Coastal Aquaculture and Mariculture (2+1)

Unit 3 - Methods of coastal aquaculture and mariculture

3.2. Cages

• Rearing fish in cages is a traditional practice in some Asian countries. It appears to have originated in Kampuchea two centuries ago. It has spread to other places in more advanced form recently.
• Salmons in Norway, yellow tail in Japan and groupers and sea bass in Hong Kong, Vietnam and Singapore are grown in cage farms.
• Cages offer great opportunities for aquaculture in open waters of the seas. In recent years cage aquaculture has become a major source of aquaculture production, particularly of high value fish like salmon, trout, sea bass and groupers. Several types and designs of cages and cage farms have been developed and commercially used.
• Majority of the cages consist of a floating unit, a framework and a flexible mesh net suspend under it. The floating unit can consist of empty barrels, Styrofoam polyethylene pipes, or ready made pontoons of plastic or metal.
• The buoy units are often built into a framework. The framework can be made of impregnated wood, bamboo spars, galvanized scaffolding or welded aluminum bars. Nylon is commonly used for net, but weld-mesh or even woven split bamboo is also used. Cage flotillas provide safer working conditions and enable storage of feed on site, as well as installation of automatic feeders.
• Reasonably sheltered areas, with sufficient water movement to effect adequate mixing and aeration are selected as sites for cage farms.
• The occurrence of typhoons, hurricanes and cyclones in the area and the vulnerability of the sites are also major considerations in the design of cage farms. Polluted sites are generally avoided.
• In cold climates, areas that receive safe heated water effluents are preferred, as higher water temperatures generally improve growth and productivity.
• Unused feed and fish faeces fall from the bottom of floating net cages on to the floor of the waters bodies. Accumulated wastes decompose and cause oxygen depletion or generation of methane or other toxic gasses under anaerobic conditions.
• Cages also increase deposition of silt on the bottom of the site. It is therefore necessary to have enough movement of clean water below the floating cages. Carrying capacities of the areas should be determined to avoid overcrowding of cages.
• The most sophisticated design of cages is use in cage farms of Norway where salmon are grown extensively. Cages of size 100 to 500 m3 are used. A simple unit holds a net of four vertical sides and is rectangular in cross section. The more popular ones are circular in cross section.
• There are many ways of arranging cages in a cage farm. Where possible, it is preferable to moor cages to a jetty with direct access to a quay, in order to facilitate work and reduce labour cost.
• However, environmental and site conditions may require them to be located farther away from the coast, in which case a work boat will be needed for access. Cages should be installed on the sides of a central walkway to facilitate day to day work on the farm.
• Feed dispensers are installed above each cage; in others, manual feeding is done. Mooring blocks have to be sufficiently heavy and are usually made of concrete with heavy galvanized bolts.
• Most of the presently available cages are designed for use in protected bays and fjords. In order to utilize more open waters and high seas, special cages with a flexible rubber framework have recently been developed.