4. Floating cages

There are many possibilities for obtaining floating cages for growing fish, from locally-made ones (usually cheaper but more fragile) to high technology cages (more expensive and more robust) which are usually used in exposed sites. The selection can only be made according to the overall practical considerations concerning the site, the production targets, and the project's economics.

While the optimal size for a tilapia cage is unknown, one should consider that the bigger the cage, the cheaper it costs in both material and equipment elements. Choice will be determined by considering the ease of management and the final production cost.

It appears that a cage of a dimension of 100 m² is a good compromise. Concerning the depth of the nets, the. technical possibilities and maintenance expertise will determine the choice. If the net is deep, special diving equipment will be required for daily checking of the nets and eventual repairs. If the depth does not exceed 3 to 3,5 m., repairs & maintenance can be made through free diving.

Materials used for the floating structure.

1. Locally made cages. The floating structure of cages can be made out of metal or polyethylene floats, on top of which some walkways will be fixed, in order to obtain a square floating structure from which nets will be hung. The size of the cage will be adapted according to the material's strength and the physical constraints faced (waves, storms, and wind).

6. Industrial cages. With the development of salmon and other marine species farming, equipment manufacturers have developed a large range of cages made from different materials and components. High-density polyethylene is often used for the retaining structure, this material offering the advantages of strength and longevity. Cage structures can be square or circular and of many different sizes. Circular cages are invariably cheaper to buy (relation between diameter and surface) while square cages are easy to assemble and facilitate work and fish handling tasks.

7. Finally, some cages are made from assembled polyethylene cubes, easy to mount and adaptable to different sizes and forms.

Nets.

Nets are; the subject of very careful selection. Their quality must be beyond reproach since the security of the fish stocks depends largely on the net strength. Mesh size must be adapted to the fish size and. must not allow the fish to trap itself by the gills. On the other hand, small size mesh is subject to fouling which restricts the water flow through the cage. Nets are sold by weight and, therefore, those with a small size mesh are more expensive.

Large farms will also invest in net-washing equipment, which imposes the need to have cages with adequate net-removal facilities.

Predation

Predation risks must also be studied, where these can be birds or aquatic animals. Dependent on the predator species, a physical barrier may need to be installed to avoid contact with the 'rearing' net. Strong nylon nets can be fixed at a distance of about 50 cm. from the rearing net. For birds, light nets (large mesh) are stretched above the water surface.

Mooring systems.

Good care must be paid for the mooring (anchoring) system. This will be selected following the speed of the current, winds and any effect that could move the cage and require a special study. Note that mooring systems for square cages are cheaper than those for round cages.