Cage production increases as the initial biomass (Bj kg/m3) at stocking
increases until it reaches an optimum value. For this Bi optimum,. the final
biomass (Bf) will equal the MCC at the end of the production cycle. In the
Ivory Coast, in 1 m3 cages, the MCC = 90 kg/m3 was reached in 4 months
from Bi of 20 kg/m3 S. niloticus (at 250 to 350 fish/m3). The correspond-
ing maximum production was about 70 kg/m3 or a monthly average of
17 kg/m3 (Coche 1977).
It is well known that male tilapias grow faster than females. Any shift
in the sex ratio of the cultivated population towards a male predominance
will therefore accelerate production. In cage culture, increases in growth
rates and production as well as decreases in FCR's have been recorded for
monosex male populations (Coche 1977). Campbell (1978a) has also observed
good production with 84% male S. niloticus following size grading of the
juveniles. In both monosex male and mixed sex S. niloticus cage culture
at Auburn University, Alabama, male growth rates were about 2.4 times
than those of females (Anon. 1979~).
Seed Production- FRY AND FINGERLING PRODUCTION IN CAGES
Most tilapia cage culture is concerned with growout to market size and
takes advantage of the fact that reproduction, is usually suppressed by
the cage environment. Tilapias may, however, spawn in cages under certain
conditions, depending mainly on the mesh size and on the fish density. For
example, in Lake Atitlan (Guatemala), S. mossambicus have produced larvae
in floating cages (Bardach et al. 1972) and Guerrero (1975) has observed
in the Philippines female S. mossamb'icus mouthbrooding in cages with
200 fish/m3. I have also found mouthbrooding S. niloticus in floating cages
with 25 mm mesh but these were only a few individuals within a large popu-
lation and the young fry disappeared rapidly from the cages through the
netting. Suwanasart (1972) observed that S. aureus spawned successfully
at densities of 500 fish/m3 in cages with a small mesh screen placed on the
bottom. In Indonesia, Rifai (1979, 1980) has also bred S. niloticus in cages
with 3 mm mesh but "the occurrence of reproduction was relatively low'"
only 5 out of 27 cages. Ther re to use cages with the definite purpose of
producing tilapia fry is un 3 mising and a better technique is required.
In the Philippines, Guerrero (unpublished data) uses fine mesh nylon
or mosquito net cages, termed hapas, to breed S. niloticus and S. niloticus x
S. mossambicus hybrids. The broodstock live in these cages, breeding con-
tinuously, and the fry produced are collected once a month. These fry
are then grown on to fingerling size, either in another cage or in a nursery
pond.
The hapas (1.5 x 1 x 1 m) are suspended just above the pond bottom
from poles in water about 1 m deep, from a good quality supply. The water
depth inside the cages is about 0.6 m giving arearing volume of about 0.9 m3.
Table 3 summarizes some results of fry production in hapas for 5 weeks. Fo/r