COCHE: In relation to the two previous questions, the prey-predator relationship and
sexing, at the pilot commercial fish farm in the Ivory Coast sexing of S. niloticus is done
by hand on 25 g fish with about 10% error. Then to control this error, a small predator
(Hemichromis fasciatus) is used in the ponds without any calculation of prey:predator
ratio. One kilogram of predators is used per 1,000 m2 pond. They remain very small and
can be reused several times. When the pond is drained, the predators are taken out and
transferred to another pond. When some tilapia recruitment is needed, the predators can
be removed and tilapias can be bred in the ponds. This combination of predators and
early hand sexing seems to work well.
HEPHER: May I comment on the response of tilapias to pond fertilization? Fertilization
may be organic or inorganic. Tilapias are much more responsive than other fish to organic
fertilization. This may be because of their feeding habits, for example, consumption of
detritus. A wide variety of different manures are readily divided into small colloidal
particles, for instance, liquid cattle or chicken manure, and even the sludge remaining
after processing manure for biogas. Tilapias are very responsive to all of these. We manure
ponds every day, in some cases in very big quantities, up to 150 to 180 kg dry matter/ha/
day. With respect to feeding, we have found that the feeding rate of tilapias is almost half
that of carps. Using feeding charts, we can feed half the amount of supplemental feed for
the same standing crop of tilapias as for carps.MORIARTY: You are using S. aureus and a niloticus x aureus hybrid. Presumably, S.
aureus feeds on phytoplankton in nature. Are you encouraging blooms of algae in the
ponds?HEPHER: No, more detrital feeding.MORIARTY: Are they really feeding on detritus? Has anyone actually looked at the
contents of the stomach?HEPHER: Yes, Spataru has analyzed the stomach contents of S. aureus and S. galilaeus
from Lake Kinneret. The majority of the contents is detritus. This is probably why they
are so responsive to manures.MORIARTY: The manures will also encourage the algal blooms as well.HEPHER: Yes.MIRES: Our carp ponds always suffered very much from blue-green algae blooms in the
past. Then, with the production of tilapias and with a combination of tilapia and silver
carp, this problem has been totally eliminated. Although what is found today in the
stomach of tilapias may not suggest that they consume much blue-green algae, we have
had the impression in the past that they can clear these algae from the pond.CHERVINSKI: We experimented at Texas A&M University growing Macrobmchium
rosenbergii by itself or in combination with S. aureus. They don't suffer from blooms
of blue-green algae in the tilapia pond, but they do suffer from these when cultured
alone. We also tried culturing S. galilaeus in dirt ponds, but they did not perform well
because they are almost exclusively plankton feeders whereas S. aureus, S. niloticus and
hybrids will bottom-feed ae well.BOWEN: There appear to be two feeding categories amongst the tilapias and sarothero-
dons. Those adapted to feeding on coarse material, like macrophytes, and the micro-
phagous group. The comments made here emphasize the fact that the microphagous
group is highly flexible in its feeding mode. They can feed from suspension or they can
feed from substrates, depending entirely on food availability. Certainly, there are some
species which tend to feed either always from suspension or always from the bottom in
the natural environment, but even in the natural environment, some species are flexible.