Some experimental results are also available from Laguna de Bay (Pantas-
tico and Baldia 1979) for S. mossambicus. Three floating cages of about
9 m3 capacity each (5.4 mm mesh) were stocked with fingerlings of mean
weight 10 g at 50 fish/m3 and grown for 3 months. The results were as
follows: average growth rate of individual fish, 0.36 g/d; average monthly
specific growth as a percentage of the original weight at the start of the
month, 110%; average monthly production, 0.825 kg/m3; daily rate of
increase of the biomass,* 1.31%. Growth and production were relatively low
when compared to those obtained in fertilized ponds with S. aureus (Table 5).
- CAGE CULTURE IN EFFLUENTS, FERTILIZED PONDS AND CANALS
In Tihange (Belgium), a series of experiments were conducted in a pond
fed by the heated effluent of a nuclear power plant (25 to 37OC) to quantify
the monosex production of male S. niloticus in cages (Philippart et al. 1979).
Populations of 200 males/m3 were reared in floating 0.5 m3 cages for 15
days. Three size classes of fish were used, giving a range of initial biomass (Bi)
of 5.1 to 10.4 kg/m3. The main results are given in Table 6. The quantities
of plankton available were not recorded but the water exchange rate was
high and it may be assumed that there were no blooms. The food supply was
therefore probably inadequate and the results were rather poor, considering
that only males were used: see Table 5 for comparison. These data can,
however, be taken as a basis for obtaining true FCR's of trials using artificial
diets by subtracting the production due to natural feeding.
In Alabama, U.S.A., S. aureus were cultured in experimental floating cages
in ponds in which other fish were also cultivated. These ponds were fertilized
regularly to develop either moderate or dense plankton blooms: average
Secchi disc visibility depths were 64 and 37 cm. Table 5 summarizes the data.
Dense plankton blooms produce much higher fish biomasses, especially with
the small fish. For the larger fish, it appears that the MCC of the cages might
have been exceeded (where Bi = 12.5 kg/m3) which would explain the
reduced MSG and DRIB values (compared to where Bi = 5 kg/m3) but the
Pmi difference also has an effect. The smaller the fish, the greater the benefit
they are likely to derive from an algal diet. All these results defnonstrate the
real potential of S. aureus cage culture in the presence of plankton blooms.
In West Java (Indonesia), common carp (Cypn'nus carpio) are cultured
in fixed cages placed in streams and canals heavily enriched by domestic
and agro-industrial effluents on a commercial scale. This technology is
*The daily rate of increase of the biomass is given the acronym DRIB. The compound
interest formula describing this is BT = Bi (1 + i)", where Bf is the biomass at harvest;
Bi is the initial biomass at stocking; n is the culture period in days and i = DRIB.
100