The Asian green mussel (Perna viridis), also known as the Philippine green mussel or Tahong, is an economically important mussel, a bivalve belonging to the family Mytilidae. The Asian green mussel is a large bivalve, with a smooth, elongate shell typical of several mytilids. It is harvested in the Philippines as food source because of its fast growth. It is rich in vitamins, minerals, protein and carbohydrates.
Green mussels were originally regarded as pests before World War II because they competed with food and space in oyster farms. In 1950, it was recognized as a primary bivalve food. The first mussel commercial farm started in Bacoor, Cavite, in 1955.
How to culture Green Mussel or Tahong
Site selection
Mussels are known to exist in bays, coves and inlets. They are found in Manila bay, east coast of Panay, Negros Occidental southwest coast and in Maqueda Bay and Jiabong, Samar.
Green mussels may be transplanted in new areas with seeds (juveniles) or breeders for growing and multiplication.
Mussels grow best in brackish to salty waters, with salinity ranging from 20-35 ppt. Good culture sites are indicated by the presence of indigenous existing mussels in the area to have sufficient breeding stock and spatfall. The water must be greenish in color which is an indication of abundant natural food of mussels.
The site must be free from pollution, with enough tidal exchange and moderate currents for the transport of food. It must be protected from strong winds and waves and must be relatively deep from two to four meters. Muddy to sandy bottoms that is semihard and sticky usually produce high yields.
Species, Reproduction, Food and Growth
There are two species used as food in the Philippines, namely: the green mussel and the brown mussel (Modiolus philippinarum). The green mussel commonly referred as tahong is the commercial species.
The male mussels’s mantle or meat is milky white to creamy and the female is orange to red orange. Since they have stationary forms of life, either one can change sex for the purpose of reproduction.
Spawners release eggs and sperms into the water where fertilization takes place in a few seconds. Eggs hatch into free swimming larvae within 24 hours and remain at this stage for 15-20 days. After the larvae are ready to settle, they secrete hair-like threads called byssal filaments to attach themselves. This ability to secrete new byssal when cut will allow thinning and transplanting operations. The settlement of larvae is called spatfall and the young mussels are called spats. Spawning normally occurs every two months, but the peak spatfall season in Manila Bay (Bacoor) occurs from April to May and October to November; February to March and September in Eastern Panay; and January to March and July to September in Western Negros Occidental. The spat is about the size of a grain of beach sand.
Mussels eat waterborne phytoplankton and minute organic materials by sucking and filtering water through its four rows of gills that is directed to the mouth. The gills serve both as a respiratory or breathing organ and as a filter-feeding organ.
Spats or larvae are attracted by filamentous objects and later move on to solid substrates or objects. Coconut coir and abaca coir are the best materials that can lure the spats.
Mature mussels can reach the size of 15 cm. in length, but they can be harvested in four to six months’ time. Frequent visit, at least every three days, is recommended to check the growth of filamentous algae and the presence of starfish and crabs that prey on the spats. It is best to place bottom nets for crabs or crab traps as an added income to mussel farming. Usually, there are plenty of blue crabs in oyster and mussel culture areas.
Methods of culture
There are five common methods of culture, like: stake (tulos), wigwam, raft or hanging, tray and rope web. The best and most popular ones, however, are the stake and rope-web methods.
Logs, hard bamboo (Bambusa sp.) and light bamboo or bagakay (Schizostachyum lumampao) can be used. However, hard bamboo is more popular because they are readily available and cheaper than logs.
Sharpen the bottom tips of the bamboo and drive it (pile) at the bottom by about one-half meter. Place a hole at the upper section of the bamboo segment to reduce buoyancy. The distance of the bamboos as post is 1-1½ m. Tic or nail the 2-3 m row of horizontal braces (bila). Connect every two rows of poles with short horizontal supports (baral) forming a square with the long bila.
Light bamboos can be used for the supports and braces. The series of squares forms the plot. Leave 1½-2 m in between 2 rows or plots for the wooden boat (banca) to pass through. Tie spat collectors to the poles. Preferably use abaca rope with coconut or abaca coir in the rope lay at 7.62-12.7 cm distance.
The best time to construct plots is the month before the occurrence of peak spatfall in the area to insure higher production. Mussels settle in the plot materials. The depth of the plots should be 1-2 m below zero tide level at the upper portion and about 0.50 m from the bottom. An average of 1,000-2,000 seeds or pieces of mussels per meter can be attained at these levels.
The rope web method is recommended in areas where there is heavy occurrence of spatfall. A sharpened hard bamboo pole is driven by piling at the bottom by at least ½ m. The upper portion of each segment of the bamboo must have at least 2.54 cm hole to prevent too much pressure from floating. The bamboo poles are 4.0-5.0 m apart. Abaca or polypropylene (nylon) ropes of 12mm-15mm are tied horizontally at the upper portion of at least ½-1 m from zero tide level. Another rope is tied horizontally at the lower portion with at least ½ m from the bottom. The upper and lower rope tied between the 5.0 m distance pole will form a parallel. The distance between the parallel ropes can be 1.5-2 m.
Abaca or nylon ropes of 10.0-12.0 mm diameter are made into webs that are tied vertically in zigzag fashion to the parallel ropes between the bamboo poles. The interval of the zigzag ropes or webs must be at least 30-40 cm. Web-rope lines can be spaced at 1.0-1.50 m rows for space to work on and for wooden boat to pass through.
Bamboo pegs of 20 cm length and 2.0-3.0 cm width are inserted into the zigzag rope lay at spacing of 30-40 cm to serve as spat collectors. Abaca or coconut coir can also be used because mussel spats prefer filamentous or hairy objects as cultches. The two rows of rope webs will form the plots.
Juvenile mussels can be placed in wet gunny sacks in clusters and transported to new areas to be transplanted. Transplanting is done by tying the young or small mussel clusters into the culture medium, like rope web or stakes.
Harvesting
Market preparation and knowledge of peak demands and high prices season are important in the timing of harvesting. Selective harvesting can also be done by harvesting the bigger ones first and leaving the smaller ones to grow further.
Divers are hired to do harvesting in the stake method. This is done by scraping the mussel clusters from the stakes and supports with the use of sharp knife or bolo. Care is taken not to pull the mussel shells because the byssus or beard when detached, can kill the mussels. The byssals are parts of the muscle structure of the green mussel. The mussels are placed in bamboo or rattan baskets and cleaned by continuously dipping them in the sea water. The mussels are separated or detached with the use of scissors.
Marketing of green mussels has never been a problem because of its demand as a source of low-cost protein. Also, it has generated employment in the coastal-producing areas. Green mussels can live three to four days after harvesting by continuously wetting them with sea water. It his a high nutritional and medicinal value.
Mussels are versatile aquatic products. Bivalves culture has a relatively simple technology and the labor is not intensive. The cost of investment is minimal but high profits can be expected.
Source: Oyster and Mussel in Western Visayas, Greenfields, April 1993. Photo: USCG, Photograpger Buck Albert; Panoramio.com