- MATERIALS AND THE WORKING LIFE OF CAGES
The choice of materials for the construction of cages varies greatly from
country to country. Local materials such as wood and bamboo may be
used, but generally their working life is short when continuously submerged.
Boring insects such as Povilla adusta in the Ivory Coast (Coche 1979) readily
attack light wooden frames. Mahogany frames are more resistant but rather
heavy. Some bamboos are more resistant to attack than others.
For the subsurface structure, there is a tendency to eliminate supports and
to use a net bag either of synthetic fibre or plastic.
Fibre netting with a nylon twine size R 470 tex (e.g., 210118) has been
successfully used in the Ivory Coast, mounted on nylon ropes at 33 per
hundred meshes and spread at the bottom by a steel frame (Campbell
1978a). Knotless netting with square rather than diamond-shape meshes is
preferred. In brackishwater, considerable damage may be experienced due
to crabs. Chua and Teng (1977) recommend in such a case an R 1150 to
1300 tex polyethylene netting (21 to 24-ply threads). Compared to nylon
netting such material is not only much cheaper but it also appears to be
able to better withstand the tropical sun for a considerable period of time.
Treating the netting with tar may also increase its resistance and reduce
fouling (Coche 1979).
Plastic netting combines the advantages of being light and durable with
some extra rigidity. It should, however, contain ultra-violet stabilizers for
longer-lasting, performance. Its only drawback might be its price, especially
in countries where it has to be imported. Even then, careful consideration
should be given to plastic netting because of its inherent advantages, partic-
ularly durability.
The working life of cages and their depreciation period vary greatly
with the materials used for their construction and the local conditions, e.g.,
climate, limnology, handling, maintenance, etc. Under careful management
the components of floating cages used in Lake Kossou (Ivory Coast) had the
following estimated working life (Campbell 1978a): surface floating frame,
(ordinary wood, 6 x 6 cm) 5 years; floats, (empty plastic barrels, 2Q to 60 1)
3 years; subsurface wooden frames (mahogany, 5 x 5 cm) 3 years; subsurface
wooden frames (ordinary wood) 1 year; nylon fibre netting (210118 twine,
14 mm mesh) 3 to 5 years and plastic netting (8 and 25 mm mesh, Netlon) 5
to 10 years.
The prices of some cages being used for tilapia culture illustrate the mag-
nitude of the initial investment to be made (Table 2). The average cost/m3
varies from US$55 to US$8.50 according to the size of the cage md the
material used. The larger the size, the cheaper the unit volume cost but
also the lower the recommended (see below) fish density. The average fish
production/m3 therefore decreases as the cage volume increases.
Campbell (1978a) therefore recommends the following cage dimensions
for artisanal tilapia culture in the- Ivory Coast: for 15 to 30 g"finger1ings