Tilapia Farming in Cages

In public waters, cage culture faces many competing interest and its legal status is not well defined. Not all bodies of water offer proper conditions for cage culture.

Design and construction

Both floating surface cages and standing surface cages are used for tilapia culture. Standing cages are tied to stakes driven into the bottom substrate, whereas floating cages require a flotation device to stay at the surface. Flotation can be provided by metal or plastic drums, sealed PVC pipe, or styrofoam.

Cages should be constructed from materials that are durable, light- weight and inexpensive, such as galvanized and plastic coated welded wire mesh, plastic netting and nylon netting. Welded wire mesh is durable, rigid, more resistant to biological fouling, and easier to clean than flexible material, but is relatively heavy and cumbersome. Plastic net- ting is durable, semi-rigid, light- weight and less expensive than wire mesh. Cages made of nylon netting are not subject to the size constraints imposed by other construction materials. Nylon mesh is inexpensive, moderately durable, lightweight and easy to handle. Nylon is susceptible to damage from predators such as turtles, otters, crocodiles and crabs. An additional cage of larger mesh and stronger twine may be needed around nylon cages.

Mesh size has a significant impact on production. Mesh sizes for tilapia cages should be at least 1/2 inch, but 3/4 inch is preferred. These mesh sizes provide adequate open space for good water circulation through the cage to renew the oxygen supply and remove waste. The use of large mesh size requires a larger fingerling size to prevent gill entanglement or escape. For example, a 3/4-inch plastic mesh will retain 9-gram tilapia finger- lings while a l-inch mesh requires a fingerling weighing at least 25 grams with plastic netting and 50 to 70  grams with nylon netting. Larger mesh size facilitates the entry of wild fish into the cage. These fish will grow too large to swim out of the cage, but they do not grow large enough to reach marketable size, thereby representing a waste of feed.

Cage size may vary from 1 to more than 1,000 cubic meters. As cage size increases, costs per unit volume decrease, but production per unit volume also decreases, resulting from a reduction in the rate of water exchange.

Cages should be equipped with covers to prevent fish losses from jumping or bird predation.  Covers are often eliminated on large nylon cages if the top edges of the cage walls are supported 1 to 2 feet above the water surface.

Feeding rings are usually used in smaller cages to retain floating feed and prevent wastage. The rings consist of small-mesh (1/8 inch or less) screens suspended to a depth of 18 inches or more. Feeding rings should enclose only a portion of the surface area because rings surrounding the entire cage perimeter may reduce water movement through the cage.

However, feeding rings that are too small will allow the more aggressive fish to control access to the feed. If sinking feed is used, small cages may require a feed tray to minimize loss. These rectangular trays can be made of galvanized sheet metal or mesh (1/8 inch; galvanized or plastic) and are suspended from the cover to a depth of 6 to 18 inches.

Site selection and placement of cages

Large bodies of water tend to be better suited for cage culture than small ponds because the water quality is generally more stable and affected less by fish waste. Exceptions are eutrophic waters rich in nutrients and organic matter. Small (1 to 5 acre) ponds can be used for cage culture, but provisions for water exchange or emergency aeration may be required.

Cages should be placed where water currents are greatest, usually to the windward side. Calm, stagnant areas should be avoided. However, areas with rough water and strong currents also present problems.