It is important to know the feeding preferences of the species being cultured so the correct food can be provided at all times in order to maximize growth. The feeding preferences are determined by observing the fish as well as by examining the gut of naturally occurring fish.
Fish (and other animals) can be divided into
different feeding classes:
Herbivores – feed only on plants (e.g. Tilapia rendalli, grass carp)
Carnivores – feed only on other animals or meat (e.g. trout, bass)
Omnivores – feed on both animals and plants (e.g. common carp)
Planktivores – feed on the very small plants and animals in water (e.g. mullet, silver carp) Detritivores – feed dead plant or animal material on the bottom (e.g. Oreochromis mossambicus).
Natural foods are the best foods for fish and include algae (phytoplankton), zooplankton, detritus, snails, worms, insects and insect larvae, small plants like duckweeds and various other weeds and grasses that are found in a fish pond. If the fish is carnivorous, smaller fish can be a
good fish growth. The best supplementary foods a farmer can put into the pond are extra natural foods. But there are a great number of other foods that fish will eat.
Depending on the type of fish in the pond, almost anything can be used as a supplementary food. Common supplementary foods are: bread crumbs, rice bran, fish meal, ground-up maize, oats, barley, rye, potatoes, broken rice, soy bean cakes, peanut cakes, corn meal, cottonseed oil cakes, coconut cakes, sweet potatoes, guinea grass, napier grass, water hyacinth, wheat, and leftover animal feeds and some animal manures.
Raw materials containing high amounts of animal protein, such as fishmeal and blood meal, are scarce and expensive. It is therefore easier to obtain the relatively high protein requirements for catfish by using feedstuffs that contain higher quantities of vegetable protein such as plant oilseed cakes and meals. These
by-products from the agriculture industry are more common, cheaper and generally available in large quantities.
As there is little large-scale intensive aquaculture in most African countries, the present demand for raw materials comes mainly from domestic poultry and livestock industries. Consequently, there are generally no specific vitamin and mineral supplements available for aquaculture species.
The type of extra food supplied depends on the kind of fish. For example, tilapia will eat many vegetable or grain-sourced waste products, including the supplementary foods listed above, which is why they are such good pond fish.
The silver carp, on the other hand, will eat only phytoplankton, even when it is a fish of marketable size. It is therefore important that the farmer knows what the fish will accept before putting extra food into the pond.
When the stocking density of fish is increased
to levels beyond those capable of surviving on
the natural food in the pond, the growth of the fish can only be maintained by supplementing the natural food with some artificial feed. This is the single most important management element to increase the pond’s fish production. The
type and quantity of food fed must be carefully considered as it may negatively affect the health
of the fish in the pond (i.e. by causing a drop in oxygen levels, possible pollution, etc.). Feeding should be performed at the same times (early morning and late afternoon) to improve food consumption and to teach the fish to come to the same area of the pond.
Food should be fed in the shallow end of the pond or around wooden stakes, which encourages the fish to come to the same place in the pond to feed. This allows the fish farmer to check the health of his fish at each feeding. If ponds are too big to feed by hand, an open-
bottomed boat can be used to supply a stream of feed around the pond. The build-up of waste on the bottom of the pond depends to a large degree on the amount of artificial food fed. It is therefore important that any excess or uneaten food is removed if the fish are not feeding. If the waste builds up, it will lead to high levels
of ammonia and nitrate, which will cause a drop in dissolved oxygen levels, all of which are dangerous to the fish.
The amount of feed used per day is generally calculated for a two-week period and adjusted every four to six weeks after catching a few fish with a cast net and recording their average body weight. The biomass of the fish and the daily quantity of feed are then calculated according to the recorded average body weight (see example for catfish, in table above) and estimated survival rate. It may be difficult to predict growth and survival rates as they depend on many factors (such as fish density, feed quality, temperature, predation, etc.).
If the temperature changes by 4˚C (up or down) from
the preferred range of the species, the feeding rate should be reduced by 25%. If the temperature changes by 8˚C, the rate should be half (50%) of normal. It is very important that the fish are not fed more food than they can eat. Not only is this a waste of money but it also leads to poor water quality and possible death of all of the pond’s fish.
If the level of dissolved oxygen drops between
3-5 mg/l, the amount of food added to the pond should be halved. No feed should be given if the dissolved oxygen falls below levels of 2 mg/l.
Every effort should be made to bring the oxygen level up to an acceptable level before feeding resumes.
If the fish should stop feeding, the cause of this should be determined immediately. The illustration below depicts various reasons why fish may stop feeding. All of these possibilities should be considered and investigated, otherwise the fish in the pond may die.
As in any business, it is important to reduce costs while trying to increase production. In aquaculture, fish farmers try to feed just the right amount and type of food to allow the fish to grow but not waste food as it is expensive. Some foods, such as those with a higher protein level (e.g. fishmeal) may produce better growth than plant-protein diets. However, the high protein content may be more expensive and therefore the cost of the diet must be carefully considered in relation to its contribution
to improving fish growth. The concept of food conversion ratio (FCR) is important to
understand as it helps describe how well a diet is used by the fish to grow. FCR is defined as the mass of food consumed divided by the mass
of fish produced (see examples in the boxes,
If the cost of the feeds were the same per kg, the diet with the lower FCR (of 1) would be
cheaper to use as you only use 1 kg feed instead of 2 kg of feed to produce 1 kg of fish. However, the cost of the plant-based diet in this example is R8/kg, and therefore:
The cost of producing 1 kg of fish = R8/kg x 2 (FCR) = R16/kg produced.
Similarly, the cost of the fishmeal diet = R20/kg x 1 (FCR) = R20/kg fish produced.
Therefore, although the fishmeal diet has a better FCR than the plant-based diet, the cost of the plant-based diet is cheaper per kg of fish produced and is therefore the more cost-effective diet to use.