Lovshin et al. (1977) experimented with a duoculture of 8,96O/ha all-
male hybrid tilapia (S. niloticus x S. hornorum) and 1,785/ha common carp.
They found that the total yield was somewhat lower than that for tilapia
hybrid monoculture. However, the feed added to the duoculture pond was
much lower than that added to the tilapia monoculture pond.
In ponds with very high stocking densities, oxygen usually becomes a
limiting factor. Aeration is therefore mandatory, at least during the night.
Other drawbacks of the system are the accumulation of metabolites in the
water and organic matter in the bottom mud. The latter causes a reduction
in the redox potential and the appearance of H2S which is toxic to fish and
fish food organisms. These two facto-rs curtail the growing season to about
100 days, after which &e water has to be changed.
In densely-stocked ponds, fish are fed protein-rich pellets which seem to
be sufficient to sustain their growth at 1.5 to 2 g/d. In spite of the high
density, the FCR usually remains relatively low. Table 5 presents the yield in
two ponds under such a culture system.
Inorganic Fertilization and ManuringThe tilapia species dealt with here are all microphagous: feeding either on
plankton or detritus. When reared at low densities they can obtain a major
part of their nutritional requirements from natural sources. Increasing the
production of this natural food by inorganic fertilization and/or manuring,
when coupled with increased stocking density, usually results in a con-
siderable increase in yield. Van der Lingen (1959a) found that the yield of S.
mossambicus could be increased 2.5 times over that from natural production
alone by means of inorganic fertilization. George (1975) conducted an
experiment on the effect of fertilizers on the yield of S. niloticus in ponds of
0.02 and 0.15 ha. Although no replications were carried out for treatments,
the effect of both inorganic fertilizers (triple superphosphate) and organic
manures (cow and chicken) were clearly demonstrated. Superphosphate
increased production 3.4-fold, cow manure 1.7-fold and poultry manure
3.3-fold over natural production. The highest yield was obtained using a
combination of superphosphate and cow manure, which increased the yield
5-fold.
Organic manures stimulate food production in a different way to inorganic
fertilizers. They add detritus which stimulates the heterotrophic food chains,
producing more bacteria and zooplankton (Schroeder 1978). The micro-
phagous tilapias are very responsive to such treatment and yields can be
increased considerably. Chimits (1955) describes a method of rearing of S.
mossambicus in Thailand where considerable amounts of manure (pig,
cow and chicken) were added to ponds. The annual yield reported from
these ponds was about 4 t/ha. Moav et al. (1977) experimented with the
effect of liquid cow manure on fish in polycultures which included tilapia.
They found that high yields of tilapia could be obtained by manuring, even
at high stocking densities, thus replacing supplementary feed to a large
extent. Feeding tilapia under these conditions did not have much additional
effect on their growth rate. Table 6 gives the relevant results.