Authors: Andrew S. McGinty (University of Puerto Rico) and James E. Rakocy (University of the Virgin Islands)
Cage culture, the practice of rearing fish in cages, can be applied in existing bodies of water that cannot be drained or seined and would otherwise not be suitable for aquaculture. These include lakes, large reservoirs, farm ponds, rivers, cooling water discharge canals, estuaries and coastal embayments. In the southern U. S., tilapia are among the most suitable fishes for cage culture.
The most appropriate species or strains of tilapia for cage culture are Tilapia nilotica (Nile Tilapia), T. aurea (blue Tilapia), Florida red tilapia, Taiwan red tilapia, and hybrids between these species and strains. The choice of a species for culture depends mainly on availability, legal status, growth rate and cold tolerance. Many states prohibit the culture of certain species. Unfortunately, T. nilotica, which has the fastest growth rate, is frequently restricted. The ranking for growth rate of the remaining species or strains are Florida red tilapia > Taiwan red tilapia > T. aurea. Hybrids of T. nilotica x Taiwan red tilapia grow as fast as T. nilotica. Hybrids of T. aurea x Florida red tilapia grow at an intermediate rate between Florida and Taiwan red tilapia. Cold tolerance, important in northerly latitudes, is greatest in T.aurea.
Tilapia can be cultured at high densities in mesh cages that maintain free circulation of water. Cage culture offers several important advantages. The breeding cycle of tilapia is disrupted in cages, and therefore mixed-sex populations can be reared in cages without the problems of recruitment and stunting, which are major constraints in pond culture. Eggs fall through the cage bottom or do not develop if they are fertilized. (Reproduction will occur in cages with 1/10-inch mesh or less, which is small enough to retain eggs.)
Other cage advantages include:
- flexibility of management,
- ease and low cost of harvesting,
- close observation of fish feeding response and health,
- ease and economical treatment of parasites and diseases,
- and relatively low capital investment compared to ponds and raceways.
Some disadvantages are:
- risk of 1oss from poaching or damage to cages from predators or storms,
- less tolerance of fish to poor water quality,
- dependence on nutritionally-complete diets,
- and greater risk of disease outbreaks.
In public waters, cage culture faces many competing interests 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, lightweight 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 netting is durable, semi-rigid, lightweight 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, lightweightand easy to handle. Nylon is susceptible to damage from predators such as turtles, otters, alligators 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 fingerlings 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.
Continue to Cage Culture of Tilapia (part 2)