|Technical Guide for Tilapia Farming (CDI, 1998, 51 p.)|
|PART ONE: FISH FARMING NOTIONS|
|E. Technical characteristics of the facilities|
The following information should serve to help the estimation of the technical value of the facilities described when visiting existing fish farms. For further details, it is necessary to refer to the appropriate technical handbooks (see Bibliography).
Earthen ponds are more commonly used in tropical fish farming and represent the oldest fish farming facility. A good pond will show the following characteristics:
· A well designed water supply but also easy drainage. This means a slight slope (0,5 %) of the pond floor from the water inlet to the drainage point.
· The drainage outlet (standpipe or monk) must be studied with care.
· Water inlet and outlet will be at opposite ends in order to ease water exchange within the pond.
· Good impermeability of the pond as a whole and strength/integrity of the pond's walls and edges.
· Access and possibility to work around the pond which must be accessible to vehicles (tractors/trucks & trailers etc.).
· Size of ponds must be adapted to the species but also be easy to manage. If the width exceeds 50 metres, it becomes difficult to pull nets for harvest (e.g. 3 to 4 men on each side are necessary to pull a 60 m. net) rendering the ponds impractical.
Concrete ponds are used for intensive fish farming; concrete walls/banks eliminate erosion due to currents caused by mechanical aeration, waves generated by the wind and fish activity (notably nesting behaviour). This type of pond is more expensive to build and, therefore, should be made profitable by a higher production per volume utilised. Conversely, the firmer walling reduces maintenance and re-building costs that will be necessary after a few years of operation,
This type of pond is smaller than earthen ponds and should not exceed 1,000 m2 surface area. The bottom can also be in concrete but for reasons of construction costs, only if the pond size does not exceed 200 m2. Brick or stone walls must have strong foundations and, if they are built with bricks or blocks, they must be plastered, in order to avoid the effects of erosion.
Raceways are concrete canals that can be partitioned with screens. This type of facility is often used for trout farming but is not as suitable for tilapia that prefers the calmer waters of a pond. One must note that is necessary to aerate the water along the entire length of a raceway because the oxygen can often be totally consumed by the fish that are sited at the beginning of the raceway. The complete drainage of a raceway is often difficult because different batches of fish are grown simultaneously in the different sections and the whole of the raceway must be completely empty of fish.
Raceway, top view
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 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 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.
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.