The stocking rate for fingerlings varies according to the availability of fry.
The mesh size is 8 mm. With intensive hand feeding (5 times daily), 25 g
fingerlings are produced in one month.Extensive Culture- COMMERCIAL CAGE CULTURE, INCLUDING EUTROPHIC LAKE SITES
In extensive cage culture there is no supplemental feeding. In most cases,
the tilapias feed on the plankton either as natural blooms in eutrophic waters
or in fertilized fish ponds.
The species used are microphagous S. aureus, S. niloticus and S. mossam-
bicus whose natural diet normally includes algae.
The only large-scale, extensive, commercial tilapia cage culture in eutrophic
waters is that of S. mossambicus in the Philippines in eutrophic natural lakes
and in reservoirs. Guerrero (1980b) states, however, that S. niloticus is now
the preferred species in Laguna de Bay where it may be raised at the density
of 20 to 25 fish/m2 without supplementary feeding, from 3 to 4 cm juveniles
to 100 g adults in 4 to 5 months.
The eutrophication of these water bodies derives from the richness of
local volcanic soils, e.g., Lake Bunot and Lake Sampaloc, or the large inflow
of nutrients (N,P), e.g., Laguna de Bay. A rapid expansion of the tilapia cage
culture industry is expected (Guerrero 1979a). In Lake Bunot more than 70
commercial-size cages were added from 1975 to 1978. Tilapia cage culture is
now practiced also in Laguna de Bay, Lake Calibato, Lake Gunao, Lake
Paoay, the Pantabangan Reservoir, Lake Sarnpaloc and Lake Sebu.
Some examples of the technology used and its results are grouped in
Table 4. Very large floating cages (fibre netting) are used: more than 6,000
m3. Small fingerlings (5 to 10 g) are generally preferred for stocking because
of their greater availability and lower price. The stocking rate is relatively
low: usually less than 0.5 kg/m3, less tnan 70 fish/m3. The average produc-
tion rarely exceeds 1 kg/m3/month, because of the large size of the cages,
but up to 10 to 15 t of S. mossambicus can be harvested every 6 months, pro-
viding the farmer with a net income of more than US$3,000.
The success of the first cage farmers accelerated the development of
the industry so much that it got out of control within a few years in most
of these eutrophic lakes. In Lake Sampaloc for example, the water surface
has become so congested with floating cages that the average original produc-
tion of 3.8 kg/m3/yr has drastically dropped today to 0.8 kg/m31yr and the
growth rate of the tilapias has decreased by a factor of 9. It takes now as long
as 12 months to produce 50 to 60 g fishes compared with earlier culture of
fish to 150 to 200 g in 4 months. Some control has been introduced and a
license is now required for cage culture.