Methods of spawning
Attempts have been made to develop a suitable fry production system for tilapias. Campbell (1978) described a relatively simple method of producing large numbers of T. nilotica fry using 600 m2 earth ponds of about 0.4m depth. Female fish of about 700 g weight and males of 200 g are stocked in one pond at an average density of one per 2 m2 in the sex ratio of one male to four or five females. They are fed on a high-protein diet for about a month, by which time they will have started spawning. The brood fish are then transferred to a second pond, where they are fed in the same way as in the first pond. Feeding is continued in the first pond as well for another month, by the end of which the fry will have reached a size of about 4cm and on average about 5000 fry are available for harvest. By this time, spawning will have occurred in the second pond and the brood fish can be transferred back to the first pond for further spawning. The production per month by this method is reported to be about 4.2 fry per m2 or 10.4 fry per female.
In Israel, ponds ranging in size from a few square metres to 5 ha, with gently sloping bottoms, are used for spawning T. nilotica and T. aurea. The ponds are dried prior to spawningto eradicate weed-fish and pests. They are filled to a depth of 50–60 cm, which is the preferred depth for spawning of these species. As the number of eggs per spawning depends on the size of the females, the stocking rate is varied according to their size. While a 100 g T. nilotica spawns about 100 eggs, a 600–1000 g fish will spawn about 1000–1500 eggs. A female T. aurea of about 1000 g weight may spawn about 2000 eggs each time. The stocking rate for males is generally 100–250 per ha.
In the Philippines, land-based spawning ponds as well as open-water-based cages or hapas are used for spawning and fry rearing. Many farms use hapas made of nylon mosquito netting to breed T. nilotica and hybrids of T. nilotica x T. mossambica. The brood fish are maintained in hapas installed in ponds with about 1 m depth of water. The fish continue to breed throughout the year. A 1 : 3 male to female sex ratio has been found to be suitable. The fry are collected at intervals of about a month and grown to fingerling stage in nursery ponds or cloth tanks. For cross-breeding, the best sex ratio has been found to be one male to three females. The average monthly production in computed to be about 1466/m3.
Open-water-based cage hatcheries used in the Philippines consist of double-walled net cages very much like the double-walled hapas used for carp hatching in India. The inner coarse-mesh (30 mm) net measures 10 x 2 x 1 m, and the outer fine-mesh net, 12 x 4 x 1.5 m. They are installed in protected calm areas of lakes, such as the ones found in Laguna de Bay. Breeders are stocked at a density of four per m2 with the same 1 : 3 sex ratio as in ponds, and fed with fine rice bran at 3 per cent body weight per day. Spawning occurs at regular intervals and the fry are collected and stocked in rearing hapas (10 x 2 x 1.5 m) at the rate of 1000 per m2. Fine rice bran is used for feeding the fry at the rate of about 6–8 per cent of body weight. After two weeks of rearing in hapas, the fingerlings are transferred to larger-meshed (6.5 mm) cages at the rate of 250–500 per m2 and fed with fine rice bran at the rate of 4–6 per cent of body weight per day.
Systems that allow a high degree of environmental control make year-round spawning of tilapias possible in temperate climates. Removal of eggs from incubating females and hatching and rearing them separately in special containers helps to increase spawning frequency and thereby overall fry production. Another important advantage of spawning under controlled conditions is that genetic purity of lines can be maintained, and this is of special importance in hybrid production. The sex ratio of females to males is generally 3: 1 or 4 : 1. As tilapia spawn at frequent intervals, harvesting has to be carried out every fortnight, when the fry are about 0.5 g in weight.
The aggressive behaviour of the male in an aquarium or tank manifests when mature fish are introduced at a size of about 100 g. Long aquarium tanks (200 x 50 x 40 cm) are stocked with immature, four-to-five-month-old fish. One male and seven to ten females form a ‘family’ in each aquarium. When they become sexually mature, the males of mouth-brooding tilapia species dig nests at the bottom if there is sand or gravel there. Even if the bottom is bare they exhibit digging movements. The male chooses the ripest female and, after a period of courtship which may last several days, spawning and fertilization take place in the nest or the bottom of the aquarium. Soon after, the female picks up the eggs in her mouth. The male then
chooses another ripe female for courtship and spawning. The eggs are removed from the female’s mouth after three to five days for further incubation. This helps in preventing cannibalism and early preparation of the female for further spawning. Zuge jars or containers placed on a shaking platform (for keeping the eggs separate and in continuous movement) are used in incubation. The eggs hatch out in about 50 hours at temperatures of 25–27°C. The larvae remain in the incubating containers until the yolk sacs are absorbed, which may take about 8–10 days.
Nursing of normal fry or mono-sex hybrids is carried out in nursery or rearing ponds. Stocking densities vary from 50 000 to 100 000 per ha, depending on the size of fingerlings to be raised. When manual sexing of fry is needed, it is necessary to grow them to a size of at least 20–50 g so as to distinguish secondary sexual characteristics with ease. Even a larger size of 100 g is often recommended, but it has to be ensured that the stock are removed before they reach maturity.