fingerlings was made at the end of the first month and a second one month
later. Such selections based on growth rate resulted in populations which
were on average 84% males. The slower growing fry, mostly females, were
discarded after two months. Up to 50 kg of fingerlings was produced per
1 m3 cage.
On the basis of this experience Campbell (1978a) has suggested some
improvements. The use of smaller mesh (4 mm) cages would allow stocking
with 10 to 15 day-old fry, about 1.5 cm long. This would make it possible
to remove fry earlier from the ponds and increase their efficiency. With such
small mesh, however, the water exchange inside the cages would be greatly
reduced. Therefore, smaller cages should be used (0.5 m3) and the stocking
rate should be reduced accordingly. As soon as the fry reach 3 to 4 cm, they
should be transferred to 1 m3 cages with 8 mm mesh as above.
Some experiments towards this were performed in the Philippines (Guer-
rero 1980a) but at very low fish densities (100 fish/m3). Fry of mean weight
2.6 g were stocked at 0.26 kg/m3. They were fed daily for 56 days at 5% of
the biomass with a mixture of fish meal and rice bran in the form of a mash.
The mean weight at harvest was 15 to 20 g. The best feeding efficiency
(FCR=1.7) was obtained with 25% fish meal. Similar results could probably
have been obtained with higher densities.
Mass production of fry and fingerlings of S. niloticus and S. melanotheron
(T. heudelotii) is now being attempted in slightly brackish (4 to 8%) water
in Jacqueville (Ivory Coast) on the western Ebrie Lagoon (Campbell pers.
comm.). The main characteristics of the culture system, which is still in
the developing stage, are summarized here.
Four 3 x 18.3 m tanks are used for spawning. These have a water depth
of 30 to 40 cm. The water supply (26 to 32°C) is sufficient to give a total
exchange every 6 hours. Aeration equipment and automatic feeders are used.
Examples of stocking rates for broodstock are as follows: S. niloticus, 4.4 to
6.5 200-400 g fish/m2 with sex ratio 1:5; S. melanotheron (T, heudelotii),
14.5 150-200 g fish/m2 with sex ratio 1: 1.
Every day at noon, about 1,250 to 2,500 fry (still in cloud formation)
are harvested from the lower part of each tank and transferred to a plastic
tank. The four tanks produce on average 5,000 to 10,000 fry/day.
The fry are grown on in 4 x 4 m plastic tanks with a lateral water inlet
and central drain. The water depth is gradually increased as the fry grow
over a 2-month period. Intensive hand feeding is first employed followed by
automatic feeding. The water exchange rate is high. The fry harvest from
one week (35,000 to 70,000) is concentrated at first into one tank. As they
grow, they are regularly graded by size and transferred to other tanks.
The stocking density therefore gradually decreases. Regular prophylactic
treatment is given against parasites. The normal survival rate is 80 to 90%.
Up to 30,000 fry, mean weight 2 to 3 g, can be grown in each tank. These
are then transferred to fingerling production cages.