Mussel Culture

Mussels are among the many invertebrates under the Phylum Mollusca.  Their wide distribution in the coastal areas of the Indo-Pacific region makes them the most easily gathered seafood organisms, contributing a significant percentage to the world marine bivalve production. Culture of edible molluscs is now recognized as an effective way of enhancing food production and sea mussels among all the known cultivable organisms, give the highest production rate for two reasons:
 (1) They feed directly on the primary producers namely the phytoplankton
 (2) It is farmed three dimensionally in the water column at the farm site.
The world mussel production during 2006 was 1.89 million tons valued at 1.2 billion US dollars (FAO, 2008). The world production of Perna viridis during 2006 was 305,321 tons valued at 2.74 million US dollars.
The total production of green mussels in India (2008) is about 15,000 tons (CMFRI, 2009). China ranks first in the production of cultured mussels in the world, followed by Spain, Italy, Netherlands, Denmark and France.

TAXANOMY

Kingdom:  Animalia

Phylum:    Mollusca

Class:     Bivalvia

Subclass:     Pteriomorphia

Order:       Mytiloida

Family:    Mytilidae

Genus: Perna

Species: viridis

BIOLOGY

Mussels are sedentary animals with elongate, equivalved and in equilateral shells. The two valves are hinged at the anterior end with terminal umbo. External colour of the shell is green in Perna viridis and dark brown in Perna indica. Interior of the shell is margaritaceous and shining, muscle scar deeply impressed. Foot is finger-shaped, thick and extensible. Byssus threads emanate from the byssus stem and threads are long, thick, strong with a well developed attachment disc at their distal end. The mussels can discard the byssus threads and secrete new ones for enabling it to change its position. Phytoplankton forms the food of mussels and like oysters they are filter feeders.

Fig. green mussel (P. viridis)           Fig. brown mussel (P. indica)

 Reproduction

In mussels sexes are separate and reproduction and larval development are similar to that of edible oysters. The male gonad is creamy white and in females it is pink or reddish. The mussels attain maturity at very small lengths. The green mussel attains maturity at 15.5 to 28.0 mm. 

Larval development of mussels

Release gonad material (sperm and eggs) into the water i.e external fertilization. The fertilized eggs develop into trochophore in 6-7 hrs.  "D" shaped veliger in 20 hrs. Larvae are free swimming for 15-20 days. Locomotion is with the help of velum.  As the larvae metamorphose, the pedal organ developes. On formation of this the pediveliger larvae look out for a suitable substratum to settle. 

The larvae attach to the substratum by means of the byssus. The metamorphosis takes place and the secretion begins. The young metamorphosed larvae (plantigrade) is generally called "Spat" the ability of the animal to regenerate the byssus is an advantage for transplanting the animal to new areas in mussel farming operation.

                 

Technology of mussel farming

Site location

In prospecting sites for mussel cultivation, well-protected or sheltered coves and bays are preferred than open un-protected areas. Sites affected by strong wind and big waves could damage the stock and culture materials and, therefore, must be avoided. Flood waters would instantly change the temperature and salinity of the seawater, which is detrimental to the mussel. Sites accessible by land or water transportation are preferred so that culture materials and harvests can be transported easily.

Water quality

Areas rich in plankton, usually greenish in color, should be selected. Water should be clean and free from pollution.  Sites near densely populated areas should not be selected in order to avoid domestic pollution. In addition, the culture areas should be far from dumping activities of industrial wastes and agricultural pesticides and herbicides. Waters too rich in nutrients, which may cause dinoflagellate blooms and render the mussels temporarily dangerous for human consumption, causing either gastro-intestinal troubles or sometimes paralytic poisoning, should be avoided.

Water physio-chemical parameters are also important factors to be considered. 

The area selected should have a water temperature ranging from 27–30 °C, which is the optimum range required for mussel growth. Water salinity of 27–35 ppt is ideal. A water current of 17–25 cm per second during flood tide and 25–35 cm per second at ebb-tide should be observed. Favourable water depth for culture is 2 m and above, both for spat collection and cultivation.

Seed collection and seeding

The major part of the seed required for farming is collected on ropes suspended from the rafts during the peak spawning period.  The spat settled on ropes are allowed to grow to a length of 30-40 mm. These half grown mussels are collected and transplanted to fresh ropes to avoid overcrowding and help further rapid growth and fattening. Seed mussels are also collected from the natural mussel beds during low tide. A well experienced farmer collects up to 100 kg of seed in one tide. These seeds are wrapped around the ropes with a loosely woven synthetic netting, which is specially manufactured for Spanish mussel farming. The seeded length of the rope is 8-10 m and these ropes are suspended from rafts 60-70 cm apart. In the raft culture, the seeds reach harvestable size of 70-100 mm by 18-24 months in the temperate waters.

Mussel seeds are collected from the intertidal and submerged mussel beds after the peak spawning season (September-November). Normally an experienced person can easily collect 20- 30 kg of mussel in one hour. 

The average seed size for farming is 15-25 mm and 600 g seeds are required for seeding 1 m length of the rope. Synthetic and coir ropes of 15-20 mm diameter are suitable for growing mussels from the rafts. The seeds are placed around the rope and securely wraped with knitted cotton cloth. The seeded ropes are suspended from the rafts, 0.5-1 m apart, with the lower free end of the rope about 2 m above the sea bed. An optimum of 60 ropes, each having 6 m seeded length can be suspended from a raft of 6 x 6 m size. The seed mussels get attached to the ropes by means of freshly secreted byssus threads in two to three days and the cloth disintegrates in seawater within about 10 days. After the suspension of seeded ropes the mussel culture farm needs only minimum attention to see that the rafts are in good shape and the ropes with growing mussels are hung properly.

Fig. spat collector nylon rope

Fig. spat collector pipes

Bottom type

Bottom consisting of a mixture of sand and mud has been observed to give better yields of mussel than firm ones. It also provides less effort in driving the stakes into the bottom. Shifting bottoms must be avoided.

Cultured mussel species

Among the mussels proliferating in the coastal areas of the tropical zone, the green mussel, Perna viridis (= Mytilus smaragdinus), called tahong in the Philippines, is the only species farmed commercially. In the temperate zone, it is the blue mussel, Mytilus edulis, as this species can grow at low seawater temperatures. The brown mussel, Modiolus metcalfei and M. philippinarum which form dense mats on muddy bottoms in shallow bays (Yap, 1978) are simply gathered.

Culture methods

Mussel culture, as practiced in many countries, is carried out by using a variety of culture methods based on the prevailing hydrographical, social and economic conditions.

Bottom culture

Bottom culture as the name implies is growing mussels directly on the bottom . In this culture system a firm bottom is required with adequate tidal flow to prevent silt deposition, removal of excreta, and to provide sufficient oxygen for the cultured animals. Mussel bottom culture is extensively practiced in The Netherlands, where the production of seeds is completely left to nature. If the natural spatfall grounds are unsatisfactory for growing, the seedlings are transferred by the farmer to safer and richer ground or to his private growing plots, until the marketable size is attained. Natural conditions control the quality and quantity of food in the water flowing over the farming plots. Marketable mussels are fished from the plots and undergo cleansing before being sold. This method requires a minimum investment. Disadvantages, however, of this type of culture is the heavy predation by oyster drills, starfish, crabs, etc. Also, siltation, poor growth and relatively low yields per unit culture area.

Intertidal and shallow water culture

The culture methods that fall under this category are usually practiced in the intertidal zone. The culture facilities are set in such a way that the mussels are submerged at all times. Culture methods are:

 Rack culture

This is an off-bottom type of mussel culture. Rack culture is predominantly practiced in the Philippines and Italy where sea bottom is usually soft and muddy, and tidal range is narrow. The process involves setting of artificial collectors on poles or horizontal structures built over or near natural spawning grounds of the shellfish. In the Philippines, this is called the hanging method of mussel farming. The different variations used are as follows:

Hanging method.

 The process starts with the preparation of the spat collectors or cultches. Nylon ropes or strings, No. 4, are threaded with coco fibre supported by bamboo pegs or empty oyster shells at 10 cm intervals. These collectors are hung on horizontal bamboo poles at 0.5 m apart (Fig. 2). A piece of steel or stone is attached at the end of the rope to prevent the collector to float to the surface. Setting of collectors is timed with the spawning season of the mussels. Spats collected are allowed to grow on the collectors until marketable size. Other materials utilized as collectors are rubber sheets and strips from old tires. Mussels are harvested by taking out from the water the ropes or strings and bringing them to the shore on a banca. The same collectors can be re-used after being cleaned of fouling organisms. Harvested mussels are cleansed of the dirt and mud by dipping the collectors several times in the water. The process maybe laborious, but the ease in harvesting and availability of local materials for culture purposes makes it very adaptable under local conditions.

Mussel hanging (bitin) culture

Mussel stake (tulos culture)

Stake (tulos) method.

 The stake method is midway between the rack and bottom methods. Bamboo poles, 4–6 m in length are staked firmly at the bottom in rows, 0.5–1 m apart during low tide in areas about 3.0 m deep and above (Fig. 3). In areas where water current is strong, bamboo poles are kept in place by nailing long horizontal bamboo supports between rows. Since mussels need to be submerged at all times, it is not necessary that the tip of the poles protrude above the low water level after staking. However, boundary poles should extend above the high water level. In staking, enough space between plots is allowed for the passage of the farmer's banca during maintenance.

Collected spats are allowed to grow in-situ until marketable size, 5–10 cm after 6–10 months. It has been observed, that about 2,000–3,000 seeds attach on 1 metre of stake, 1–2 m below low water level. The mussels are harvested by pulling out the poles and bringing them ashore on a banca. Some poles may still be sturdy and can be re-used during the next season.

Tray culture. 

Tray culture of mussels is limited to detached clusters of mussels. Bamboo or metal trays, 1.5 m × 1 m × 15 cm sidings are used (Fig. 4). The tray is either hang between poles of the hanging or stake methods or suspended on four bamboo posts.

Wig-wam culture. 

The wig-wam method requires a central bamboo pole serving as the pivot from which 8 full-length bamboo poles are made to radiate by firmly staking the butt ends into the bottom and nailing the ends to the central pole, in a wigwam fashion. The stakes are driven 1.5 m apart and 2 m away from the pivot. To further support the structure, horizontal bamboo braces are nailed to the outside frame above the low tide mark .Spats settle on the bamboos and are allowed to grow to the marketable size in 8–10 months.

Mussels are harvested by taking the poles out of water, or in cases that there are plenty of undersized bivalves, marketable mussels are detached by divers.

Rope-web culture.

 The rope-web method of mussel culture was first tried in Sapian Bay, Capiz, in 1975 by a private company. It is an expensive type of culture utilizing synthetic nylon ropes, 12 mm in diameter. The ropes are made into webs tied vertically to bamboo poles. A web consists of two parallel ropes with a length of 5 m each and positioned 2 m apart. They are connected to each other by a 40 m long rope tied or fastened in a zigzag fashion at an interval of 40 cm between knots along each of the parallel ropes . Bamboo pegs, 20 cm in length and 1 cm width are inserted into the rope at 40 cm interval to prevent sliding of the crop as it grows bigger. In harvesting, the rope webs are untied and the clusters of mussels are detached.

The method is laborious and expensive, but the durability of the ropes which could last for several years might render it economical on the long run. However, the effect of the culture method on the culture ground is detrimental as gradual shallowing of the culture area has been observed up to the point that the areas become no longer suitable for mussel farming.

   “Bouchot” culture

“Bouchot” culture is mainly undertaken in France. This is also called the “pole culture” or stake culture. The poles, used are big branches or trunks of oak tree, 4–6 m in length, which are staked in rows, 0.7 m apart on soft and muddy bottoms of the intertidal zone during low tide.�Mussel seeds are collected on coco-fibre ropes which are stretched out horizontally on poles. Young adults, 3–5 mm in size are placed in long netlon tubes (10 m in length) and attached around the oak poles in a spiral fashion, until marketable size.�Korringa (1976) reported that for an estimated length of about 600 km “bouchot” netlon, an approximate production of 7000 tons of marketable mussels yearly or an average production of 25 kg/pole/year can be harvested.

Deep water culture

-   Raft culture

Mussel raft culture has been practiced in Spain for a long time. Mussel seeds that settle freely on rocks or on rope collectors are suspended from a raft. When the weight of the bivalves on a given rope exceeds a certain limit, the rope is taken out and again distributed over a greater length until marketable size. It is a continuous thinning of the mussel stock to provide ample space to grow. Marketable shellfish are detached from the rope, purified in basins before marketing. The raft may be an old wooden boat with a system of outrigger built around it. Other kinds of rafts could be a catamaran-type boat carrying some 1000 rope hangings, or just an ordinary plain wooden raft with floats and anchors . Floats can be made of plastic, wood, oil drums, etc. The raft are transferred from one place to another using a motor boat. Production of mussels from this type of culture is high. Advantages of this type of culture are: reduce predation, utilization of planktonic food at all levels of water, and minimum siltation.

Long-line culture

Long-line culture is an alternative to raft culture in areas less protected from wave action. A long-line supported by a series of small floats joined by a cable or chain and anchored at the bottom on both end is employed. Collected mussel spats on ropes or strings are suspended on the line. The structure is fairly flexible.

Mussel culture- cycle

Harvesting procedure

Harvesters should be aware of the stress caused during the harvesting process. 

In harvesting mussels special care is needed. Pulling them or using a dull scraper may tear the byssal thread. This will result in loss of moisture after harvest or cause physical damage causing early death of the bivalve. The right procedure is to cut the byssal thread and leave it intact to the body. 

Exposure to sun, bagging and transport also increases the stress of the mussels.

PURIFICATION OF MUSSELS

Mussel is a sedentary bivalve growing attached to rocks or any other hard substratum in coastal waters by means of self secreted byssus threads. They are filter feeders and therefore at any given time their stomachs are likely to be loaded with mud/sand particles and bacteria. Hence depuration of mussels is very important. Legislation regarding sanitary requirements in marketing mussels can be classified into two groups. In most countries mussels can be marketed only when originating from clean waters, where regular analysis ensures that quality is maintained. However, in southern Europe (Spain and Italy) all mussels must be depurated, wherever they originate from. As a result there is less stringent enforcement as to water quality, where the mussels are grown. It is rather easy to purify mussels of bacterial pollution, because they cleanse themselves of this type of pollution bacteria if kept in clean filtered sea water for 24-48 hours. This treatment can also clean the mussels of mud and sand particles in the stomach.

The depuration process consists of three aspects: (1) pumping clear sea water into large storage tanks (2) sterilization of water using 2-3 ppm chlorine or 1-2 mg ozone/litre and (3)keeping mussels in sterilized water for 24-48 hours. Recently ozone is widely used for sterilization of water, as it is a powerful oxidizing agent capable of killing bacteria and viruses rapidly.

CONCLUSION

The fast growth rate of mussel makes it an ideal species for culture. 

The production can be augmented if mussel farming is taken up on a large scale. 

It offers scope for employment to the coastal fishers, increases foreign exchange earnings and an additional occupation for the fishing community with income potential.

The advantage of mussel culture in our waters when compared to the temperate region is that the rate of production here is very high. Here it takes only 5-6 months, because of the high productivity of the tropical waters. The peak spawning season of green mussel along the west coast of India is from August to October and along the east coast it is May-July.  Mussel culture, begin with be viewed as social necessity with a bias on nutritional improvement of the people and employment potential.