Bivalve molluscs like mussels and oysters are good sources of cheap and high quality protein, and their shells are used for ornaments and other industrial products. They are ideal species to culture because of their low position in the food chain. Farming of bivalves can also increase the income of small-scale fishermen faced with dwindling catches, as well as provide livelihood for unemployed people in coastal areas.
In the cultivation of bivalve molluscs, a suitable culture site must have the following characteristics: (a) sufficient breeding stock to insure adequate spatfall; (b) protected from prolonged flooding, strong winds and waves; (c) high natural productivity of the water; (d) moderate tidal current flow for transport of phytoplankton and oxygen and elimination of wastes; (e) physio-chemical conditions of the area must be suitable for growth and survival, especially salinity and temperature; and (f) must be free from industrial wastes, sewage and other pollutants (Fujiya, 1970; PCARRD, 1983).
2.0 CULTURE METHODS
The basic methods of farming bivalve molluscs are the bottom and the off-bottom culture methods. The bottom culture system is also called the broadcast or the bottom sewing technique. For the off-bottom culture system, this includes the stake or pole method, rack, raft and long-line method. The rack, raft and long-line method are also called the hanging or suspended culture technique. The stake and rack method are mainly used in shallow, intertidal waters while the raft and long-line methods are generally utilized in deeper, open waters. In the following discussion, the culture procedures practiced in the different countries will be described. Also, some data on the economics of the different farming methods will be presented.
2.1 Bottom culture
In the Philippines, the simplest way of farming oyster is the bottom culture technique. Numerous collectors like oyster shells, stones, gravel, tin cans, or any hard object are scattered over bottoms where spatfall is known to occur. The oyster beds are sandy or rocky, non-shifting and usually near the farmer's house. In some places, a mat made of bamboo splits tied together is used as a spat collector. The principal advantage of this method is the minimal amount of investment. A disadvantage, however, is that the collectors in the bottom can be washed away by strong currents or the cultured oysters can be buried under mud or sediments (PCARRD, 1983).
The oysters, Crassostrea virginica, and the clams, Mercenaria mercenaria and Mya arenaria are also cultured on the bottom in the United States of America (Castagna, 1970; Milne, 1979). The culture sites have a firm bottom and a moderate water current flow. In the oyster beds in Long Island, New York, siltation is one of the major problems facing the oyster farmers. Silt is removed by regular suction dredging and through flushing with water jets operated from work boats. The effect of siltation is also minimized by transplanting the stock once or twice during the culture period. For clam culture in Virginia, loses are incurred due to predation by crabs and starfish. Clams are protected through the use of screened boxes and trays, fenced enclosures and net coverings. Another method is by spreading a layer of shell, gravel or aggregate 25–75 mm in height. The clam seed eventually burrows through the layer which protects it from large predators.
Bottom culture of mussels, Mytilus edulis, has attained a high degree of mechanization in Holland. Farmers are allocated areas in the sea bed of the Waddenzee. These culture plots have a depth ranging from 3–6 meters. Prior to transplanting, unwanted predators are removed from the plots using a special roller dredge. Juvenile mussels, 8– 13 mm in length, are dredged from public grounds and spread over the private mussel beds where subsequent thinning is done to ensure better growth. The marketable size of 7 cm shell length is reached after 20 months of culture. Mussels are also transferred to the Rhine estuary to clean themselves of ingested silt and stored until market prices become favorable. A Dutch mussel farmer can produce 10 tons of live mussels per hectare per year (Hulburt and Hulburt, 1980).
2.2 Pole or stake culture
In France the mussel, Mytilus edulis, are grown using oak poles (bouchot culture). Poles, 3 m long and 20 cm in diameter, are driven into the sea bed with ½–2 m exposed above the ground. The bottom 30 cm of the exposed portion is wrapped with smooth plastic to minimize predation by starfish and crabs. The poles are spaced 1 m apart and arranged in rows with 3 m distance between rows. Seed mussels are collected in ropes placed in natural mussel beds and then wrapped around the oak poles. The mussels grow rapidly and fill the poles to several layers thick. The farmer resorts to periodical thinning of the stock and small specimens are placed in plastic net tubes, 2 m long and 15 cm in diameter. These flexible net tubes are wrapped around bare poles to start the growth process again. It takes 12–18 months for a mussel to reach the marketable size of 6–7 cm. Mussel growing areas are leased from the government and most bouchot operations are conducted as family enterprises, each having 15,000–20,000 poles. Each pole can yield 9–11 kg of mussels/yr and one acre can yield about 5 metric tons of mussels per year (Hulburt and Hulburt, 1980).
In the Philippines, mussels are grown on bamboo poles staked at ½ meter depth and one meter apart in soft, muddy bottoms. Mussels settle on the submerged bamboo stakes at a rate of 2,000–3,000 seeds per meter. Bamboo poles are regularly inspected to monitor growth as well as to eliminate predators like starfish and crabs. Mussels are harvested 6–10 months after stocking or when the animals reach 5–10 cm in length. Harvesting is done by pulling up the bamboo poles and loading them into a raft where the mussels are stripped off using an iron rod. In other cases, divers are hired to pick out the bigger mussels and the small ones left for the next cropping season. This selective harvesting results in two or more croppings within the 6–8 months of the culture period. Harvested mussels are initially cleaned by spading or stamping, then placed in baskets and shaken vigorously in seawater until they are clean of barnacles and dirt. Bamboo poles that are worn out are disposed while the good ones are cleaned for the next farming season. the stake method is a cheap and simple way of growing mussels but has also some disadvantages. The bamboos decay easily and it is sometimes difficult to coincide staking operations with spatfall. This culture system also facilitate siltation which makes bays and estuaries too shallow for mussel culture (PCARRD, 1983).
2.3 Rack culture
The basic design in the rack culture method is a structure fixed in the seabed which supports collectors like trap, ropes or shells tied on strings. In Japan, oyster seeds are collected by rens, which consist of 2 m length of No. 16 galvanized wire containing 100 scallop or oyster shells. The rens are suspended from horizontal bamboo poles which are set below the low spring tide mark. Two types of frames are used to support the horizontal poles. The first consists of two crossed bamboo poles driven into the sea bed in the form of an X where the horizontal pole is tied in the crotch. The second framework consists of rectangular bamboo platform 1 m wide and 3–6 m long which is supported by four corner posts driven into the bottom. The collecting rens are then draped closely packed together over the horizontal bamboo poles. In the various prefectural laboratories, biologists monitor the plankton and put out the test collectors to forecast the setting peak of the oyster spats. The oyster farmers are then advised on the best time to put out their spat collectors (Fijiya, 1970; Koganezawa, 1979).
The spat collectors in the rack method used in Brittany, France are made of semi-cylindrical ceramic tiles. The tiles, 30 cm long and 12.5 cm in diameter, are arranged in pairs and stacked at right angles to one another with the concave surface facing downward. A stack is made up of 5–6 pairs of tiles measuring 1 m high and wired together for ease of handling. The tiles are covered with lime for easy removal of oyster seeds which are then transplanted to the oyster parks. A new light weight spat collector, consisting of a plastic mesh material of similar size and shape is now being used. These plastic collectors are immersed in a mixture of six parts lime to one part cement to improve adhesion of the oyster larvae. The spats are easily removed by simply bending the plastic collectors and then transferred to the oyster parks in Brittany. On the southern coast of Brittany, there are 500 individual parks of 2–4 hectares in size while the south coast has 200 oyster parks of 20 hectares each (Milne, 1979).
For the Australian rack method, wooden collectors are used for the culture of the Sydney rock-oyster, Crassostrea commercialis. The wooden collectors are constructed by nailing four 2 m long, 2.5 cm2 hardwood sticks between two 1 m rails. These frames are tarred to protect the sticks from teredos and borers as well as to provide a clear and smooth surface for the spats. The tarred frames are stacked together and wired to very sturdy racks made of two parallel rows of posts with 5 × 2.5 cm hardwood rails along each row. From the spatfall areas, the wooden collectors are transferred to the maturing areas until the oysters reach marketable size (three years later). The average annual production from Australian oyster beds is 0.91 ton per hectare. However, good areas under intensive cultivation have produced 5.2 tons of live oysters per hectare/year using the tray method, and 2 tons/hectare/year using the stick method (Bardach et. al., 1982; Milne, 1979).
In the Philippines, a variation of the rack method is made using polypropylene ropes of 12 mm diameter, arranged into webs and tied vertically to bamboo posts. A web consists of two 5 m parallel ropes positioned 2 m apart to which a 40 m rope is tied in a zigzag fashion at intervals of 40 cm between knots along each of the parallel ropes. Bamboo pegs, 20 cm in length and 1 cm in width, are inserted into the zigzag rope at a spacing of 40 cm between pegs. These pegs prevent the crop from sliding down the rope when it becomes heavy. Full stretched rope-web collectors are positioned 3 m apart along the rows. During harvest, rope webs are untied from the poles and lifted into the raft. Clusters of mussels are detached by cutting the byssal filaments with a sharp knife. Mussels are then placed in baskets and washed repeatedly with seawater before bringing them to the market. Web collectors are cleaned and dried before being used for the next culture season (PCARRD, 1983).
2.4 Raft culture
The raft used in mussel culture in Spain is made of a wooden framework of 5 cm2 timbers and floats of concrete, steel, styrofoam or fiberglass material. The average size of the raft is 23×23 m which supports 700 ropes, each 9–m long. The rafts are anchored along the sides with large concrete moorings. Seed mussels, 6–8 mm, are gathered from natural mussel beds and placed inside a water-soluble rayon tube netting. The netting is wrapped around the ropes and by the time the netting has disintegrated, the mussels have attached themselves to the ropes suspended from the raft. The marketable size of 8–10 cm is reached after one year of culture. During harvest, mussel - laden ropes are hoisted aboard a work-boat with a large wire-mesh basket lowered under the ropes. The ropes are given a vigorous shake to remove the mussels. Small-sized mussels are wrapped into new ropes for transplanting while marketable mussels are transported directly to the processing facilities. Mussels sold fresh are required by Spanish law to be depurated for at least 48 hours. Production of mussels in 9–m rope is 113 kg live mussels per year and a single raft produces 80 metric tons of live mussels per year (Hulburt and Hulburt, 1980).
The Japanese raft for oyster farming has an average size of 8×16 m and carries 500–600 wire collectors. It is made of 10–15 cm diameter bamboo or cedar wood poles and bighead either by hollow concrete drums, tarred wooden floats or styrofoam cylinders. Most of the newly constructed rafts use styrofoam materials and these floats are usually encased in polyethylene bags to protect them from barnacles and other boring organisms. Rafts are tied together in rows 5–10 m apart and anchored at each end of the line. The Japanese oyster culture system is divided into two categories, one and two-year farming. In the one-year farming, oyster spats are immediately transferred to rafts where they remain until they reach marketable size. For the two-year method, oyster seeds are grown in racks placed in shallow waters for the first year and then transferred to the rafts for the second year. The average annual production is 223,000 metric tons of live oysters from the Japanese raft culture method (Fujiya, 1970; Koganezawa, 1979).
In the Philippines, raft culture is not widely practiced. A raft, 6×8 m, is made of a bamboo lattice structure from which ropes are hun and kept afloat by either metal or plastic drums, styrofoam blocks or ferro-concrete buoys. Spats are collected by nylon ropes hung from the bamboo framework at ½ meter apart with weights at the end of the ropes. When spats are 10–15 mm in length, they are thinned by hand and transplanted to growing ropes made of polypropylene, polyethylene, cabo negro and abaca. The growing ropes are hung from the raft at 1 m apart and their length would depend on the water depth at low tide. To avoid transplanting, mussels are collected and cultured on the same growing ropes (PCARRD, 1983).
3.0 ECONOMICS OF BIVALVE MOLLUSC CULTURE
The capital investment, expenses and returns from a bivalve farming activity are related to the size of farm and the culture methods used. In a BFAR study, the annual production cost of a 1,000 m2 mussel farm was estimated to be 20,225.00, the gross income was 32,640.00 and the net income was 12,415 (Table 1). For a one-hectare mussel farm the production cost was estimated to be 162,750.00, the gross income was 321,600.00 and the net income was 158,850.00 (Table 2). On the basis of culture methods, the stake method for a one-half hectare oyster farm had an annual production cost of 19,465, a gross income of 23,750.00 and a net income of 4,285.00 (Table 3), while the hanging method for the same area of oyster farm had a production cost of 48,010.00, a gross income of 116,660.00 and a net income of 68,650.00 (Table 4).
In a study by Orduna and Librero (1976) of 30 mussel farms with an average size of 1,784 m2, they found that the gross receipts amount to 5,634.00, the total expenses was 2,059.00 and the net income was 975.00. In a survey of 163 mussel farms with an average size of 2,460 m2, the annual farm receipts was found to be 3,737.00, the total farm expenses was 1,290.00 and the net farm earnings was 2,447.00. On the basis of three culture methods such as the broadcast, stake and hanging method, the highest net farm earnings for one year was obtained with the hanging method, 13,419 while the lowest as in the broadcast method, 148 (Librero et. al., 1976).
Korringa (1976, 1979) also reviewed the economic aspects of the different mussel farming methods in Europe such as the bottom culture in The Netherlands, bouchot culture in France, rack culture in Italy and raft culture in Spain. Information on the rent, inventory items, expendable items, labor scales and production for the different culture methods was presented. It was found that production costs varied considerably depending on the degree of mechanization, and the cost of labour and materials.
Table 1. Estimated costs and returns for one year (two croppings) of a family size (1,000 m2) mussel farm on the stake methoda
|Item||Cost ()||Life (yr)|
|1.||1 banca (dugout)||1,000||5|
|3.||stake and stake preparation||10,860||2|
|a)||1,020 pc bamboos (7–8 m) at 9.00 eachb||(9.180)|
|b)||towing charges at 180/day for 2 days||(360)|
|c)||raft rental at 20/day for 2 days||(40)|
|d)||staking charges at 1.25/bamboo for 1,020 bamboo poles||(1,280)|
|4.||bolo and other tools||100||3|
|Total fixed cost||12,460|
|1.||1. Operating costs|
|a)||harvesting charges at 2.00/gal for 4,080 gal (4 gal/stake)||8,160|
|b)||caretaker at 500/mo (12 months)c||6,000|
|c)||100 pc sacks at 1.50/sack||150|
|Total production cost||20,225|
Sale of 4,080 gal of mussels at 8.00/gal
|B.||Net Income (gross income minus total production cost)d||12,415|
|C.||Returns per Peso Invested||1.61|
Table 2. Estimated costs and returns for one year (2 croppings) of a one. hectare mussel farm on the stake methoda
|Item||Cost ()||Life (yr)|
|a)||50 pc bamboo (8 m) at 12.00 each||(600)||3|
|b)||Contract labor for making of raftb||(100)|
|2.||Stake & stake preparation||103,210||2|
|a)||10,050 pc bamboo (7–8 m) at 9.00 eachc||(90,450)|
|b)||staking charges at 1.25/bamboo||(12,560)|
|c)||other labor during construction||(200)|
|3.||1 caretaker hut||3,000||5|
|4.||1 motor engine 16 HP||4,000||5|
|5.||1 banca “kinabite” type||4,000||5|
|6.||1 air compressor complete with accessories||2,900||5|
|7.||2 pairs flippers (US Rubber) at 300 each||600||3|
|8.||2 pc diving mask at 250 each||500||3|
|9.||2 pc raincoat at 40.00 each||80||2|
|10.||2 pc flashlight (6 batteries) at 45.00/flashlight||90||3|
|12.||2 pc life vest jacket at 200 each||400||3|
|Total fixed cost||119.600|
|a)||1,825 liters regular gasoline (5 liters/day) at 5.05/liter||9,220|
|b)||12 qt motor oil (1 qt/month) at 10.00/qt.||120|
|c)||harvesting charges for 40,200 gal (4 gal/stake) at 2.00/gal||80.400|
|d)||2 caretakers at 500/month (12 months)||12,000|
|e)||500 sacks at 1.50/sack||750|
|l)||life vest jacket||130|
|b)||Repair and other expenses||5.000|
|Total production cost||162.750|
|A.||Gross Income: Sale of 40.200 gal of mussels at 8.00/gal||321,600|
|B.||Net Income (gross income minus total production costs)d||158.850|
|C.||Returns per Peso Investede||1.98|
Table 3. Estimated costs and returns of a one-half hectare oyster farm on the stake method for one yeara
|Item||Cost ())||Life (Yr)|
|1.||1 native banca (dugout)||1,000||3|
|3.||tools and diving paraphernalia||300||5|
|4.||stake and stake preparation||23,950||2.5|
|a)||5,000 pc bamboo poles (puno) at 4.50/poleb||(22,250)|
|b)||hired labor for staking||( 1,250)|
|c)||raft rental and towing charges||( 200)|
Total fixed costs
|a)||1 caretaker at 500/month for 12 monthsc||6,000|
|b)||harvesting charges for 625 kaings at 5.00/kaingd||3,125|
|b)||Repairs and other expenses||170|
Total production costs
|A.||Gross Income: sale of 625 kaings at 38.00 each||23,750|
|B.||Net Income: (gross income minus total production cost)e||4,285|
|C.||Returns per Peso Invested||1.22|
Table 4. Estimated costs and returns of a one-half hectare oyster farm on the hanging method for one yeara
|Item||Cost ()||Life (Yr)|
|1.||Materials and labor for plot construction of 125 plotsb||49,250||2|
|a)||42 pc bamboo poles (“puno”) at 4.50/pole||(189)|
|b)||21 pc bamboo poles (“baral”) at 2.25/pole||( 47)|
|c)||10 pc bamboo poles (“bila”) at 10.00/pole||(100)|
|d)||1 kg nail No. 4 at 8.00/kg||( 8)|
|e)||Contract labor to prepare plot||( 50)|
Cost for one plot
|2.||1 native banca (dugout)||1,000||3|
|4.||Tools and diving paraphernalia||500||5|
Total fixed cost
|a)||1 caretaker at 500/mo for 12 monthsc||6,000|
|b)||440 kaings of empty oyster shells at 4.50/kaingd||1,980|
|c)||240 rolls of nylon rope No. 4 at 12.00/rolle||2,880|
|d)||contract labor to make “bitin” or collectorsf||2,190|
|e)||harvesting charges at 3.00/kaingg||9,210|
|b)||repairs and other expenses||500|
Total production cost
|A.||Gross Income: sale of 3,070 kaings at 38.00/kaing||116,660|
|B.||Net Income: (gross income minus total production cost)h||68.650|
|C.||Returns per Peso Invested||2.43|
Bardach, J., J. Ryther and W. McLarney. 1972. Aquaculture, farming and husbandry of freshwater and marine organism. Wiley Interscience. New York. 868 p.
Bureau of Fisheries and Aquatic Resources. 1981a. Prospectus for a one-hectare oyster farm. Binakayan Station. Kawit, Cavite, Philippines. 3 p.
Castagna, M. 1970. Hard clam culture method developed at VIMS. Virginia Inst. Mar. Sc. Advisory Series No. 4. 4 p.
Fujiya, M. 1970. Oyster farming in Japan. Hel. Wiss. Meevesonters. 20:464–479.
Koganezawa, A. 1979. The status of Pacific oyster culture in Japan. In T.V.R. Pillay and W. Dill (eds.), Advances in Aquaculture. Fishing News Books Ltd. Survey. England. Chapter IV: 332–337.
Korringa, P. 1976. Farming marine organisms low in the food chain. A multidisciplinary approach to edible seaweed, mussel and clam production. Elsevier Sc. Publ. Co. 264 p.
Korringa, P. 1979. Economic aspects of mussel farming. In T.V.R. Pillay and W. Dill (eds.), Advances in Aquaculture. Fishing News Books Ltd. Survey. England. Chapter IV: 371–379.
Librero, A. R. Callo, S. Dizon and E. Pamulaklakin. 1976. Oyster farming in the Philippines: a socio-economic study. SEAFDEC-PCARR Res. Paper No. 6. Los Banos, Laguna, 101 p.
Milne, P. 1979. Fish and shellfish in coastal waters. Fishing News Books Ltd. Survey, England 208 p.