2.1 Energy
2.2 Feeding Habits of Fish
2.3 Types of Natural Food
2.4 Fertilization
2.5 Effect of Feeding
'What is this manual all about? Fish already grow in my ponds. I have managed to increase production by fertilization. Why should I spend more money on feed? Will I be able to produce more or make more profit'?
These are the sort of questions this section will try to answer. Having done so, the rest of the manual deals with the type of feed that can be used, how to make mixed (compound) feeds, and how to store and use feed on your farm.
In this section, information on energy and the feeding habits of fish is summarized. The types of food available to them are then introduced. The ways in which the food available to the fish can be increased are discussed. Finally there are some notes on the economic effects of feeding.
The object of aquaculture should be to produce the maximum weight of marketable fish or shrimp from a given volume of water in the shortest time at the least cost.
Fish and shrimp require food to supply the energy that they need for movement and all the other activities in which they engage, and the 'building blocks' for growth. In this they do not differ from other farm animals, or humans. However, aquatic animals are 'cold-blooded'. Their body temperature is the same as the water in which they are living. They do not therefore have to consume energy to maintain a steady body temperature and they tend to be more efficient users of food than other farm animals. Their metabolic rate however depends very largely on the temperature of the water in which they are living. The optimum temperature (that at which they will grow best) is different for each species. Within the range of temperatures of which they are tolerant (those at which they will survive, eat and grow) metabolic rate, and the need for food, increases as the optimum temperature is reached. Thus, in areas where there is a wide range of water temperature seasonally, fish will eat much more food in the summer than in the winter.
Energy can be defined as the capacity to do work. Energy is required to do mechanical work (for example, muscle activity for movement), chemical work (the chemical processes which take place in the body), electrical work (nerve activity) and osmotic work (maintaining the body fluids in an equilibrium with each other and with the medium, whether fresh, brackish or seawater in which the animal lives). Free energy is that which is left available for biological activity and growth after the energy requirements for maintaining body temperature (not necessary for fish) is satisfied. Excess energy is dissipated as heat.
From the point of view of the fish or shrimp farmer, the most economically important thing is the quantity and cost of the energy which is available for the growth of the animal being cultured. Food supplies this energy. The food requirements of different species vary in quantity and quality according to the nature of the animal, its feeding habits, its size, its environment, its reproductive state, etc.
The gross energy (or gross calorific value) of a food, sometimes designated as GE, is the total energy contained in it. Not all of it is available to the animal. Different components of the diet have different energy availabilities. This topic will be dealt with further in section 3 of this manual. The digestible energy (DE) of a food is the GE of the food less the energy of the faeces excreted. The energy available for the 'building blocks' of growth is what remains after the energy for metabolism 1/, reproduction, etc., has been supplied.
1/Metabolism is the sum of all the chemical and energy transactions of the body. It is the process by which nutritive material is built into living matter. Metabolism includes the storage of energy (anabolism) as fat, protein and carbohydrate, and its conversion (catabolism) into free energy for work and growth.
The metabolic rate of small fish and shrimp is greater than that of large animals. Small animals grow faster than large ones in terms of percentage increase in weight per day. Thus the feed requirements of small fish and shrimp are different to those of larger animals; small animals require a higher feeding rate (Appendix XIII). At a certain body size, growth rate starts to decline rapidly. The optimum marketable size of an aquaculture species normally occurs at this point, unless market factors dictate otherwise.
The topic of energy requirements is more fully dealt with in the papers listed under 'further reading' at the end of section 2.
Fish can be grouped into four main categories, according to the type of food which they prefer under natural conditions. These are herbivores, detritus feeders, carnivores, and omnivores.
Herbivores feed directly on the green plants which are the primary source of all food energy. Plants use the energy from sunlight to convert water, carbon dioxide from the air, and nutrients dissolved in the water into organic matter. This process is called photosynthesis. Herbivorous fish and shrimp may feed on microscopic plants - the smaller algae, or 'phytoplankton', or on larger (macroscopic) plants in the pond. Animals which consume plants are the most efficient users of the energy which ultimately arises from the sun. The more intermediate organisms there are between the energy 'fixation' of plants into food and the final consuming animal, the more opportunities for energy loss there are.
Detritus feeders are also very efficient because they feed mainly on dead organic matter (and any associated live organisms) at the bottom of the pond. Much of their food consists of the fungae and bacteria concerned in the breakdown of dead plant and animal matter. The pond detritus may originate from within the pond or from outside (dead leaves from overhanging trees, etc.).
Carnivorous fish and shrimp feed on other, usually smaller, animals. Those which eat the microscopic animals present in the water (the zooplankton) are usually classed as plankton eaters and included in the detritus eaters or omnivores. The larger animals consumed by carnivores include insects and their larvae, frogs, snails, molluscs, and other fish and crustaceans. Fish which feed on other fish are sometimes referred to as piscivores. Within a pond there is a whole series of animals eating other animals, which eat other animals, etc., all down what is known as the food chain until those that eat plants directly are reached. The higher in this food chain that the preferred food for a particular species of fish or shrimp appears, the less efficient that species is as a converter of natural energy into flesh. Piscivores, such as trout, feeding on natural food, are one of the least efficient fish in this sense 1/. The food chain of a carnivorous fish has at least four links i.e.,
1/Carnivores, because of their feeding habit, require a higher protein level in their diet. This is reflected in the higher costs of artificial feeds, protein being the most expensive major feed component. Carnivores require more energy to eliminate the high levels of nitrogenous wastes which build up when animal protein, rather than plant protein, is digested.
ALGAE¯
ALGAE EATING CRUSTACEANS (ZOOPLANKTON)¯
ZOOPLANKTON EATERS¯
CARNIVOROUS FISH
Omnivores feed fairly unchoosingly on both plant, detritus and animal sources of food, depending on what is available. They are less efficient converters of plant energy than herbivores or detritus eaters, but more efficient than carnivores. Examples of these four categories of aquatic animals are shown in Table 1.
Probably the most efficient use of the natural food available in a pond is obtained by polyculture (the culture of more than one species of fish or shrimp simultaneously). If the species cultured are carefully selected, each will have the food it thrives on available to it without it having to compete with the other species present for its nutriment. The best known example of this in aquaculture is that of mixed (Indian or Chinese) carp culture, where up to four or five species of carp, each with a different feeding habit, are reared together.
Table 1 Examples of the Feeding Habits of Aquatic Animals
|
Herbivores |
Detritus Feeders |
Omnivores |
Carnivores |
|
Grass Carp |
Mud Carp |
Common carp |
Trout and salmon |
|
Molluscs |
|
F/w prawns |
Kuruma shrimp |
|
Silver carp 2/ |
|
Channel catfish |
Groupers |
|
Milkfish |
|
|
Sea bass |
|
Many tilapias |
|
|
|
1/Plankton or zooplankton eaters
2/Phytoplankton eaters
The feeding habits of fish are reflected in their digestive anatomy. Typically, carnivores have a short gut with an extendable stomach for large prey. Omnivores which tend towards animal food still have a large stomach but also a longer intestine. Omnivores, such as common carp, which tend towards a plant diet have pharyngeal teeth, no stomach and a long intestine. The gut of herbivores, particularly those which consume phytoplankton, is the longest and most complex type. Anatomical details are given in Smith (1980).
The terms primary, secondary, and tertiary productivity are often applied to pond culture. The primary producers, which convert nutrients, carbon dioxide, and water into organic matter (food) using sunlight energy, are algae. Larger plants are also primary producers but, unless eaten by fish, they do not contribute to the productivity of the pond; indeed, by creating shade and competing for nutrients, they may reduce it. Consuming organisms comprise the secondary or tertiary productivity of the pond. Secondary, or intermediate producers are animals which consume plant food and 'elevate' it to a secondary level. Only part of the food is used for growth, as much as 90% being lost in metabolism. The tertiary producers are those organisms which feed on other animals, resulting in a further, similar, loss of energy. The fish or shrimp which are cultured for food are the 'terminal' producers of the pond (although under less controlled conditions there may be other fish or aquatic animals which prey on them).
It is obvious therefore that the light and nutrients available in a pond are the limiting factor in the quantity of plant material (and thus that of all the other links in the chain - sometimes referred to as trophic levels) that can be produced. The introduction of fish or shrimp into a pond can speed up the production process by thinning out the population of food organisms, allowing more to grow in the space made available. Fish return some nutrients to the pond in the form of faecal matter. However, the maximum basic (primary) productivity of the pond still depends ultimately on the available nutrients. The maximum level of fish production from a pond on a given level of primary productivity will be achieved by an optimum population density. Too low a stocking density will result in a few, fast-growing fish but will leave excess, unconsumed, food. Too high a stocking density will result in little or no growth of individuals because insufficient food is available. Some species of fish will not breed in normal pond conditions - thus the number of fish harvested will never exceed the number stocked. Others breed prolifically before they are harvested - thus, unless management techniques are employed to prevent this, the number of fish present may escalate rapidly. This may result in there being too high a biomass for optimum growth rate to be achieved.
The food habits of fish and shrimp vary, not only between species, but also according to age. Though they may have characteristically different habits between species when they have grown to the postlarval or fingerling stage (i.e., they are then herbivores, carnivores, omnivores, or detritus eaters) the food types consumed by small fry and larvae of most species tend to be similar. The structure of the mouth and digestive tract and the gills of 'adult' animals becomes adapted to the type of food normally eaten. Fry of fish and shrimp larvae mostly consume algae and small planktonic animals such as copepods, cladocera, aquatic larvae, rotifers, etc. (see section 2.3). Food habits may also vary according to environmental changes. The classifications given to fish are indicative, not definitive.
Before continuing to talk about ways in which the food available in a pond or other enclosure can be increased, either by directly influencing the level of primary productivity or by other means, this sub-section briefly describes the different forms of 'natural' foods which are available to fish and shrimp. Only those low in the food chain (or at a low trophic level), not the larger animals which some fish eat, are mentioned here.
UNICELLULAR ALGAE
These are microscopic single-celled plants which are the major primary producers, photosynthesing food from pond nutrients utilizing light energy. They reproduce very quickly, given a good supply of nutrients.
FILAMENTOUS ALGAE
These plants also photosynthesise in the same way as unicellular algae. They consist of colonies of small algal cells attached together. Often regarded as relatively inedible and therefore wasteful in ponds for some aquaculture species, the growth of some types (the green filamentous algae) is actively encouraged (see LUMUT below) for shrimp and milkfish culture in some countries. Filamentous algal colonies can be very large and may form dense masses of material causing physical problems in pond management.
LAB-LAB
A community consisting mainly of benthic (bottom dwelling) blue-green algae and diatoms, together with other plants and animals.
LUMUT
A community consisting mainly of filamentous green algae, together with other algae and animals.
BACTERIA AND FUNGI
Microbial organisms which grow very rapidly and live on detritus at the bottom of the pond - dead phytoplankton cells, leaves, and dead animal tissue.
ZOOPLANKTON
Whereas the microscopic plants are referred to as phytoplankton, zooplankton is a term which refers to microscopic animals. Zooplankton and phytoplankton are together referred to by the general name plankton. The zooplankton include rotifers 1/, of which Brachionus plicatilis is the best known example, which eat algal cells; cladocerans, which include the water fleas, of which Daphnia spp. and Moina spp. are the best known examples; copepods like Calanus spp. and Cyclops spp. and anostracans 2/, of which Artemia spp. are well known in aquaculture.
1/Also known as 'rotatoria' or wheel animalcules
2/Also known as 'fairy shrimp'
MUD EATERS
Animals like insect larvae such as chironomids and various worms, which ingest mud and detritus and derive nutriment from the bacteria and fungi living on it, are in this category.
OTHER INSECT LARVAE
Living in the body of the water, these prey on other food animals, as well as the fry of shrimp and cultured fish, but themselves form a food for larger animals.
The maximum biomass (total weight of animals) which can be produced in a production unit (pond, tank, pen or cage) depends on the quantity of suitable food available. Simply stocking more fish into a pond will not result in a greater production of marketable fish. It may result in more fish but they will not grow so fast and their average weight at any given time will be lower than it would be if the stocking rate were lower. The yield of a production unit can be very significantly increased if more food is made available, providing other parameters, such as water quality, are maintained in an optimum condition. One way of increasing the food availability in production units is by fertilization.
Specific techniques of fertilization are not covered in this manual, which deals in this sub-section only with the principles involved.
The fertilization of ponds, like that of agricultural soil, is simply a means of increasing the nutrients available to the plants growing in it. In the case of ponds, increasing the primary productivity of the pond in this way will also increase the availability of other organisms which are food for shrimp or fish, which graze on the algae in the pond.
Fertilizers may consist of natural organic fertilizers, such as animal manures, or of inorganic chemicals, such as urea. The major nutrients supplied by fertilizers are nitrogen, phosphorus and potassium (and, in manures, organic carbon).
Examples of typical fertilizers include:
INDIVIDUAL CHEMICALS
|
|
N |
Available P |
Soluble K |
Other |
|
(%) |
(as P2O5) |
(as K2O) |
|
|
|
(%) |
(%) |
|||
|
Urea |
46 |
- |
- |
- |
|
Single superphosphate |
- |
18-21 |
- |
Ca |
|
Triple superphosphate (TSP) |
- |
43-50 |
- |
Ca |
|
Orthophosphoric acid |
- |
30-54 |
- |
- |
|
Ammonium sulphate |
21-22 |
- |
- |
24% S |
|
Ammonium chloride |
25 |
- |
- |
66% Cl |
|
Ammonium nitrate |
35 |
- |
- |
- |
|
Potash |
- |
- |
60 |
Cl |
|
15-15-15 ('mixed') fertilizer |
15 |
15 |
15 |
- |
MIXTURES OF CHEMICALS
These have a numerical system given to them which refers first to the percentage of nitrogen (N), then to the percentage of available phosphorous (as P2O5) and finally to the percentage of potassium (as potash, K2O).
Sometimes these are referred to as NPK Fertilizers.
12-24-12 therefore contains 12% N, 24% available P2O5 and 12% K2O16-20-0 is 16% N and 20% available P2O5
45-0-0 is 45% N only
ORGANIC FERTILIZERS
Examples are:
1. Chicken manure
2. Cattle manure
3. Pig manure
4. Compost
Organic manures can be applied directly, in the same way as inorganic (chemical) fertilizers, or they can form part of an integrated animal-fish farming unit. In the latter, terrestrial or avian species are grown together with one or more aquatic species. For example, duck-fish culture, poultry-fish systems or pig-fish systems are practised in many countries. Normally, only the terrestrial or avian species is fed; the fish feed on the aquatic organisms in the pond, whose growth has been encouraged by the manure, or directly on the droppings themselves (copography).
Fertilization thus, by various means, is a way of increasing the available natural food in a pond. Different fertilizing programmes encourage different types of food production. For example, phytoplankton production is favoured by a high ratio of nitrogen to phosphorus in the fertilizer. Organic manure, which is high in phosphorus, favours the growth of 'lab-lab'. If not carefully applied, organic fertilizers may deplete the oxygen supply in the water. Generally, organic fertilizers encourage the production of phytoplankton. Often, a combination of both is used. The fertilization programme can be tailored to fit the feeding habits of the fish being cultured.
Fertilization is often part of a programme to increase the natural productivity of the pond which includes the addition of lime to reduce the acidity of the water, liberate bound nutrients already in the soil and provide a reserve supply of carbon dioxide for photosynthesis.
It is difficult to generalize on the effects of fertilization on the productivity of aquaculture ponds because that productivity depends on so many factors, including the species being cultured, the quality of the soil and the water, temperature, etc. The major point is that the increase over the natural productivity of the pond is very great. The natural productivity could be as low as 100-200 kg/ha/year (even this is 2-4 times that which might be obtained in open waters). In such a pond, fertilization could be expected to increase productivity by a factor of 4 or 5. The following example, extracted from a paper by Kavalec (1976), illustrates the potential effect of fertilization very well, even though the ponds compared were of different construction. In one location, where ponds were built in poor soil and there was a rapid flow-through of water (which resulted in loss of fertilizer) fertilization of 460 kg/ha of sodium nitrate, 360 kg/ha of triple super phosphate (TSP) and 15 kg/ha of potassium chloride gave a production of tilapia equivalent to 820 kg/ha/year, compared with 185 kg/ha/year without fertilization. In another location, using the same water supply, where the soil quality was better and water management was possible, fertilization rates 50% of those quoted above increased the production of tilapia from about 800 kg/ha/year to 1 332 kg/ha/year, 1 938 kg/ha/year and 2 516 kg/ha/year for stocking rates of 5 000, 10 000 and 16 000/ha, respectively. The natural productivity in this example at this site was already very high.
The benefit of a fertilization programme can be measured in a number of different ways. One is the INCIDENCE OF COST, a formula proposed by Vincke (1969):
A second method of comparison is the PROFIT INDEX (Miller, 1976):
PROFIT INDEX VALUE OF FISH CROP PROFIT INDEX COST OF FERTILIZER
These formulae can also be applied to the effect of feeding (section 2.5.)
However, there is another factor besides the cost of the treatment, whether it is fertilizer or feed, and the weight or value of the fish or shrimp produced. This is the time taken for the animals to reach marketable size. If the treatment shortens the length of the growing period (increases the growth rate) in a location where the pond can be re-stocked earlier, a further benefit is achieved.
Actual data on the cost benefit of fertilization is rare (Leopold, 1981).
Feeding consists of supplying the animals being cultured with additional or 'artificial' food. This is food which is not 'natural', i.e., grown in the pond. The 'artificial' food may consist of single (individual) raw materials such as rice bran or groundnut cake, simple mixtures of ingredients, or complex formulated 'compound' feeds compacted by hand or by machine.
Feed may be presented to the animals by hand or from automatic feeders (Appendix XIV).
Feeds for aquaculture may be regarded as 'supplementary' or 'complete'. Supplementary feeds are those which supplement the natural food available in the pond. In other words the animals being cultured are receiving part of their nutrient requirement from natural sources and part from the 'artificial' feed being added to the pond. Feeding supplementary feed increases the production available from the pond. A 'complete' feed, as its name implies, must supply all the nutrient requirements of the animal. Complete feeds are essential in intensive systems of culture where (as in tanks) no natural food is available at all or where the proportion of natural food available is small in relation to the total required. For example, if a pond is expected to produce 4 000 kg/ha/year of shrimp and the natural food available, stimulated by fertilization, can only support a shrimp output of 1 000 kg/ha/year, a 'complete' artificial feed will be necessary for the pond. If the productivity goal is lower a 'supplementary' feed, which adds to the nutrients available but does not completely satisfy all the nutrient requirements of the animals, would be sufficient. Supplementary feeds are cheaper than complete feeds. Supplementary feeds are low-cost agricultural and animal by-products which can be directly consumed by fish but which, in excess, may also act as organic fertilizers. These materials (such as mill sweepings or brewery waste) may be fed alone or in combination as a mash or pellet.
The rest of this manual (section 3 onwards) describes how ingredients are selected for feeding, what the nutritional requirements of different species of farmed fish and shrimp are, and how to prepare, store and present feeds.
As has been exemplified earlier in this section of the manual, fertilization is a means of increasing output from a pond or other aquaculture production unit over that achievable through the consumption of the natural foods present (the natural productivity of the pond). However, the increase made possible by fertilization is limited. Organic fertilizers particularly tend to cause water quality problems.
'Artificial' feed (i.e., feed placed in the pond by humans), which will in future simply be referred to as 'feed', is generally a more direct and efficient way of increasing the food available to the fish than fertilization. Except for those fish that consume faecal material from organic manure directly (which makes that technique a form of feeding), fertilization acts by increasing the primary productivity of the pond. As has been shown earlier, there may be many links in the food chain (trophic levels) between the primary producers and the organisms actually ingested by the targeted aquacultural fish or shrimp. At each trophic level, perhaps 90% of the energy originally supplied is lost. Nevertheless, fertilizers are normally more readily available and cheaper than feeds on a unit weight basis, and should be used to increase production as far as possible. Beyond that, direct feeding is more appropriate. In some situations, for example in tanks or ponds with a rapid exchange of water, fertilization can be wasteful because much of the material washes out of the pond before it is used. Feed is consumed immediately by most fish and relatively quickly by shrimp and prawns and in the latter case, if the water stability is adequate, little feed is wasted.
Increasing the stocking density and thus the fish biomass in a pond places additional strains on water quality. Good water quality has to be maintained by greater water exchange and artificial aeration. These devices remove soluble excretory products, which are toxic, and maintain dissolved oxygen at the levels necessary for growth and survival. These conditions are not ideal for fertilizer application.
Both fertilizer and feed have a role to play in the intensification of aquaculture. Both increase the productivity of the system - fertilization up to a certain maximum point, beyond which feeding must be employed to achieve higher results. Fertilization is more appropriate for the rearing of herbivorous fish which are lower down the food chain, such as the tilapia in the example in section 2.4. (unless grown in high density) rather than carnivorous species, such as marine fish, salmonids and most shrimp.
Production levels achievable in shrimp ponds are approximately 100-300 kg/ha/year without fertilization and 600-1 000 kg/ha/year with. However, with feeding, maximum productivity levels have probably not yet been achieved. Intensive methods of culture in Taiwan are consistently giving production levels of up to 20 000 kg/ha/year on some farms. In Japan, levels up to 50% higher than this have been reached. These cases are extreme examples of the potential results of feeding. Enormous increases in unit productivity are possible but total costs (not just those of the feeding operation itself but other costs, such as pumping and aeration equipment, fingerling costs, etc.,) also obviously rise. Increasing productivity results in increased income and, despite increased total expenditure, should mean increased returns too, as will be seen later.
The efficiency of a feed is normally measured by the amount necessary to produce a unit weight of fish. This is called the feed conversion ration (FCR). The feed conversion ratio is the unit weight of feed given, divided by the live weight (or wet weight) of animal produced.
For example, if 1 250 kg of fish were produced (harvested) from a pond to which 2 000 kg of pelleted feed had been presented during the growing cycle, the FCR would be
Often the FCR is written 1.6:1 (i.e., 1.6 units of feed are necessary to produce 1 unit of fish). The higher the value of the FCR, the less efficient the feed is. For example, a feed which has an FCR of 2.2:1 is worse (less efficient) than one which has an FCR of 1.6:1. Moisture levels in the animal or in the feed are not normally taken into account when the FCR of a feed is calculated. This means that wet or moist feeds automatically have much higher FCR's than dry feeds because so much of the feed consists of water. This does not mean that they are less efficient, simply that you should compare like with like. Moist feeds are normally proportionately cheaper than the equivalent dry feeds, if made on the farm. Strictly it is only possible to compare the FCR of a moist diet with that of another diet of similar moisture content unless a further calculation is done. To compare moist feeds with dry diets the FCR's must be reduced to a common level, either to a dry matter basis or to an assumed 10% moisture basis 1/. For example, let us assume that the FCR of a dry feed is 2.3:1 and that of a moist feed is 3.8:1. We know that the moisture content of the moist feed is 35%. It is reasonable to assume (since we do not know it and most 'dry' feeds are in this range) that the dry feed has a moisture content of 10%. Therefore the FCR of the moist feed can be brought to a comparable level as follows:
1/Wet weight is still used for the weight of animal produced.
Then the two feeds can be compared together - the dry feed, with an FCR of 2.3 is more efficient than the moist feed in this case, which has an FCR of 2.74. Strictly the true moisture of each feed should be measured and the FCR's compared on a dry matter basis.
The FCR, as calculated above, is only the true FCR of the feed in a situation where the 'artificial' feed is the only or 'complete' feed that the animals are receiving. Even in very intensive systems of aquaculture there is some natural feed available, because of the natural productivity of the water, which may have been stimulated, either by true fertilization or because of waste feed. I therefore prefer to use the expression 'apparent feed conversion ratio' (AFCR) because the true FCR of a feed can only be measured where there is no other feed available to the animal. From the farmer's point of view it is the AFCR which is important rather than the FCR since the AFCR is the figure that he can use to measure the cost effectiveness of the use of that feed in his particular farm environment.
The AFCR is a way of comparing two feeds used in similar circumstances. However, several other factors or also important. The first is the relative cost of the two feeds. Thus, if diet 'A' gives an AFCR of 2:1 it may appear better than diet 'B', with an AFCR of 2.5:1. However, if diet 'B' costs 75% of diet 'A' it is obvious that, economically, diet 'B' is better assuming that all other factors are the same. Putting numbers on this example, let us assume that diet 'A' costs US$ 1.20/kg and diet 'B' US$ 0.90/kg. Then, to produce 1 kg of fish with diet 'A' will cost 2 × US$ 1.2 = US$ 2.4 whereas the production of 1 kg of fish with diet 'B' would cost 2.5 × US$ 0.9 = US$ 2.25. Thus diet 'B', though it has a poorer AFCR, is more economical to use. What we have arrived at here, comparing the product of unit feed cost and AFCR, is the same as the INCIDENCE OF COST used by Vincke (1969) and mentioned earlier in section 2.4.2:
Though AFCR and feed (or fertilizer) cost are the two most important factors, others also have a bearing to play on economic viability. Diets 'A' and 'B' in the above example are only comparable in that way if the resultant fish have equal values, for example. If diet 'B' produced fish that are less marketable because of poor appearance or flavour, then the comparative efficiencies of the two diets would have to be re-examined. Here the PROFIT INDEX, mentioned by Miller (1976), is a more useful means of comparison:
Here, the cost of the whole feed/fertilizer treatment is being compared with the value rather than the amount of fish produced.
Similarly the efficiency of several diets can only be directly compared if the time taken for the crop to reach the desired market size is the same (i.e., the growth rate is the same). If diet 'X' results in the animals taking 4 months to reach market size while diet 'Y' gets the animals to that size in 3 months, diet 'X' is less efficient than diet 'Y' even if they have same cost and the same AFCR and the animals fetch the same market price.
Assessing the relative efficiency of two diets therefore requires information of several different types:
AFCR
Feed cost
Growth rate
Product market value 1/
Cost of feeding 2/1/The 'dress-out' weight of fish or the head:tail ratio of shrimp is not mentioned here becuase these are factors which govern the market value (at farm gate) of the whole animals.2/One feed may cost more than another to feed because of some inherent quality, such as different storage or transport costs, different labour costs for feeding, etc.
Feed costs increase according to the increase in intensification of the aquaculture system (Leopold, 1981). The importance of natural feed decreases as intensification of production increases. Although the total feed costs increase, within certain broad limits, return on capital does not decrease but may in fact increase because an increase in intensification results in a relative decrease in the level of fixed operating costs per unit of production. Leopold (1981) quotes a case. (Table 2) where decreasing fixed costs cancelled out the increased cost of feeding to such an extent that the unit cost of fish production also decreased.
Table 2 Example of the Effect of Feeding on the Production of Carp In Poland in 1962-1966
|
Amount of Feeds used (kg/ha)
|
Average Fish Yield (kg/ha)
|
Costs of Production of 1 kg of Fish (in zloty) |
||
|
Feed Costs |
Fixed Costs |
Total |
||
|
up to 400 |
191 |
5.54 |
14.00 |
19.54 |
|
400 - 500 |
221 |
6.56 |
12.10 |
18.66 |
|
500 - 600 |
237 |
7.17 |
11.29 |
18.46 |
|
600 - 700 |
292 |
7.10 |
9.16 |
16.26 |
|
700 - 800 |
330 |
7.17 |
8.11 |
15.28 |
|
1000-1500 |
512 |
7.36 |
5.22 |
12.58 |
|
1500-2100 |
711 |
8.42 |
3.76 |
12.18 |
Source: Leopold, 1981
In the above example, other variable costs (including fingerling cost, labour, etc.,) were excluded as they only constituted 15% of the total production costs. It is clear from the example that the AFCR (apparent feed conversion efficiency) of the feeds used were better at low levels of application, probably because the contribution of natural food was still significant at this level. This example well illustrates the effect of intensification through feeding increasing levels of feed per unit area. These include a much more efficient use of the facilities available and a very significant reduction in the total cost of producing each unit weight of fish.
The economics of compound feed use are discussed further in section 5.4.6.
Feeding has a most significant potential role to play in increasing the revenue and profitability of your aquaculture unit. This manual seeks to introduce you to ways of starting to use feed on your farm.
Further reading (section 2):
Frey (1960); New and Singholka (1982); Woynarovich (1975); Leopold (1981); Hopkins and Cruz (1982); Cook (1978); Stickney (1979); Miller (1976); Vincke (1976); Kavalec (1976); George (1976); NRC (1973); Jauncey and Ross (1982); Wright and Kenmuir (1981); Boyd (1982).
