Cage and pen culture
Coche (1982) made a very extensive review of cage culture attempts in different countries. The early interest in cage culture of tilapia was on the assumption that wild spawning would not occur in cages or, if it did, that the progeny would not remain in the cages and cause over-population, as in ponds. Later the value of cage culture in utilizing open bodies of water, particularly those of eutrophic lakes, coastal areas and running waters, was recognized. Though of widespread interest, most of the cage culture presently practised is still on an experimental scale, with only a few exceptions as in some areas of the Philippines, Ivory Coast, Costa Rica and El Salvador. The commonly used species are T. mossambica, T. nilotica and T. aurea.
Cages are mainly used for grow-out, and the necessary fry or fingerlings are produced in land-based facilities like ponds, cisterns, etc., or in hapas installed in ponds.As described earlier, special double-walled cages can be used for spawning of tilapia in open waters. Fixed and floating cages are used in the open waters of lakes in the Philippines for tilapia grow-out. The fixed cages are used in shallow eutrophic lakes and the floating cages in deep lakes. The stocking density varies with the size of the cage, but in floating cages up to 25 fingerlings/m2 of 3–4 cm length are common. Artificial feeding is generally not practised, except in waters with low productivity. In six months, from February to July, the fingerlings grow to a size of 200– 250 g each, and in the nine months from August to April to 250–300 g each. The growth rate is largely based on the primary productivity of the lake and the management practices, which include the density of cages in the lake and the distance between cages.
The stocking rate of T. nilotica in fixed cages ranges from 15 to 50 fingerlings/m2, and the duration of culture from 4 to 12 months. The growth rate depends on the productivity of the lake. Without supplemental feeding, 5cm fingerlings stocked at 15 per m2 in Laguna de Bay attained 150–180 g in four months. The average production of 3.5–7 kg/m3 of 100–150 g fish has in recent years been greatly reduced as a result of crowding of cages in the lake (Coche, 1982).
Pen culture of tilapia appears to be practised only in the Philippines in the Laguna de Bay. The same materials used for abandoned milk-fish pens are used for the construction of tilapia pens, but smaller pens of 0.5–1 ha are preferred. The pens are stocked at the rate of 20–50 fingerlings/m2 and fed with rice bran or wheat bran at 2–3 per cent of body weight per day. The growth rate varies according to the productivity of the lake, and in productive waters they can grow to a size of 170–250 g in four to five months, even without feeding. One of the major problems in pen culture of tilapia is the poor catchability of the species used, namely T. nilotica. The harvesting rate with seine and gillnets has been reported to be only about 15–30 per cent.