rearing environment (cage size, water exchange rate, DO and temperature).
To maximize the feeding efficiency, every one of these factors should be
maintained at its optimum level for the particular species being cultured.
Most of these aspects have already been reviewed above.
Feed losses through the cage walls should be kept to a minimum by
adapting feeds and feeding to the particular conditions prevailing. Excessive
water movement either caused by the fish themselves or due to a strong
water current may wash a great proportion of the feed out of the cages. A
dense wild fish population may even learn how to create the water current
necessary to achieve this. The fish density should be kept below a threshold
value, above which the FCR increases. For S. niloticus in 1 m3 cages this was
400 fish/m3 (Coche 1977).
With protein-rich compounded feeds, good feeding efficiencies in tilapia
cage culture are usually demonstrated by FCR's close to or lower than 2.
FCR's between 1.0 and 2.0 are now attained by Campbell (pers. comm.) in
the Ivory Coast with 25% protein sinking pellets and S. niloticus. With 36%
protein floating pellets (about 10% moisture), FCR's below 1.0 have been
attained with S. aureus in Puerto Rico (Jordan and Pagh 1973), probably in
the presence also of additional natural food.
The presence of such additional food in the environment may, however,
have a negative rather than a positive effect on the efficiency of utilization
of the artificial feed. In the fertilized Alabama ponds (Table 12), 40%
protein floating pellets fed at DFR 3% B to caged S. aureus gave a net FCR
of 1.1 to 1.5 in the presence of moderate plankton blooms, but with dense
blooms the net FCR rose to 3.3 to 6.3 (Armbrester 1972). As far as possible,
one should, therefore, also take into account the natural productivity of the
environment when fixing the DFR.
Some data for the intensive cage culture of S. aureus are summarized in
Table 12. The fish densities used were usually high (above 400/m3) and
relatively small fish were harvested (130 g or less) because of the small sizes
stocked (3 to 10 g) and/or the short culture period (2.5 to 3.0 months).
Growth rates and production were definitely higher in fertile fish ponds
than in the rock quarry pond (Table 12D) although the feeding efficiency
was best in the latter. It is worth noting that the FCR's were below 2.0,
for all the examples given except in heavily fertilized ponds with dense
algal blooms (Secchi disc depth visibility, 37 cm) and with 2.9 g fry as
stocking material (Table 12A).
Table 13 summarizes data on the intensive cage culture of S. niloticus.
The data for 20 m3 cages (Campbell 1978b) are the first on intensive tilapia
cage culture at the artisanal level. In all cases except the Philippine work
(Guerrero 1979a, 1980a) sinking pellets were fed with a medium (20%) to
high (40% or over) protein level mostly at DFR 4 to 6% B. The fish densities
were generally low to medium: below 300 fish/m3. In the Ivory Coast, a
particular effort was made to produce commercial size fishes, averaging at
least 200 g, which called for culture periods in the growout cages of 4 to 5
months. Some brief notes follow concerning each set of trials (Table 13).