The Biology and Culture of Tilapias

Surface standing cages, resting on the bottom, are used in shallow water
bodies such as ponds and streams. Floating cages are preferable, however,
wherever the water depth permits such as in lakes and rivers. In all such
cases, the floor of the cage should be kept at least 0.5 to 1.0 m above the
bottom sediment, where wastes may accumulate and dissolved oxygen (DO)
is lowered. A water depth of 5 to 10 m is recommended to reduce parasitism
and disease outbreaks.
The size of cages varies for different operations. Breeding cages and
fingerling production cages are smaller than growout cages. Experimental
cages do not generally exceed a few cubic metres until the pilot-scale stage is
reached. At the subsistence level relatively small cages are also preferred.
For commercial exploitation, medium-sized cages (6 to 20 m3) should first
be used at the artisanal level while larger cages (50 to 100 m3) may be
envisaged for the industrial level. Very large cages (1,000 m3 or more)
have also been used (Table 2).

Table 2. Construction costs for tilapia cages in the Philippines and Ivory Coast, 1976 to
1978.

(a) Philippines, 1978 (Guerrero 197%): us$

1) Experimental cage, capacity^1 m3, wooden frame with poly-

ethylene netting of 25 mm mesh using styrofoam floats 10

2) Commercial cage, capacity 6,250 m3 (50 x 25 x^5 m), bamboo

and wood with nylon netting bag of 12.7 mm mesh 2,000

(b) Ivory Coast,^1976 (De Kimpe 1978):

1) Experimental cage, capacity 20 m3, wooden surface structure

with nylon netting bag of 14 mm mesh and 200 1 metal drums

as floats 185

(c) Ivory Coast,^1978 (Campbell 1978a):

1) Experimental cage, capacity^1 m3, wooden frame with plastic

netting of 8 mm mesh and stymfoam floats 55

2) Experimental cage, capacity 6 m3, floating wooden frame with

plastic netting bag of 25 mm mesh and 20 1 plastic barrels as

floats 100

3) Experimental cage, capacity 20 m3, floating wooden frame with

nylon netting bag of 14 nun mesh in 210118 twine mounted

33 per hundred meshes and 60 l plastic barrels as floats 170

The size chosen for cages should reflect the level of technology available.

In principle very large cages can result in the loss of several inherent advan-

tages of cage culture, mainly flexibility and maneuverability. With tilapias

however, a relatively large cage environment results in better growth rates, in

reduced feed losses and in improved survival at very low DO'S (Campbell

1978a). It seems that there is also a minimum cage size for guaranteeing a

good feed conversion ratio (FCR). Cages have to be sufficiently large to