reduce feed losses through the walls from the turbulence created as the
tilapias feed voraciously.
The water depth in the cages has been shown to influence growth and
reproduction (Maruyama and Ishida 1976). When comparing the growth of
S. mossambicus in water depths ranging from 0.5 to 1.5 m in square cages
of 6 m side, the best growth and the highest fry production were observed
in the deepest cage. A depth of at least 0.75 m was recommended.
For S. niloticus in the Ivory Coast, Campbell (1978a) recommends the
simultaneous use of various sizes of floating cage. For fingerling production,
0.5 and 1.0 m3 cages should be successively used as the fish grow. For
market fish production at the artisanal level 20 m3 cages (3 x 3 x 2.5 m) are
the most suitable. He believes that the maximum cage sizes above which
handling becomes a problem without special equipment are 22.5 m3 (3 x 3 x
2.5 m) for plastic netting and 30 m3 (3.5 x 3.5 x 2.5 m) for synthetic-fibre
netting.- CAGE CONSTRUCTION
Standing cages have a supporting frame extending 0.2 to 0.3 m below the
cage floor to keep it away from the bottom sediment. The mesh walls are
attached to the upper part of this frame. Floating cages are made of two
components. The surface structure consists of a floating rigid frame and the
subsurface structure of either a rigid frame with mesh walls or a flexible
mesh bag designed to retain a rectangular shape. In the presence of strong
water currents (above 20 to 30 cmlsec), a rigid construction is preferred over
a flexible bag with heavy corner anchors. The choice of materials is important
(see below), but the mesh size of the walls remains the most important
factor. This should be as large as possible, according to the size of the fish
being raised, to allow a free circulation of water through the cage at all
times.
Accessory items are used according to each particular situation. For S.
niloticus culture, Campbell (1978a) observed no advantage in using cages
with either a solid bottom or an opaque top cover. A feeding ring is also
unnecessary under normal circumstances. Against bird predation, a light
covering net should be used. If the subsurface structure includes fibre
netting (rather than more resistant plastic or metal netting), additional
protection against aquatic predators may be required either as a second
stronger fibre net with larger meshes added to each cage or a larger anti-
predator net around the culture site.
There are definite advantages in grouping several cages (e.g., four to six)
together with a stable working platform, to form a raft. When poaching is
a major problem, a watchman can be housed on this, as is commonly done
in Asia. As the structure increases in size, however, more attention should be
paid to the design of an effective anchoring system, adapted to the local
water conditions.