundity and feeds on epiphytic algae. It has a meat rate of 22–25 per cent and a food-conversion ratio of 40 for black carp. Corbicula fluminea lives in the clay bottom of rivers and lakes and is collected with Viviparus quadratus. It has a meat rate of 13 per cent and a food conversion ratio of 60 for black carp. Both species are commonly fed to black carp.

Table 3.11. Composition (%) of common animal feeds.

In Japan and China, silkworm pupae are commonly used as fish feed. Fresh pupae are more effective but harder to preserve than dry pupae, which are rich in fats; however, these fats deteriorate easily through oxidization. Furthermore, the fish fed with silkworm pupae have off-flavour; therefore, pupae feeding is stopped 2–3 weeks before harvesting.

Pig blood is used as a binding agent for pelleted feeds. These feeds are effectively used in rainbow trout culture.

Formulated feeds

Advantages — Formulated feeds are composed of several materials in various proportions. In fish farming, formulated feeds have the following advantages:

• The ingredients of formulated feeds can complement one another and raise the food utilization rate.

• Proteins can supplement one another improving the essential amino acid conformation of the feed and raising the protein utilization rate.

• Food sources can be broadened by mixing feeds disliked by the fish with other preferred feeds.

• By adding a binding agent to produce pelleted feeds, the solution of nutrients in water is diminished and wastage is reduced.

• Drugs may be mixed into the feeds (indicated feeds) to control fish diseases.

• Formulated feeds are convenient to transport and preserve; they are suited to automatic feeding, which can lead toward the mechanization of fish farming.

Preparation — The feeding habits and nutritional requirements of the fish and the nutritional content of the food must be considered when making a nutritionally balanced feed. To achieve complete nutrition, many feedstuffs should be combined. The economics of formulating the feed should be considered; a good selection of cheap materials is preferable. Different formulas should reflect the different nutritional requirements of various species at different developmental stages; e.g., more protein for black carp than for grass carp; more protein for finger-lings than for adults. A moderate amount of binder should be added to ensure a high water stability of the feeds.

Some formulated pellet feeds have been tried on black carp because of a lack of snails and Corbicula.

• “320” straw powder pellet feed: “320” straw powder is made by fermenting “320” Basidiomycetes: straw powder, 50 per cent; bean cake powder, 10 per cent; fish meal powder, 5 per cent; barley flour, 15 per cent; wheat bran 10 per cent; rapeseed cake 10 per cent. The crude protein content is 20.14 per cent and the recommended daily feeding rate is 3–5 per cent of body weight. The food conversion ratio for 2-year-old black carp in monoculture is 2.

• Bean cake powder, 10 per cent; pupae, 10 per cent; barley flour, 30 per cent; rapeseed cake, 50 per cent; bone powder, 2 per cent; table salt, 1 per cent. The crude protein content is 28.1 per cent and the carbohydrate content is 38.9 per cent. The food conversion ratio for 2-year-old black carp in monoculture is 2.4.

• Bean cake powder, 25 per cent; fish meal, 4 per cent; barley flour, 16 per cent; rapeseed cake, 24 per cent; wheat bran, 26.5 per cent; mineral mixture, 1.5 per cent; plant oil, 3 per cent.

Formulated feeds have also been used with grass carp.

• Straw powder, 25 per cent; sesame stem powder, 25 per cent; bean cake powder, 25 per cent; ricebran, 25 per cent; waste flour (Binder), 10 per cent. The crude protein content is 23.35 per cent. The food conversion ratio is 2.8.

• Straw powder, 70 per cent; bean cake powder, 15 per cent; ricebran, 10 per cent; waste flour, 5 per cent; bone powder, 1 per cent; table salt, 0.5 per cent. The crude protein content is 15.07 per cent. The food conversion factor is 4.4.

• “320” green grass powder pellet feed: green grasspowder, 40 per cent; rapeseed cake, 20 per cent; fish meal, 5 per cent; bean cake, 15 per cent; silkworm pupae, 5 per cent; barley flour, 15 per cent. The crude protein content is 22.6 per cent. The recommended daily feeding rate is 3.5 per cent of body weight. The food conversion rate for grass carp and wuchang fish is 2.5–3.

If only pellet feeds are applied, the muscle, intestine and liver of grass carp accumulate a high amount of fat. This can have a pathological effect on the liver and, consequently, adversely affect growth. When green grass is also provided (5 days of pellet feeds followed by 2 days of grass application), this situation improves: no pathological change is observed. Nevertheless, every substance demanded by the fish cannot be included in a specific feed, no matter how complete the feed is. Therefore, it is important to also supply fresh, natural fodders (e.g., green grass for carp and snails for black carp) to promote the digestion of the artificial diet.