Chapter III Pond fertilization and fish feeds

I. Pond fertilization

1) Significance of fertilization in pond

China has a long history of pond fertilization for fish culture ages ago. The farmers adopted the methods of manuring to rear fry. For instance, Dacao (green manure) has been applied in Guangdong and Guangxi Provinces and night soil has been used as manures in Jiangxi and Human Provinces so as to nurture fry into summer fingerlings. Rich experience has been accumulated. In fingerling rearing ponds and table-fish rearing ponds, fertilization is aimed at developing natural food and saving artificial feeds. This method has been widely adopted in China.

Phytoplankton, elementary producers in ponds take in all essential inorganic materials from the water and conduct phytosynthesis with sunlight, producing organic nourishment needed for their own growth and reproduction.

Fertilization means to supply phytoplankton with nutrients for phytosynthesis and to promote the growth of phytoplankton, by which zooplankton and other aquatic animals are fed on for their growth and propagation. Fish feed on plankton and other hydrobios. Pond fertilization lies in cultivation of various food organisms and their propagation in large quantities in fish ponds to provide fish with abundant natural feeds, by which they can grow faster. The yield of fish pond may be raised thereby.

Among the fish food organisms in ponds, there are a series of interrelations between the predator and the prey. The biological term is “food chain”. Fish is the last link of the food chain of hydrobios in ponds.

e.g. phytoplankton→Silver carp

phytoplankton→zooplankton→Bighead

aquatic plant→Grass carp

plankton→benthos→Black carp

Usually, animals could use only 5–20% energy of both animal and plant feeds. Utilization of energy is related to the length of food chain. The shorter the food chain is, the higher the rate of energy transfer will be, in other words, the higher the utilization rate of energy transfer is, the higher the fish production will be.

The hydrobios in ponds are in a constant progress of growth and death. The dead bodies of organsims will turn from complex organic materials into simple inorganic materials through decomposition by bacteria, then dissolve in water, which can again be utilized by phytoplankton to produce new individuals. Hence, the materials in ponds are in a constant state of circulation mainly through the food relationship between living organisms. This kind of circulation is called pond material circulation.

The relationship may be illustrated with Fig. 3---1

Fig 3---1 A diagram of material circulation in ponds

The process of pond material circulation is just that of the production of fish and their food organisms in ponds. The circulation originates from the soluble minerals in water. Under certain conditions of light and heat, the propagation of phytoplankton depends upon the amount of nutrients in water. In turn, the production of all food organisms depends upon the proliferation of phytoplankton. Fertilization is just to enhance the quantity of nutrients and to nurture food organisms in ponds so as to create favourable conditions for raising fish yield.

2) Kinds of organic manures and methods of application

Organic manures mainly refer to excrements of farm animals. It is a general term of manures containing organic matter. Nowadays, manures applied in fish ponds in China are mainly organic manures. In production, the following are often used, faeces and urine of livestock and poultry, night soil, green manure, compost and silkworm faeces etc.. Only through decomposition by microorganisms, organic manures may be converted to nutrients, which plants can absorb, and then, organic manures are full of nutritional elements with long effects.

(1) Kinds of organic manures

(i) Faeces and urine of livestock and poultry

A. Pig manure

Composition and characteristics: pig manure includes much organic matter and other nutritional elements like nitrogen, phosphorus and potassium and is a kind of fine, complete manure. Pig faeces are delicate, containing more nitrogen with a c/n proportion of 14/1, smaller than that of other livestock faeces, so they are easy to rot. The major portion in pig urine is nitrogen in urea form. It is easy to decompose. See Table 3--1 about pig faeces elements.

Table 3---1 Nutritional elements in pig manure

excretory amount: The excretory amount of pig is greatly associated with its body weight and food intake. From the measurements, a pig of 50 kg in body weight discharges about 10 kg per day, about 20% of body weight. A pig offers 1000 kg of faeces, 1200 kg of urine in the culturing period of 8 months from pigling to the adult Pig's daily excretory amount is less than cow's or horse's. But, pigs have the merits of faster growth, shorter fattening period, and suitableness for pen culture, whereas the scale of pig raising is much larger; so it is more beneficial to collect their manure.

B. Cattle manure

Composition and characteristics: The elements are similar to those in pig manure, but cattle are ruminants and the food-stuffs are repeatedly masticated, so that the excrement is delicate. There is less nitrogen in the composition of cattle manure. The C/N proportion is 25:1. The cattle urine contains more nitrogenous element in hippuric acid form, (C₆H₅CONHCH₂COOH), so that cattle excreta are slow to decompose.

See Table 3---2 about the nutritional elements in cattle excrement.

Table 3--2 Nutritional elements in cattle excrement (%)

Excretory amount: The average daily excreta is 25 kg each, in which the ratio of faeces and urine is about 3:2. The yearly total amount of excrement for each is 9000 kg.

C. Poultry manures

Composition and characteristics: Poultry manures include faeces of chicken, duck and geese, which contain much more nutrients. For the nutritional elements in poultry manures, see Table 3---3

Table 3--3 Aoverage amount of nutrients in poultry manures

Poultry manures are easy to rot and the nitrogen in poultry manures is chiefly in uric acid form, which could not be absorbed directly by plants, accordingly, poultry manures are better to be used after fermentation.

Excretory amount: As for each fowl, the yearly amount of excrement is not large, chicken 5---5.7 kg; duck 7.5---10 kg and goose 12.5--15 kg. Though the excretory amount of each is small, the quantity of poultry culture is often great, therefore, every fowl faece is one of manure sources. The total amount is quite a lot.

(ii) Nightsoil

Composition and characteristics: The composition of nightsoil is greatly dependent on the food, with a larger quantity of nitrogen, of which 70--80% is in urea form. It is apt to be absorbed with good effect. C/N proportion of human excreta is three to one. They are easy to ferment. The nutritional elements in human excreta are listed in Table 3---4

Table 3---4 Nutritional elements in human excreta (% in wet weight)

Excretory amount: Nightsoil is a popularly-applied manure. From the measurements, a yearly amount from an adult is shown in Table 3---5

Table 3---5 Yearly amount of an adult's excreta (kg)

Nightsoil, used as manure has to be fermented before application. It can be rotten under anaerobic conditions in storage for 2---4 weeks. The decomposition of human waste produces ammonia. Under airtight conditions, after reaching to a certain concentration, ammonia can sterilize human waste. 1---2% quicklime or 0.1---0.2% formalin can also kill harmful insects and bacteria in nightsoil.

(iii) Silkworm dregs

Composition and characteristics : Silkworm dregs are composed of the faeces and sloughs of silkworm as well as mulberry residues. Silkworm dregs are full of organic matter. Dried ones contain 87% organic materials and 3% nitrogen. They are also good feeds for fish. 8 kg of silkworm dregs can produce 1 kg of fish.

Mulberry-dike-fish-ponds are popular in Guangdong Province. In Shunde county, there are 260,000 mu of fish ponds with mulberry trees planted on the dikes. The ratio of area of dike and water surface is 1:1. Mulberry leaves go for silkworm raising; silkworm dregs for fish and pond humus for mulberry trees.

This approach was formed 600---700 years ago. In Pearl River Delta each mu of mulberry plants may produce about 2000 kg of mulberry leaves, which can produce 160 kg silkworm cocoons and 1200 kg silkworm dregs.

(iv) Green manures

All wild grasses and cultivated plants, if used as manure, are called green manures, which are apt to rot and decompose, providing ideal environments for bacteria propagation, so they are good for appliation in ponds. A few common green manures are listed in Table 3--6

Table 3---6 Nutritional elements in green manures (% wet weight)

(v) Compost

Mixed compost is made of green manures and animal wastes. The mixture of several manures may make the ingredients of manure all the more suitable to the reproduction of plankton. The ratio of the constituents of manures depends upon the local sources of manures. From experiments, the following proportion of the ingredients of compost is suitable for plankton reproduction.

A. Green grass 8:cattle faeces 8: human excreta 1:lime 0.17.

B. green grass 1:cattle faeces 1:lime 0.02

There are two methods to make compost, that is, heaping and soaking. Heaping method: The manure heap is to be made under aerobic conditions. Spread one layer of green grass, then sprinkle some lime on it. Then add another layer of faece manure and do it again, layer by layer upto 1.5---2 m. At last, cover the heap with 5---6 cm thick of mud. The ingredients of the compost will rot and decompose. After 3---4 weeks the compost can be used.

Soaking method: dig a pit near fish ponds, put in green grass, lime and faeces one layer after another and then add some water to soak all manures with no leakage at all. The use of lime is to neutralize the organic acids turned out in the process of rotting & decomposition of the compost so that they could not attenuate the activities of microorganisms, whereas they could promote the decompostion of organic materials. The compost can be taken out for use after 10---20 day's fermentation at a temperature of 20°---30°C.

(2) Methods of the application of organic manures

(i) Application of Dacao: In Guangdong and Guangxi Provinces, Dacao is habitually used to fertilize pond water. The so-called Dacao means several composite plants. In reality the grass is not limited to composite plants; some graminous plants and leguminous plants are sometimes used as well. In application, Dacao is just heaped at a corner of pond and is turned once every other day or every two days. The rotten part will go spreading in water and at last, the roots and stems which are hard to rot, will be dredged up out of ponds. The decomposition of green manure in water consumes a great amount of oxygen. In light of experiments, if 1000 kg of grasses are applied in a pond of 1 mu with a waterdepth of one meter, all fish may die of lacking oxygen as the pond is in a state of no oxygen from the second day to the sixth day. The peak of oxygen consumption is on the second and third day. And it will be reducing later on. It is appropriate to apply green manures frequently in a small amount, or to pour fresh water into ponds or adopt aerators, in order to guarantee plenty of oxygen in pond water.

(ii) Application of nightsoil.

In Jiangxi and Hunan Provinces, nightsoil is applied in fish ponds and the application amount varies with fish sizes. Before application, one portion of nightsoil is diluted with double portions of water. The dilution is then sprayed along the pond dikes. It is added once a day and the added amount is dependent on the fertility of water.

(iii) Application of livestock manures

The application of livestock manure as base manure is similar to that of green manures: heap the manures at a corner of pond or put them in small heaps in shallow water with a sunny exposure so as to make them decompose and spread gradually in water. If the manure is used as an additive, it is added in small heaps every 7---10 days.

(iv) Application of mixed compost

After fermentation, the compost is taken out, and given a flush. The liquid is collected and the residues are removed away. Spray the liquid evenly into ponds. In case the area of the pond is rather big, you may load the needed amount of manure on a boat, flush it with pond water in batches and then spray the liquid evenly in ponds. The manure dregs can be used to fertilize crops. For small ponds, there is no need to use a boat. The liquid is spread round the dikes. Another method is to put one side of the compost to the other in the pit and expose the liquid and then the manure liquid can be ladled out and spread into ponds according to the required amount.

The nutrients of the compost can be quickly absorbed by phytoplankton once the compost is applied in ponds. It consumes less dissolved oxygen in ponds since the organic materials are already decomposed after full fermentation.

3) Effects of manure application on food organisms

First of all, the application of organic manure causes the propagation of bacteria in large quantities. On one hand, the bacteria decompose organic materials, mineralizing them into nutritional inorganic materials, which can be utilized by phytoplankton; on the other hand, they themselves take part of the nutrients as their own structural materials, reproducing a large population of bacteria.

After organic manure being applied, the organic detritus in water is increasing in quantity. The organic detritus carries dense bacteria on its surface; which are the important food for lower aquatic animals and filtering fish.

After manuring, the predominant species of plankton at their initial appearance in water depend closely on the properties of the manure. If organic manure is applied, those that are fond of organic materials will show the first population bloom------phytoplankton: Ochromonas spp. Cryptomonas; zooplankton: Urotrichia spp. etc. For inorganic manure, the predominant species which will be seen at first are centric diatoms Centrales and Scenedesmus acuminatus.

There is a close relationship between the amount of manure applied and the plankton community. Huge amount of manure will lead to the presence of some species of green algae Chlorophyta and blue algae Cyanophyta while small amount of manure will lead to the presence of Navicula rostellata and Cyclotella stelligera etc.

After each application, the nutrient content in the water increases, resulting in a planktonic peak. Phytoplankton which are easily digested by silver carp reach a peak after 4 days, whereas those that are not so easily digested will 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. Since protozoans multiply by binary fission, by which the population increases very rapidly, it will, therefore, be the first to reach a peak. Rotifers undergo parthenogenesis under normal condition. The eggs produced is not very many, with an average of 10---20 eggs during their life span. Thus, the rotifer population reaches a peak slightly later than the protozoans. Cladocerans also reproduce parthenogenically, but the span between hatching and sexual maturity is longer. So the cladoceran peak appear later than that of the rotifer. The timing of manure application is most important. Ideally, the peak should appear at a time required to meet the demand of the fish fry.

4) Kinds of inorganic manures and methods of application

(1) Kinds of inorganic manures

Inorganic manures mean chemical fertilizers. According to the composition, the chemical fertilizers can be divided into three groups, that is, nitrogenous, phosphoric and potash fertilizers. The advantages of inorganic fertilizers are exact constituents, fast effect, slight pollution, no consumption of dissolved oxygen in water, small application amount and convenient operation. However, when chemical fertilizers are applied in ponds, the first link of the food chain in ponds is principally phytoplankton which, if taken as food for zooplankton, is nutritionally not so nice as bacteria. Therefore, the zooplankton amount in ponds applied with inorganic manure often lags far behind that in ponds applied with organic manure. Moreover, in most ponds applied with chemical fertilizers, the predominant species of phytoplankton are Chlorophyta, which is nutritionally worse than the predominant species in ponds applied with organic manure, such as Chrysophyceae, Bacillatiophyceae and Cryptophyceae. In addition, the manure effect is rather short and it is difficult to control the water quality, therefore, the result of the application of chemical fertilizers there is not better than that of the application of organic fertilizers.

(i) Nitrogenous fertilizers

1. Liquid ammonia : Molecular formula, NH₄OH or NH₃ x H₂O, with a nitrogen content of 12---16%. It is a water solution of ammonia, which is an important product of small-scaled nitrogenous fertilizer factory with simple synthesizing procedure and low cost. Ammonia is in an unsteady state when in water, easy to volatilize, so it could almost lose its effect through the volatilization if it is exposed to the air for a long period of time.

2. Ammonium sulphate: Molecular formula, (NH₄)₂SO₄, with a nitrogen content of 20-21%. It is produced from the liquid ammonia directly neutralized with diluted sulphuric acid. It is white crystals when pure, apt to dissolve in water. 100 kg of water can dissolve 75 kg of ammonium sulphate. With a little absorption of moisture, it is convenient to preserve and apply.

3. Urea: Molecular formula, Co(NH₂)₂, with a nitrogen content of 44-46%. Ammonia and carbon dioxide are interacted and synthesized into urea under high heat & pressure. It is white crystals with a strong absorption of moisture. After dissolution in water, urea does not turn out ions and is unable to be absorbed directly by plants. It can be utilized by plants only after it is decomposed by urease excreted by urea-decomposing bacteria and transformed into ammonium carbonate. The conversion rate of urea is related with the temperature. It can be totally transformed into ammonium carbonate in 4---5 days at 20°C and in 2 days at 30°C.

(ii) Phosphoric fertilizers

Calcium superphosphate: Main contents, Ca(H₂PO₄)₂ H₂O with 12-18% of P₂O₅; subsidiary-contents, CaSO₄. 2H₂O, about 50%. Usually, it is white powder. It is corrosive, and is apt to absorb moisture, smelling acidic since there is some free acid in the products.

(2) Methods of the application of inorganic manure

Nitrogen is one of the essential nutritional elements in a plant. It is also the principal content in protein, and can accelerate the formation of plant chlorophyll and strengthen photosythesis. For this reason, the nitrogen content is one of the decisive factors of phytoplankton production. Nitrogen is always short in pond water, so the application of nitrogenous fertilizer may achieve better results.

Nitrogenous fertilizer is generally better to be used as an additive because of its quick-effectiveness, and the nitrogenous fertilizer with an ammonium content must not be mixed with strong-alkaline materials lest it should convert into gas and lose effect through volatilization. In the utilization of nitrogenous fertilizer in ammonium form, due attention must be paid to toxicity of ammonia. Either ammonium or liquid ammonia will become ionic (NH₄)⁺ and nonionic (NH₃) once they are placed in water. This phenomenon may be expressed in the following formula:

NH₃ + H₂O ⇌ NH⁺₄ + OH^-

In an acidic state, the balance inclines to the right side with the concentration of ammonium ions enhancing; and in an alkaline state, NH₃ concentration will raise high. As water temperature comes up to 25°C with pH6, the concentration of NH₃ holds 0.05% in total nitrogen; pH7, 0.49%; pH8, 4.7%; pH9, 32.9%.

NH₃ is toxic to fish. It poisons juvenile rainbow trout at 0.3--0.4 mg/L. The Chinese carp have a higher toleration to ammonia anyhow, ammonia inhibits the growth of fish. The mortal concentration is beyond 13 mg/L. The highest concentration permitted in the water body for fish farming is 0.1 mg/L. For this reason, the amount must be strictly controlled in one application. What is more, caution must be given to pH value to avoid being applied in strong alkaline water (e.g. just after pond clearing with lime) since liquid ammonia is alkaline itself. Besides, the amount of nonionic ammonia is rising with the increasing of water temperature, so special care is needed when nitrogenous fertilizer in ammonia form is used in hot summer and antumn.

The application amount of nitrogenous fertilizer depends upon the nitrogen content of the fertilizer applied. In a pond with the area of 1 mu and the water depth of about 1.5 m, 1.5-2 kg may be given as base manure. From then on, the additive of the fertilizer is applied 3-4 times monthly with 0.5 kg/mu each time. Thus about 10 kg are needed for the whole year. For example, if nitrogen content in ammonium sulphate is about 20% and 2 kg are given to 1 mu as base manure, the amount of ammonium sulphate needed is:

The annual amount needed is about 50 kg.

Applying method: make a solution of it and spread it near the dikes. In case of liquid ammonia, put the container with ammonia under water and then open the lid so as to let liquid ammonia diffuse out slowly. By this way, the volatilization can be avoided during the operation on the bank.

Most waters are lacking phosphorus, so it is important to apply phosphoric fertilizer, which can accelerate the reproduction of azotobacteria, and enrich nitrogenous fertilizer in water.

The application amount of phosphoric fertilizer is calculated in accordance with the content of phosphoric acid applied. If 0.5-1 kg/mu is used as base manure the annual amount might be about 5 kg. The method of calculation and application is the same as above.

Potassuum is also one of the principal nutritive elements for plants, but usually it is plentiful in water. There is no particular need to apply potash fertilizer in aquaculture.

2. Fish food

1) Significance of food application

In addition to the fertilization in ponds for proliferation of natural food organisms for fish, artificial feeds must be supplemented to meet the demands of various species of fish. Fish feeds are the prime material base of high-density fish culture. Applying artificial feeds in fish pond can raise the per-unit yield by a big margin. Take common carp as an example. The output does not exceed 25--30 kg/mu in extensive culture, whereas in intensive culture the output will be as high as 200–250 kg/mu. The rapid increase of yield comes from the direct effect of artificial feeding, the so-called fish yield of artificial feeding; however, the plankton-eater output is also enhanced. Especially in the system of polyculture in China, the consequence of feeding is more conspicuous. Because the food applied can be directly taken in by the so-called feed-eaters, in turn, the excreta of which can fertilize the pond water, multiplying the natural food for the plankton-eaters. In such a culture system, the yield of these species often occupies ⅓ of the total fish output.

2) Fish requirements for different nutrients

The nutrients that fish require are the same as the other animals. They can be sorted into five kinds: protein, carbohydrates, fats, vitamin and minerals. The demands of fish for these nourishing elements are the fundamental basis for selection and preparation of fish feeds.

basis for selection and preparation of fish feeds.

(1) Protein

Protein is the basis of all living things.

Just as other animals, from food, fish take in protein which's decomposed into amino acid through enzyme in the digestive organs. The amino acid is absorbed internally and synthesized into fish protein for growing mending tissues and maintaining life activities. It is used as energy for fish activities when fats and carbohyrates are not sufficient. One gram of protein can supply 4 kilo calories.

The demand of fish for protein contents in feeds is generally 25---40%. It is higher than the demand of terrestrial animals like chicken or pig or cattle, that is 12---17%. Since fish are cold-blooded animal without need of maintaining body temperature, they require lower energy comparatively. Different fish have different feeding habits, so do their requirements for protein contents in feeds. Carnivorous fish like rainbow trout and eel demand higher protein contents, while herbivorous fish lower.

The nutritional value of food depends not only on the quantity of protein but also on the quality of it, namely, the compositions of amino acids. Amid acid is the elementary unit, of which protein is constructed. The researches have proved that several amino acids are essential to fish growth. Therefore, they must be fully prepared into the feeds. These amino acids are termed essential amino acids while the other amino acids, which may not exist or may be just a little in feeds, are called dispensable amino acids because they are needed only in small amounts or can be synthesized internally.

The following ten amino acids are essential amino acids to fish: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, arginine, and histidine. These ten must be included in fish feeds so that they can satisfy the nutritional requirements of fish and guarantee the normal development and growth. Besides, the proportion between different essential amino acids in protein must conform to the nutritional requirements of fish, which is dependent on the composition of amino acids in fish protein, so the proportion of all essential amino acids of fish may be referred to the composition of fish body protein. From the above-mentioned factors, We can arrive at a conclusion that the nutritional value of protein is determined mainly by the completeness of the sorts of essential amino acids and the equilibrium of specific value between all essential amino acids to the fish you feed. As for the incomplete and unbalanced protein, we can still raise its nutritional value greatly by way of supplementing those amino acids it lacks.

(2) Carbohydrates

carbohydrates are also called saccharide, in other words, the chief source of energy. Through digestion, they are decomposed into monosaccharide absorbed and utilized by fish body. In the organic body, part of them is decomposed through oxidization into carbon dioxide and water, releasing energy. Applicable heat is 4 kilocalories each gram of carbohydrate; part of them is conveyed into livers and muscles as glycogen to be preserved for the time being and the remaining part may be converted into fats, to be cumulated for life maintenance in case of shortage of food or stoppage of food-taking.

Cellulose is also a kind of carbohydrates, the major component of plant cell wall. Among the cultivated fish, only a few species like Tilapia and milk-fish can digest cellulose at a rather low utilization rate. It is believed that Cyprinids lack the cellulolytic enzyme. In consequence, they are unable to utilize cellulose, and neither are Grass carp, the typical herbivorous fish. An appropriate amount of crude fibers in fish feeds can stimulate the digestive movements of the intestinal tracts so as to promote the digestion and absorption of other nutrients.

(3) Fats

Fats are also a source of energy. Every gram of fats deliver 9 kilocalories of applicable heat. Fats are decomposed in digestive tracts into fatty acid and glycerol which can be absorbed. After absorption, the body fats are synthesized from the excessive fatty acid and glyceral, storing in subcutaneous tissues, muscles, spaces between connective tissues and the abdominal cavity.

Fats are apt to deteriorate through oxidization, bringing about toxic substances like aldehyde and ketone etc, which are detrimental to fish and destroy Vitamin E in fish feeds. If fish take in too much deteriorated fish meal and silkworm pupae, they will suffer from thin-back disease, muscular atrophy, losing weight and higher mortality.

(4) Vitamins

Vitamins are organic trace elements indispensable for fish. There are so many sorts of these substances with various physiological functions; however, all of them are indispensable to keep fish fit with a normal growth.

A number of Vitamins take part in the process of metabolism: e.g. Vitamin B1 of body carbohydrates; Vitamin B6, of proteins; Vitamin C, that of the synthesis of body protein of animal; And Vitamin D, of the normal metabolism of calcium and phosphorus, promoting the formation of skeleton.

As it is, it is not easy to determine the exact amount of various Vitamins required by fish. On the basis of the practical state and the characteristics of the cultivated species in China, fresh feeds are added to make up the deficiency of Vitamins. If pellet feeds are applied in farming Grass carp, green grass may be supplied at intervals so as to replenish the Vitamins the fish lack and the effect is desirable.

(5) Minerals

Minerals are essential elements of fish body composition. Without minerals, the organic bodies are unable to retain normal physiological functions. Phosphorus and calcium are the important components of skeleton. The deficiency of the two substances will affect the skeleton development with a result of deformity.

The minerals demanded by fish come basically from feeds and part of them directly from environmental water. Fish absorb the calciferous and phosphorous salts and the ions of clorine and sodium through their gill rakers and skin.

Minerals in diet may enhance the utilization of carbohydrates by fish, accelerate the growth of fish tissues like skeleton and muscles, improve their appetite and speed up the growth of fish body, and therefore, in the preparation of fish feeds, the minerals must be taken into consideration. As a whole, some additives like bone powder and table salt in fish feeds are enough for the pond fish.

3) Judgement of nutritional value of fish feeds

In feeding, on one hand, we must be aware of nutritional requirements by fish; on the other hand, we must make a judgement on the nutritional value, so that the feeds applied can come up with fish nutritional needs.

Besides the analysis of chemical composition of fish feeds, other criteria like digestive rate, utilization rate and food coefficient can be taken into account for the judgement of food nutritional value.

(1) Food digestive rate

The judgement, which is made only out of the chemical ingredients could not be so exact, since the contents of nutrients may not stand for the quantity that fish digest and utilize. Food digestive rate means the percentage of the nutrients digested by fish.

Food digestive rate is affected by a number of factors and even the same ration could have various digestive rates. For example, if we feed the same ration to different species or fishes with different feeding habits, the digestive rate is different, because of their respective digestive organs enzymes. The activeness of enzyme is associated with the temperature and the digestive rate increase when the temperature rises within the adaptable temperature range. The crude fibers content in diet will reduce the digestive rate of other nutrients. The amount of food intake of fish also affects food digestive rate and a proper food intake will bring up the highest digestive rate.

(2) Food utilization rate

It refers mainly to the utilization rate of crude protein in food.

Utilization rate of crude protein in food (%) =

Since the quality of all food proteins is not identical, the efficiency of making up fish body protein from food protein differs. In this sense, food nutritional value is not only demonstrated by food digestive rate, but also by utilization rate of crude protein in food, and the quality of food protein, i.e. the biological value of protein. The food intake amount, protein utilization rate of several plant feeds taken by Grass carp and the growth after food intake in experiments are manifested in Table 3---7.

Table 3---7 Utilization rate of protein of several grasses of Grass carp

It shows that the daily crude protein of Potamogeton malainus is 0.149 higher than that 0.129 of Vallisneria spiralis; on the contrary, the growth of Grass carp with the former is worse than that with Vallisneria spiralis.

(3) Food coefficient and food efficiency

These two criteria explain food effect in farming.

(i) Food coefficient: It indicates the consumption amount of feeds needed for the per-unit fish body weight gain.

In production, it is a common method to appraise the food nutritional value by food coefficient. The cirterion is also considered as the basis for planning & preparation of food and the prediction of fish yield.

Food coefficient is affected by many factors, particularly by the management.

Improvement of management level can diminish food coefficient, therefore, the same amount of feeds could attain more fish yield. the factors affecting food coefficient are as follows:

1. Preparation before feeding: For bean cakes, they must be first throughly soaked in water for the adult fish to take in and digest or be gound up for juvenile fish. But the soaking should not be too long lest it goes fermenting and rotting. If that happens, the cakes will not be eaten by fish but will turn out to be fertilizers. In production, generally, the ground cake paste, wheat bran and rice bran are just placed on the feeding board and the food will be spread and dissolved in water in competition by fish. There might be a waste of feeds sinking to the pond bottom as fertilizers. If the brans and cakes are made into granulated feeds, the food loss might be avoided and the food consumption could be reduced.

2. Amount and method of feeding: Feeds must be evenly spread and the amount must be appropriate without any sudden change. The amount of feeding should be controlled according to weather, water quality and fish appetite with an avoidance of excessive amount or there will be a waste of food if it is not eaten up and consequently food coefficient will be higher. But it can be reduced by frequent feeding in small amounts.

3. Water quality: So far as water quality is concerned, the oxygen content in pond water is the factor affecting the food coefficient the most. Experiments show that the food coefficient is doubled in culturing Common carp if the oxygen content comes down from 3---6 mg/L to 2---0.5 mg/L.

4. Nutritional elements in food: The fish at different developmental stages or with different feeding habits demand different nutritional elements in food. In order to speed up growth and reduce food cofficient, the nutritional elements contained in the food applied must be in conformity with the physiological needs of a certain fish.

5. Species and age: Food coefficient of the same feed is variable with different species. From experiments, for Common carp, food coefficient of dry silkworm pupae is 1.3---2 while for rainbow trout, it's 6. It is different with fish ages, lower for the juvenile fish than for the adult. The usual food coefficients in present production is listed in Table 3---8.

(ii) Food efficiency

It is an upside-down figure of food coefficient, i.e. percentage of fish body weight gained in the consumed food amount.

4) Kinds of fish feeds

The artificial feeds used for fish culture are sorted into plant feeds, animal feeds and compound feeds.

(1) Plant feeds

(i) Grain food

Soybeans, wheats and maize etc are commonly used. Soybean which's a nutritional food, contains crude protein beyond 38% and substantial essential amino acids. They are usually ground up into bean milk to feed fry. About 5---7 kg of soybeans can supply the milk for 10000 fishes from fry to summer fingerlings. In nurturing fry some bean milk particles are directly taken in by fry, whereas most of them are used as manure to proliferate plankton. In culturing Grass carp brood fish, certain amount of wheat and rice sprouts are often given, which comprise lots of Vitamin, especially Vitamin E. It is good for the development of the gonads of fish. For granulated feeds, wheat powder is often used as feeds and an adhesive as well.

(ii) Cakes

Cakes are by-products of oil-plants after oil-pressing, Beancakes, peanut cakes, cottonseed cakes and rapeseed cakes are often used in fish farming. This sort of feeds includes rich crude protein with a content of 30---40%, a principal source of protein for fish feeds

If cakes are used for fry, they must be broken into pieces, soaked and ground up into milk, before feeding. About 150---200 kg of cakes are needed for nurturing 10,000 fingerlings with a body length above 10 cm.

Table 3---8 Food coefficient of several common feeds for referential use

Cottonseed cakes are popularly used in carp culture in the USA & U.S.S.R. and not so popular in China. However, China is a cotton producing country with a great potential of cottonseed cake as fish feeds. There is a little gossupol in cottonseed cakes. That is detrimental to livestock, but harmful to fish.

(iii) Wheat bran

It is a by-product of rice and wheat processing industry, containing plenty of vitamin B apart from crude proteins, fats and carbohydrates. It is an important ingredient of compound feeds for fish because of its significant value. The nutritional elements of several plant feeds are shown in Table 3---9.

Table 3---9 Nutrients of plant feeds (%)

(iv) Green fodder

Green fodder includes aquatic plants and terrestrial plants, mainly as feeds for Grass carp and Breams and some as feeds for Common carp, Crucian carp and Tilapia. There are numerous kinds. The main aquatic plants are Wolffia arrhiza, Lemna minor, Vallisneria spiralis, Potamogeton malainus, Potamogeton maackianus, Hydrilla verticillata, Eichhornia crassipes, Pistia stratiotes and Alternanthera philixeroides etc. The terrestrical plants are Echinochloa crusgalli, Pennisetum alopecuroides, Lolium pereme and Sorghum sudanense, Pennisetum purpurcum of the grass family; Lactuca tenticulata of the composite family; leaves and vines of melon crop and vegetable crop.

If Water letture (Pistia stratiotes), water hyacinth (Eichhornia crassipes) and water peanut (Alternanthera philoxeroides) are directly applied, fish do not like to eat, but after proper processing e.g. being minced into pieces or fermented, they are welcomed by fish. If 100 kg of the above mentioned aquatic plants mixed with 3---4 kg of rice bran and 0.5 kg yeast are sealed for fermentation, that can be brought out for use after 2 days at 26°C. Water peanuts containing toxic saponin are not favoured by fish. If they are processed by adding a little table salt with a 2---5% concentration to eliminate the toxicity, fish will like to eat. If these plants are mashed into grass paste with a high-speed masher, these aquatic plants are desirable in fry culture; since fry can swallow the mesophyll cells in paste, which are similar in size to zooplankton and phytoplankton. The left-over serves as manure for the reproduction of plankton and other natural food.

This kind of green fodder contains plenty of moisture and cellulose. It has a little other principal nutrients; carbohydrate, but rich Vitamins such as Vitamins C, E, K etc. It is principal feeds for Grass carp and Wuchang fish, and serves as supplemental feeds for the other cultivated fish (see Table 3---10). Nutrients of green fodder.

Table 3---10 Nutrients of green fodder (%)

(2) Animal food

Animal food have a higher nutritional value by virtue of the completeness of nutritional elements and essential amino acids with rich content of proteins. Among the cultivated species of fish China most of them are herbivorous or omnivorous with only one carnivorons species, Black carp. The animal food in aquaculture is mainly collected from Mother Nature such as Viviparus quadratus and corbicula fluminea for Black carp and partly for Common carp.

The common animal foods used in aquaculture are fish meal, fresh small wild fish, silkworm pupae, freshwater shellfish (e.g. Viviparus quadratus and Corbicula fluminea etc.), fish entrails and earthworm etc.

Viviparus quadratus, live in rivers and lakes with a strong fecundity, feeding on epiphytic algae, with meat rate 22---25% and food coefficient 40.

Corbicula fluminea, live in the clay bottom of rivers and lakes, so they will be collected together with Viviparus quadratus with meat rate 13% and food coefficient 60.

In Japan and China, silkworm pupae are usually applied to feed fish. Fresh pupae are more effective but difficult to preserve, whereas dry pupae contain rich fats, but fats are easy to deteriorate by oxidization. Furthermore, the taste of fish flesh is not so good; therefore, pupae feeding must be stopped 2---3 weeks in advance before catching.

The blood of pig can be used as a bonding agent of pelleted feeds, which are very effective for rainbow trout culture. Common animal food may be seen in Table 3---11 for their nutrients.

Table 3---11 Composition of common animal food (%)

(3) Compound feeds

(i) Advantages of compound feeds

Compound feeds are composed of several materials in certain proportions. At the angle of fish farming, the compound feeds are considered to have the following advantages:

1. The incompleteness of the nutrients of original feeds can be avoided because the ingredients of compound feeds can complement one another and can raise food utilization rate.

2. Various proteins can supplement one to another and keep the balance between the essential amino acids in compound feeds. As a result, the utilization rate of proteins can be raised.

3. To broaden the food sources, certain feeds disliked by fish in sole application can be mixed with the others.

4. With the bonding agent added to the feeds, the solution of nutrients in water may be diminished, and then, the waste may be reduced.

4. Drugs may be mixed in the feeds so as to control fish diseases.

5. It is convenient to transport and preserve the compound feed. They are fit for automatic feeding, that can lead towards mechanization in fish farming.

(ii) Preparation of compound feeds

In the preparation, the feeding habits of fish and their requirements for main nutrients and nutritional elements in food must be taken into consideration in making a food formula with a balance of nutrients: Attention should be paid to a mixture of a number of original feeds in order to arrive at a complete nutrition. Cares are also given to the economic effect in producing compound feeds with a good selection of cheap materials of broad sources. Difference in formulas ought to be made in the light of the nutritional requirements of fish at different growing stages, e.g. more proteins for Black carp, whereas a little less for Grass carp; more proteins for fingerlings and lesser for the adult fish.

In formula diet, bonding agent needs to be added in a moderate amount, such as potato alfa-starch sweat potato powder, barley flour lest the nutrient should scatter in water.

(iii) Practical samples of compound feeds.

1. Compound pellet feeds for Black carp

   It is due to lack of snails and corbicula that some formulas have been tried out, achieving certain results in experiments.

   1. “320” straw powder pellet feeds
      “320” straw powder is made by means of fermentation of “320” Basidiomycetes.

      Straw powder 50%, bean cake powder 10%, fish meal powder 5%, barley flour 15%, wheat bran 10%, rapeseed cake 10%. The crude protein of it 20.14%, daily application amount 3---5% body weight. Food coefficient of monoculture of two-year-old Black carp is .

   2. Bean cake powder 10% × pupae 10%, barley flour 30%, repeseed protein of it is 20.14%, daily application amount 3–5% body weight. Food coefficient of monoculture of two-year-old Black carp is 2.4.

   3. Beancake powder 25%, fish meal 4%, barley flour 16%, rapeseed powder 24%, wheat bran 26.5%, mineral mixture 1.5%, plant oil 3%.

2. Compound feeds for Grass carp

   1. Straw powder 25%, sesame stem powder 25%, beancake powder 25% ricebran 25%, waste flour 10% as bonding agent. Crude protein content of it is 23.35%, food coefficient 2.8.

   2. Straw powder 70%, beancake powder 15%, ricebran 10%, waste flour 5%, bone powder 1%, table-salt 0.5%. Crude protein content of it is 15.07%, food coefficient 4.4.

   “320” green grass powder pellet feeds. “320” green grass powder 40%, rapeseed cake 20%, fish meal 5% bean cake 15%, pupae 5%, barley flour 15%. Crude protein content of it is 22.6%. Daily application amount is 3.5% of body weight. Food coefficient of Grass carp, and Wuchang fish is 2.5--3.

   Experiments have proved that if we only apply pellet feeds, the Grass carp meat, intestines and livers will contain unusual high fat content, and liver cells will be abnormal with pathological change of fatty liver, consequently, the growth rate is affected. When amounts of green grass are provided (5 days of pellet feeds followed by 2 days of grass application), the state is improving.

   In case of the pellet mixed with green grass, no pathological change is discovered. Nevertheless: all active substances demanded by fish could not be totally included in a certain feed no matter how complete the nutrients of the feed are, not to mention that certain biological active substances in natural food and their functions are still a puzzle to the mankind. It is, therefore, thought that it's rather important to supply fresh natural fodders to fish at a definite time, e.g. some green grass for Grass carp and some snails for Black carp, so as to promote fish digestion of the artificial diet.