Submersible cages and cage maintenance
One of the advantages of a cage farm is that it can generally be towed away to a different location for harvesting or if unfavourable weather or other environmental conditions occur. In areas subject to typhoons or cyclones, sub-mersible types of cages can be useful. Such cages are used in Japan for yellowtail rearing. They can withstand wind and waves much better than floating cages and can often also be used in open sea areas. The headropes of the cages, constructed on more or less the same design as floating cages, are attached to taut mooring ropes suspended from plastic surface buoys. Under normal conditions, the mooring ropes will be only about 2–5 m from the surface, but in rough weather the ropes are dropped to 10m or more. By attaching weights of about 10 kg at each corner of the cage bottom, the shape of the cage is maintained. Additional weights may be added if the currents are too strong (Fujiya, 1979). The net is raised to the surface at feeding time, and the feeds conveyed through the feeding passage attached to the top of the net. A more sophisticated version of this type of cage uses variable buoyancy synthetic rubber floats that can be filled or emptied with compressed air or sea water from the surface.
A different type of submersible cage has been designed for use in hurricane-affected seas in the Caribbean. It is a spindle-shaped collapsible net cage held in position by circular PVC rings of different diameters (the largest rings in the middle and progressively smaller rings towards the ends) (figs 6.30 and 6.31). It looks very much like an enlarged and modified version of a fyke net. There are funnel-shaped pockets through which fish in the cage can be fed. Under normal weather conditions, the cage floats with its top above the surface, but when there is a hurricane warning it can be sunk to the bottom by increasing the weights and removing the floats. When the hurricane has passed, the cage can be raised again by removing the extra weights and replacing the floats. The spindle shape helps in rotating the cage andexposing the submerged parts to the sun for drying and removal of fouling organisms on the net.
Two of the major problems for cage farms are fouling of cage materials, particularly nets and mesh, and susceptibility to easy poaching. Fouling makes the net heavier and prevents easy exchange of water. Antifouling coating, which does not harm the fish, is a solution, but the most practical way at present is a regular change of nets. Clean nets are installed at regular intervals (the frequency of change will depend on the rate of fouling at the site) and the old nets cleaned and dried for further use if they are sufficiently strong. Generally, the economic life of nets in cages is two to three years, depending on local conditions. Constant watch has to be kept on deterioration in the frame-work and other structures of the farm, and repairs or replacements have to be in sufficient time to avoid unnecessary risks. Besides fouling, cage farms have to be protected fromfloating debris, rotten tree branches and drift wood. Unless these are exceptionally heavy, no damage should be done to the strong frame-works. However, they can tear the nets some-times and any such damage should be repaired immediately. Even though floating booms are sometimes erected with old netting slung beneath them to protect the cage, they can hinder routine operations.
There are different types of alarms that can be used to warn against poachers, but there appears to be no better means of looking after a cage farm than by having the owner or a watchman on the spot, possibly with the help of trained dogs. In cage farms in Singapore, there is a floating house for the operator attached to the cage complex and a good number of guard dogs, for round-the-clock watch and constant care of the cages.
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