Fish Culture in Cages

Both fish pens and fish cages are confinement structures used for rearing fish. The pen, however, is larger; it ranges from 10,000 sq m to more than a square kilometer in contrast to the cage which ranges in size from one square meter to several hundreds of square meter. Also, cage culture is done in at least one-meter water depth or in deeper waters. Thus, this type can either be stationary or a floating cage which can then be established in the sea, lake, cove or river where biophysical factors are favorable. Species of fish that are grown in fish cages are usually expensive and sold live for a certain group of consumers. Demand for live fish exports to Taiwan, Hongkong and China is fast-increasing. Seafoods that are popularly exported alive and grown in cages are grouper, humphead wrasse, lobster and seabass. These species, when cooked alive, command more than triple in prices. Wrasse and groupers are first-class fish species that are believed to also have medicinal properties for sick and recuperating people. 

How to Culture Fish in Cages Site selection 

Fish cages should be installed in suitable areas that are protected from strong waves and currents, free from pollution and accessible to the farmers and market. A minimum depth of one meter is required. 

Fish cages 

There are two common types of floating fish cages: the bamboo frame cage and the nylon net cages with frames. They are both provided with anchors and floats. Fish cage rearing can be done in freshwater and brackishwater areas. 

Bamboo frame fish cages 

They are made of hard bamboo slats tied or nailed to wooden frames. The usual measurements are: 2 m long, 2 m wide and 2 m deep, 3 × 3 × 2 m and 4 × 2 × 1.75 m. They are provided with whole bamboo floats or empty drums at the top side. Net or bamboo top cover with door and lock is provided. 

Advantages 

 · Cages are easy and cheap to construct. · Cages can be operated cooperatively. · Cages are easy to stock and feed · Fish grow fast in cages. · Cages are easy to harvest. 

Net fish cages 

They are made of fine-meshed (0.32-1.27 cm) nylon nets connected to a float frame of whole bamboo with empty drums of plastic or styrofoam to enhance buoyancy. The empty drum is optional for a small-sized net cage. The usual size is 8-10 sq m with 2 m-2.5 m depth. The net cages are provided with concrete weights that also serve as anchors. The cage is also provided with a mooring line to keep it in place, as well as reinforcement bamboo frames to spread the nets. Fish cages can be used for fish fattening or growing. For growing purposes, use fingerlings. Before stocking, weigh and count the fish. As a nursery for fingerlings, the cage should be made of fine mesh net. The size is appropriate for easy handling. There are two cages —the first is a brooder's cage with a 1.255.0 cm mesh size This is placed inside a bigger cage which functions as the confinement cage or as a conditioning cage for fishes, subject to transport. This is commonly used for rearing fingerlings to marketable-sized fish. The net's mesh size depends on the size of fish or fingerling that would be stocked initially. The cage area usually ranges from 1 sq m to 100 sq m or more and the depth ranges from 1.0-2.0 m or more. Cages are subject to fouling organisms. Clean heavily fouled nets as often as necessary to ensure efficient water change. Check the net daily for possible damages to prevent escape of the stocks. Check also the structure (e.g., bamboo structures, ropes, sinkers, floats, etc.). Guarding the fish cages should be done at all times to prevent losses from poaching. Construct a caretaker's hut at the culture site to discourage poachers. 

Rearing of the stock 

 Fish sampling is done at least every month to determine the growth rate and the proper quantity of feed to be given. Expose the fish to outside parameters that may affect their feeding performance which eventually affects their growth rate. During the wet season, water temperature usually drops; thus, decelerating growth rate. In summer, the growth rate is faster. Hence, feeding should be regulated. Practice an addition feeding to determine the actual food needs of the fish, especially before the sampling schedule. Do not feed fish subject for sampling. On the average, depending on the fish species and the kind of feeds, the feeding rate is three to five percent of the body weight. Give commercial feeds at three percent and trash fish at five percent. In the absence of commercial type, an alternative feed may be made at home, suited to fish requirement, e.g., a 70-percent rice bran, mixed with 30-percent fish meal or fine trash fish for Tilapia mosambica (hybrid). Feeding should be done early in the morning and late in the afternoon by equally dividing the feed needed. It is important that feeds are available at all times when using fish cages. Feeding trays may be used to mimize wastage of feeds. Culture period ranges from three to five months. The stocking rate can be 5-20 pieces per square meter. 

Harvesting and marketing 

 Here is one way of harvesting fish: For the net cage, untie the bindings at the corners and sides of the net from the float frame. Insert a bamboo pole at the upper edge of the net cage and push the net along in order to corner the fish at one end. Scoop the fish with hand nets. The fish, if sold live, fetches a higher price. It is, therefore, advisable to place the fish in double plastic bags containing well-oxygenated water. The bags are then placed in styrofoam or burl bag containers. Dead fishes to be sold should be packed in crushed ice at the rate of 1:4 by weight (1 kg ice to 4 kg fish) for nearer markets and 1:1 ratio for more distant markets at a temperature of 0° C which is good only for 24 hours or less. 

Source:IIRR. 1995. Livelihood options for coastal communities. International Institute of Rural Reconstruction, Silang, Cavite, Philippines. Published 1995 by the International institute of Rural Reconstruction Silang, Cavite 4118 Philippines. Printed in the PhilippinesISBN: 0-942717-64-3; photo from www.scotland.gov.uk.