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WATER HARVESTING AND AQUACULTURE
FOR RURAL DEVELOPMENT

INTRODUCTION
Net enclosures can be used in every phase of tilapia culture from fingerling production to growing market size food fish. In Southeast Asia the "hapa" net system for Oreochromis niloticus fingerling production is very popular. Hapas protect tilapia from predators and allow high survival. Produced fry and fingerlings aretransferred to ponds, other hapas or tanks for further grow-out.

Figure 1: Tilapia fry production in hapas for transfer to nurseries.
WHAT IS A NET ENCLOSURE OR "HAPA"?
Hapas are constructed of netting material which is sewn together to form a square or rectangular enclosure. Hapas differ in size and mesh according to use. Breeding hapas hold tilapia broodstock and are constructed of nettingwhich has a mesh size of 1.6 to 2.0 mm. Inverted mosquito nets are often used for this purpose, but the fine mesh will become clogged with plant growth if not cleaned frequently. Clogging prevents fresh water from circulating into the hapa and can result in a low oxygen condition which kills fish. Larger mesh sizes allow greater water exchange in the hapa, and are used for nursing fingerlingsstocked at high densities. Figure 2 illustrates the kind of hapa frequently used in Latin America. A cover is often attached over the hapa to prevent brood fish from jumping out and keeps predatory birds from injuring fish. Figure 3 illustrates a hapa typical of Southeast Asia. The support frame is lacking and a cover may be absent.

Figure 2: A typical hapa and support frame used in Latin America.Figure 3: A hapa typical of Southeast Asia.
HOW DOES THE TECHNIQUE WORK?

Step 1: Determine where to place the breeding hapas.

Breeding hapas are placed in shallow, protected areas of ponds, lakes and slow moving rivers. Water should be at least 60 cm deep and the hapa tied with the rim at least 30 to 70 cm above the water line. This prevents fish from escaping when water levels riseduring floods, and prevents brood fish, which can jump 50 cm, from escaping. The hapa floor should be tied at least 20 cm above the bottom sediments in ponds with soft mud bottoms. If sudden changes in water level of more than 20 to 30 cm are likely to occur from flooding or drainage, hapas may be tied to a floating frame rather than to stakes driven into the mud.

Figure 4: Hapa tied to supportstakes contains broodstock and fry.
A female tilapia will lay eggs on the hapa floor, and pick them up in her mouth for incubation after the male has fertilized them. If the mesh size of the floor netting is larger than 1.6 mm (1/16 inch), the eggs will pass through and be lost. To prevent the loss of eggs, materials such as fine mesh mosquito netting, a piece of plastic, a weighted board oranything with a flat surface that is not very heavy may be placed on the hapa floor to serve as a substrate for egg laying. If hapas are set up in concrete tanks or in ponds with a hard bottom the net floor may rest directly on the bottom. Water circulation will, however, be reduced.

Step 2: Determine how large the hapa system will be.

A farmer must be able to calculate how many brood fish areneeded to produce a given number of fry. Assumptions must be made about the number of fry a female will produce. The assumptions presented in this manual are general and will not give the same results in all areas of the world. They are provided only as an example which can be adjusted according to local conditions.

The number of eggs and fry produced by a female depends largely on her weight. A200 g female will produce roughly twice as many fry as a female weighing 100 g. Tilapia smaller than 50 g are normally immature and are not satisfactory as brood fish.

Brood fish are stocked into hapas at rates of about 500 g per m2 (usually 3 to 7 fish depending on size) and in a ratio of 1 male to 3 females (or 1 out of 4 fish is a male). Under these conditions, 500 g of brood fish on...