Tilapia Raising in Cages

Tilapia cage culture is growing tilapia in cages made of nylon nettings and bamboo frames that are floated, submerged or fixed at the bottom. It utilizes bodies of water such as dams, rivers, lakes, bays, reservoirs and coves. This is one of the effective technologies used in raising tilapia. It started out in 1974 in Sampaloc Lake and Laguna Bay and being practiced now in different regions like in Magat Dam Reservoir in Region II.

The following are the advantages of tilapia cage culture:

  • easier handling, inventory and harvesting of fish
  • better control of fish population
  • efficient control of fish competitors and predators
  • effective use of fish feeds
  • reduced mortality
  • high stocking rate
  • total harvesting and swift or immediate return of investment
  • less manpower requirement
  • minimum supervision

There are two types of cage design — fixed and floating. The fixed cage is suitable with a water depth of 1 to 5 meters and the usual size is 50 to 200 square meters.

The floating fish cage, on the other hand is from 5 meters deep and about 50 x 25 x 3 cubic meters depending on the area where the fish cage is placed. It is supported at the bottom with a stone weighing 40 to 100 kilos and covered with a net to prevent the fishes to escape.

The success of the project depends on the quality of breeds or species of fish as well as the production capability of the selected site in enhancing the maximum growth of the fish. Tilapia species is widely used as fish stock because it grows fast. It takes only four months for fingerlings to reach an average weight of 100 grams.

Site Selection

Water circulation. The Magat Dam has water current circulation throughout the area that gives a continuous flushing of water inside the cages, making dissolved oxygen highly available to fish and wash out metabolites. Wind direction from northeast to southeast or vice versa prevails in the months of March to August. The prevailing winds augment the distribution of natural fish food within the dam.

Protection from winds and waves. Locate the site in waters protected from strong wind action and water currents caused by flush flood or heavy runoff. In the Magat Dam, there are few floating debris; the quantity increases as the wind changes its direction and force. However, this could be checked by providing floating bamboo barricades or wave breakers facing the direction of the wind.

Dissolved oxygen concentration. The ideal range of dissolved oxygen concentration on the water must be at least 3 ppm (parts per million). For tilapia, a lesser ppm is not considered lethal. However, growth and reproduction is greatly affected.

Temperature. This is one factor that plays a major role in the growth of the fish stock. The suggested range is from 20°C to 30°C. The lethal temperature levels are 12°C and 42°C.

Pollution. The fish-farmer should know the effect of thermal, biological and chemical pollutants to the fish stock which may come from domestic, industrial and agricultural sources. pH Level. To enhance a better growth, the recommended pH range is 6.8 to 8.0.

Accessibility. The site must be accessible to land and water transportation to facilitate bringing in of inputs and marketing of produce.

Other factors. One social problem existing in any fishery establishment is poaching. Poachers get into the project at night, bore hole on nettings to let the stock escape, then set gill net on surroundings of the project. This gives a bountiful catch overnight. This problem can be remedied by the management by establishing good public relation with the people in the vicinity. Another consideration is the source of fingerlings for periodic stocking.

Structural Design and Construction of Cages

The design of fish cages is determined by the behavior of the culture species. For Tilapia nilotica, which is less active and sometimes territorial in habitat, the shape of the cage does not affect its mobility. In this case, design rectangular cages for easy assemblage and management. The arrangement of the cages is not a problem if there are only few of these. However, 8 or more should be arranged depending upon the direction of the wind.

There are many kinds of nets that could be used for cage fabrication. The most common are the B-net (1/4″ mesh), DD-net (3/8″ mesh) and CC-net (1/2# mesh). However, the most popular is the B-net because smaller fingerlings do not need a nursery cage. It is cheaper per unit area because it is wider (108 inches) than other nets, hence, labor cost in fabricating cages is much lower, and tearing of one or two meshes do not easily provide an escape route for bigger fish.

Generally, floating net fish cages are made of nylon nettings supported on all sides and corners with polyethelyne rope fixed by a nylon twine. Each is hung within a rectangular area, the top is supported by bamboo braces and the bottom is provided with lead sinkers. The size of net cages used in Magat Dam for commercial production of tilapia is 6 m deep, 6 m wide and 12 m long. This size makes possible the full utilization of bamboo poles and nets. To do it, hang the net cages in bamboo raft type frame which also serve as catwalk for workers allowing 1 m of the net above the water level and fix the synthetic ropes to four corners of the poles to prevent the fish from escaping by jumping out. The longest side of the cages is oriented perpendicular to the direction of the wind.

Construct the net fish cages in the following manner:

  1. Cut the net according to desired specification
  2. Double-lace every mesh of the four corners using nylon twine 210 d/6, double-twine beginning at the second mesh row using rolling hitch or clove hitch with a single hitch as lock at intervals of 7.62 – 10.16 cm.
  3. Double-lace the nylon salvage net to the top edges of the cage with a nylon twine, using either a rolling hitch or clove hitch with single hitch as lock. Start the second half from the second mesh row.
  4. Rig all sinkers (No. 7) to the rib lines of the bottom side and centers. Attach the rib lines on all sides using rolling or rib hitch with an interval of 7.62 – 10.16 cm.
  5. Make splices on the four corners of the hanging lines (top portion of net cage) for the attachment of four stretching ropes with weight. Continue with the other units following the same procedure.

source: STII and PCARRD, photo from article.wn.com

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