- FEED PRESENTATION
Caged tilapias have been grown using mash or pelleted feeds, the pellets
being wet or dry, sinking or floating. Guerrero (1980a) regards S. niloticus as
an avid surface feeder in cages and advises the use of mash feeds for small
fish only. With large fish, the water agitation is so strong that much of the
mash feed is washed out of the cage and lost. Pelleted feeds are therefore
more efficient for large fish but the simplicity and low cost of mash make it
still attractive for artisanal fanners. Rather than pelleting compounded feed,
they could prepare meal in advance, in the form of "balls" or mash and
airdry them. Such feed "balls" can be placed in the middle of the cage-
covering net, which is then slightly lowered under the surface of the water
to give the fish access to the food (Campbell pers. comm.). A similar feeding
method can also be used with fresh mash balls, particularly if the area of
the cage is of the order of several square meters.
Guerrero (1980a) fed S. niloticus moist and dry compounded artisanal
feeds (65% rice bran, 25% fish meal, 10% copra meal) at DFR 4% B as two
meals per day in 1 m3 cages (mesh 2.5 cm) placed in a pond for 24 days. The
results are summarized in Table 11A. He concluded that moist pellets were
better utilized by the fish. The FCR was slightly higher with the moist
pellets, but there was no significant statistical difference between the two
treatments. Moist pellets were easier and cheaper to produce.
Table 11B gives the results of feeding floating and sinking pellets (40%
protein) to S. aureus in 0.12 m3 cages placed in fertilized ponds for 87 days
at DFR 3% B for 70 days of this period. The floating pellets gave better
growth, higher production and a lower FCR. This holds true particularly for
small cages with high fish densities and biomass. The difference may be less
for larger cages (several m3), at lower densities (below 400/m3) and with a
smaller biomass (up to 40 kg/m3). In such cases, the extra cost of floating
pellets might not even prove economical, but good data are missing.
Dry pellets may be distributed in cages either by hand or mechanically.
In the latter case, automatic rather than demand feeders are preferable,
especially if the fish density is high. Again, economical considerations
should guide the choice. If labor is expensive, automatic feeders can reduce
costs. They also facilitate dividing the DFR. On the contrary, in artisanal
cage farms, hand feeding may be more advantageous. It also gives the oppor-
tunity to monitor the fish stocks mare closely. No comparative data exist for
tilapia cage culture.
- FEED CONVERSION
The efficiency of feed conversion is generally quantified as the feed
conversion ratio (FCR): the weight of feed required to produce unit weight
of live fish during a determined feeding period. Some authors calculate
a "Net FCR" where the production due to the natural feed is taken into
consideration. In intensive cage culture particularly (Coche 1978, 1979)
FCR is determined by the interactions between the fish (individual size,
sex ratio and density), the fed (quality, DFR and distribution) and the