As a fish farmer, maybe you are wondering what the importance or feeding your fishes are or you have you heard a question like this: I have discovered that fishes in Rivers and Dams are not usually given any feed and yet they develop well. Is feeding then necessary? …. Smiles!
Well, fishes in Rivers and Dams are not raised for economical reasons and so, they are usually cropped after about three years or more. In the Rivers and Dams, they have access to zooplanktons and phytoplankton’s; they do feed on one another and also have enough time to grow.
However, raising only one type of fish in an enclosure within a period of 6months will definitely need giving them what they would have had access to if they were in their natural habitat. So feed them adequately to get the best from them.
Fish need to be provided enough nutritiousfood in order to attain big sizes in a shortperiod of time under culture conditions.
There are two main types of fish feeds. A) Natural food B) Artificial feeds.
A) Natural Food: Natural food can be stimulated to developin a pond through pond fertilisation. A fertil-ised pond will have tiny plants(phytoplankton) and animals (zooplankton)that can only be seen under a microscope.
B) Artificial feeds: Artificial feeds are those feeds prepared’and given to fish. The nutrients in thesefeeds should be well balanced to meet the nutritional needs of the cultured fish species.The nutrients that should be included in fishfeeds include:
– Protein for body building
– Fat for normal functioning of the body and for energy
– Carbohydrates for energy
– Mineral salts for bone structure and body functions
– Vitamins for good health
Locally available ingredients such as fish meal,soya, maize and wheat are some ingredientsthat provide the nutrients listed above.There are two sources of artificial feeds:
i) On farm made feed
They are made on the farm by mixing ingredients into powder or dough which is fed directlyto the fish or cooked prior to feeding. However, on-farm made feeds have been found to lack some of the nutrients necessary for fast fish growth. Therefore, it is advisable to usethem as supplementary feed along with pond fertilisation.
ltis also recommended that farmers seek advice on how to formulate feeds from competent persons.
ii) Factory-based fish feed
This type of feeds are prepared by commercialproducers and sold to farmers. Normally factory based feeds contain all the nu-trients needed by the fish. The pellets can eitherbe sinking or floating pellets.
The cost of feeds is the highest cost of production in commercial fish farming. It ranges from 40% to 60% of,the total cost of production. Being’ the highest cost, a lot of care is needed to minimize over expenditure on feeds as much as possible in order to avoid loss of profit. The two most important ways by which fish farmers may lose money in fish feedsare:
i) When the feed is of poor quality
When a feed lacks or contains certain impor-tant nutrients (especially protein for bodybuilding) only in small quantities, fish doesnot grow well. It has to eat a lot more feed inorder to make little growth. Even i~the priceof such a feed is low, the farmer spends moremoney on the feed for each gain of 1 kg.
ii) When excess feed is given
Any feed that is not eaten by the fish drops into the pond water. The excess feed drop-ping into the water may be a result of two things. Either the farmer gives too much feed for the fish, or the feed is of poor quality and the fish only eat a small proportion. Eitherway, the uneaten feed drops into the pond and starts rotting (decomposing).
Anything that rots in the pond takes up thefresh air (oxygen) that the fish needs for breathing, and produces bad gases e.g. ammonia that are poisonous to fish.
Under such conditions, the fish gets stressedand stops.eating. In extreme cases the fishdies.
Therefore, it is important to supply the exact amount of feed needed by the fish and avoid wastage.
A good quality feed that is poorly administered is not better than a poor quality one.
How to deal with over feeding
The signs of overfeeding include the pondwater turning very green. Eventually stressed fish is seen coming to the surface and gasping for air, especially at night and early morning.
When these signs are observed, farmers should flush more water through the pond to dilute the effects of pollution until the green colour of the pond water is greatly reduced.
Proper fish feeding
Just as it is important not to supply excess feed, itis also important not to underfeed the fish. Therefore, farmers should seek guidance on the right amount offeed to be fed to their fish daily.
Feeding charts are available at the aquacultureresearch and development center to guide farmerson the amount of feed to give different fish speciesand age groups.
Read Also: Diseases Fishes can get from Feeds
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.
Read Also: Using Tilapia Fishes to Feed Catfishes
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.
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
Trout and salmon
Silver carp 2/
1/Plankton or zooplankton 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. Food habits may also vary according to environmental changes. The classifications given to fish are indicative, not definitive.
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