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Chapter: Aquaculture Principles and Practices: Design and Construction of Aquafarms

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Types of cages and layout of cage farms

It is obviously difficult to describe the various designs of cages presently available. Detailed descriptions of different types of cages are given in Beveridge (1987).

Types of cages and layout of cage farms

 

It is obviously difficult to describe the various designs of cages presently available. Detailed descriptions of different types of cages are given in Beveridge (1987). Although there are submersible and rigid-walled cages in use, the majority consist of a floating unit, a framework and a flexible meshnet suspended under it. There are different methods of flotation and mooring, placement and attachment of individual cages in a farm, means of approach and handling of cages. The floating unit can consist of empty barrels, styrofoam polyethylene pipes, or ready-made pontoons of plastic and metal. The buoy units are often built into a framework, the material of which can be impregnated wood, bamboo spars, galvanized scaffolding or welded aluminium bars. Nylon is commonly used for the net, but weldmesh or even woven split bamboo are also used. Cage flotillas provide safer working conditions and enable storage of feed on site, as well as installation of automatic feeders. The diversity of materials used shows that the design of the cages and cage farms should be based on conditions prevailing at the selected site. 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 site to these 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 water body. Accumulated wastes decompose and cause oxygen depletion or generation of methane or other toxic gases 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, and if the movement is not enough to clear them, provision has to be made for regular mechanical removal with suction or slush pumps and disposal of the waste at safe distances. Though the determination of precise carrying capacity of waterbodies is difficult, at least empirical estimates should be used to avoid overcrowding of cages.

 

Extensive testing of materials for construction of cages, supporting framework, floats, sinkers, walkways, etc. has been carried out. Despite differences in technical efficiency, different types of materials continue to be used depending on availability and cost. The most sophisticated designs appear to be used for sea cage farms, especially in Norway (fig. 6.25) and Scotland. Cage size can be anything up to 1000 m3, but is normally between 100 and 500 m3. A simple unit holds a net of four vertical sides and is rectangular in cross-section, but the more popular ones are circular in cross-section. When timber is used as framework it is


not easy to have a perfectly circular shape and an approximation is achieved with six- or eightsided structures. A commonly used cage in Norway has an eight-sided floating framework of timber, which is impregnated to reduce rotting. The sides are linked together by flexible joints to reduce the rigidity of the structure (fig. 6.26). Planks 12 cm x 5 cm or larger form the sides of the section and are spaced 30 cm apart by wooden slats nailed across the top and bottom. The slats on the top should be positioned close together, as the framework has also to serve as the walkway around the net. Expanded polystyrene or other flotation material is inserted between the two layers of slats and held in place by nails driven through the timber. The joint between two sections of the frame is formed by bolting together strips of heavy rubberized machine belts. For further safety, all the sections are securely fastened together by suitable nylon rope or reinforcedplastic piping. In every alternate corner of the frame a loop is provided to attach the anchoring devices. Inside the collar, four 120 cm long laths are nailed to each of the eight units. A nylon net is stretched between the laths to prevent leaping fish from escaping.

 

Another common type of cage system used in Norway employs rectangular cages suspended from a rectangular float (as in fig. 6.25). The float consists of four PVC pontoons in iron frames. The two frames, made of 25 mm galvanized iron tubes, are connected to a wooden frame made of 5 cm x 10 cm impregnated planks with galvanized bolts and nuts. The wood frame is made in two sizes (4 m x 1 m and 5 m x 1 m), depending on the length of the two types of elements. On top, the wooden frames are covered by 2.5 cm x 12.7 cm impregnated planks. The elements of the two different lengths are joined together to form a rectangular float with a network of 4 m x 4 m square openings. The bag net, equipped with headline and leadline, is fixed to the rafts with brass hooks screwed into the wooden frame. Fence poles are erected from pedestals. The raft is usually moored by its four corners and can easily be towed.


When welded tubular metal or PVC or fibre-glass tubing is used for the framework, there is greater flexibility in shapes and sizes of cages (fig. 6.27). Besides cost and safety of the structures, a major consideration in designing cages is the ease with which they can be handled.

Obviously, large cages, though cheaper to buy and install, are not very convenient in this respect and would need a larger labour force or special mechanical equipment to handle. Cages with an underwater net volume of more than 1000 m3 are not recommended; the preferred size is between 200 and 500 m3. It is common practice to have double netting: the outer one serving as a predator net to protect the inner one and the fish stock in it.

 

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 costs (fig. 6.28). 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 (fig. 6.29). In either case, the cages should be installed on the sides of a central walkway to facilitate day-to-day work on the farm. In many modern cage farms, 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. Cages should be attached to the mooring by chain or, for lighter structures, by nylon rope.

 

While the arrangement of cages in a battery is the most common practice, in cases where infection of diseases is feared they may be moored separately. Workers then use boats to attend to feeding and the care of the cages and fish stock.

Most of the presently available cages are designed for use in protected areas like bays, fjords and lakes. In order to utilize more open waters and high seas, special cages with a flexible rubber framework have recently been developed. Some of these designs have twin rubber booms for increased stability in rough seas. It is claimed that the low weight of the cage reduces strain on the moorings and contributes to all-weather capability and high durability. Other types of very large cages made of high-strength galvanized steel, fitted with all accessories including even independent power supply, have been tried and are claimed to be capable of withstanding very severe storms. If such cages prove technically and economically successful, an enormous expansion of unpolluted potential sites for cage farming can be expected.


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