The Biology and Culture of Tilapias

3. SEMI-INTENSIVE CAGE CULTURE USING LOW PROTEIN MIXED FEEDS

Various feeds based on local ingredients have been used for caged tilapias.
In some cases, such as in the eutrophic lake Laguna de Bay (Philippines),
such feeds are used to augment natural feeding on welldeveloped algal
populations. In other instances, they may constitute practically the only
nutritional source available to the fish. The protein content of these feed is
generally less than 10% of their total dry weight. Several ingredients may be
combined as diverse as rice bran, snails, plant leaves, brewery waste, oil cake
and cattle blood, according to local availability and price. In Lake Ilopango
(El Salvador), the Fisherman's Cooperative, with 210 m3 of cages, harvests
about 900 kg of S. aureus and private operators sell another 2,500 kg annually
(Street 1978) but no details are available on the exact nature of the diet.
Data related to various other tilapias are summarized in Table 8. S. nilo-
ticus and S. mossambicus were reared experimentally in floating cages in
Laguna de Bay using rice bran alone (Anon. 1979a). S, niloticus gave a much
better production (2.3 kg/m3 /month) than S. mossambicus (Pantastico and
Baldia 1979), even though the latter was fed additional chopped snails
(Stenomelania canalis and Melanoides sp.). The average initial biomass,
although not clearly ascertained, was most probably higher than for S.
mossambicus of which the production potential appears to be lower than
that of S. niloticus. McLarney (1978) obtained good results in Colombia
rearing T. rendalli in 1 m3 cages in fish ponds. Starting with 22.5 g finger-
lings and an average Bi of 2.25 kg/m3, 21.6 kg/m3 of 200 to 250 g fish were
harvested after 5 months. Alocasia macrorhiza leaves and wheat bran were
fed daily. The average individual growth (1.3 g/day) was good and the
monthly production nearly 4 kg/m3. In fact, these are the best results avail-
able for semi-intensive tilapia cage culture (Table 8).
In the first experiment with tilapia cage culture in Africa in 1972, S.
esculentus and T. zillii were reared, either separately or together in Lake
Victoria, Tanzania (Ibrahirn et al. 1975). Feeding mostly on brewery wastes
and some fish meal plus plant leaves (for T. zillii), the fish's average Bi's were
very low (0.2 to 0.4 kglm3). The resulting average growth rates and produc-
tion were relatively low to medium, probably due in part to poor feed value.
Trials to raise tilapia (probably S. mossambicus) in floating cages in
the inlet to the Negombo Lagoon, north of Colombo, Sri Lanka were started
recently with relatively good results (Sollows pers. comm.). Some water
is added to the feed components (see Table 8) to form a mash.

Intensive Cage Culture

As the quality of supplemental feed-in particular its protein content

and its nutritional balance-are improved so cage culture of tilapias may

be intensified. From the economic point of view, the cost of feeding gains

in importance, and can account for more than 50% of the production costs