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Production methods for the Mediterranean mussel

the Fish Site Editor
07 May 2009, at 1:00am

The habitat of the locations where culture takes place is very similar. Galicia (NW Spain) has been taken as a model in this fact sheet produced by the Food and Agriculture Organisation of the United Nations to explain how the different systems of mussel production work.

Habitat and biology

The Galician coast is characterized by flooded river valleys called 'rias', where farmers culture the mussels. Rias are up to 25 km long, between 2 and 25 km wide, and from 40 to 60 m deep; their bottoms are muddy, and they are bordered by hills. Annual productivity in the rias averages 10.5 mg carbon/litre/hr. The temperature ranges from 10-20°C, salinity is around 34‰, and the tidal range averages 4 m. Tidal currents are strong. There is a continuous upwelling of cold water that is rich in nutrients; these, along with nutrients which wash down from the hills during heavy rains (mean annual precipitation is 1250 mm) probably stimulate an abundance of phytoplankton. In turn, this favours the growth of mussels. These sheltered rias provide an ideal environment for culturing mussels on ropes suspended from floating rafts. The most important culture area is the ria de Arosa which is responsible for 60 per cent of Spanish mussel production; it is followed by ria de Vigo and ria de Pontevedra. Natural populations of mussels are present in large areas in the mouths of the rias and islands on intertidal rocky shores, where the mean density in the most crowded beds is around 24 000 mussels/m². They also grow along the rias mainly on rocky areas, cliffs, and boulders. Farmers collect mussel seed from these areas to suspend from their rafts. Recruitment of seed mussels occurs throughout the year with the major settlement season from May to September.

Mussels can move with the aid of the foot. A byssus gland secrets byssal threads that allow their attachment to the substrate. The gills are two pairs of broad plates composed of a large number of parallel filaments that filter food particles from the water. A mussel of 5 cm in length can filter 5 litre/hr. Digestion takes places in the digestion gland (brown-greenish in colour) situated in the centre of the body. Mussels feed on phytoplankton and organic matter. The mantle, immediately in contact with the interior of the shell, secretes the shell and contains the gametes (eggs or sperm). In Galicia, reproduction may take place at any time of the year. Mussels produce millions of eggs, losing a considerable amount of their reserve glycogen. Recently spawned mussels are so lean that they cannot be marketed. Fertilization is external. Fertilized eggs develop into a trochophore larvae, and then into a veliger that is carried by tide and currents. When they reach a shell length of 0.25 mm the pediveligers attach themselves with their byssus threads to filamentous substrates. They are able to detach themselves and reattach to other substrates.

Production

Production cycle

Mediterranean mussel Production Cycle

Production systems

The rearing of Mytilus galloprovincialis is always extensive, in all countries where it is being carried out; the steps that are described in this fact sheet therefore apply to all cases. The young mussels are collected from the sea and can be cultured on suspended ropes; these ropes, which are covered with mussel seeds kept in place by nylon nets, are suspended either from rafts, or wooden frames, or from longlines of floating plastic buoys. A substantial portion of the EU production is grown on suspended ropes, a technique which can be extended further offshore and which, although quite sensitive to plankton blooms, is the only one which could further increase production.

Seed supply

Culture begins when farmers collect mussel seed, mainly from natural beds (60-70 per cent); the remainder from the collector ropes hung from their rafts. Farmers can collect up to 1500 kg of seed per low tide in about 4 hours, from the exposed rocky shores on the ocean side of the rias and islands. They use a special steel shovel, called a 'rasqueta', which has a blade of about 10 cm², attached to a wooden handle. Farmers suspend the mussels from their own rafts or sell them to other farmers. The price is about 60 pesetas/kg (€1 = 166.386 pesetas). Farmers gather around 4500 tonnes of mussel seed (mean length = 2 cm) from these areas every culture cycle. They take the seed to the rafts, keeping it moist, and attach it to ropes within 24 hours after collection. To collect seed from the rafts, farmers use special nets made from old fish nets and suspend them during March and April.

Ongrowing techniques

In Galicia, Mytilus galloprovincialis is cultured in rafts. Raft size varies considerably from <100 m² to >500 m². These structures are supported by floats (from one to six), constructed of wood or steel covered with fibreglass or polyester, or filled with expanded polyester. Depending on the number of floats, the usable culture area can go from 80 per cent, when a central float is used, to 90 per cent when four to six floats are used. Farmers secure the rafts with one or two iron chains and a 20 tonnes concrete anchor. In protected areas with little boat traffic, they use only one mooring chain. Two chains are better in exposed areas or when the rafts are near the shore or heavy boat traffic. The rafts are located together, but separated from each other by about 80-100 m, in groups called parks. These vary in the number of rafts, and their locations are regulated by marine authorities. From the beginning of mussel culture in 1946, the number of rafts increased moderately to 400 in 1956 but, after that, they increased rapidly. The average size of the rafts increased from 297 m² in 1977 to 369 m² in 1984. Currently, farmers work from shallow-draft, wide-beam boats (9 tonnes in weight), powered by diesel engines of about 24 hp. Each has a basket and crane to raise the ropes and machines to separate and thin the mussels. Special machinery has been developed to help with the various culture practices, especially with the wrapping of spat to the ropes, and grading.

Attaching the seed

Farmers attach the seed to the ropes by hand, or with a machine which secures it with a special cotton or rayon mesh; this mesh disintegrates within a few days. By then, the mussels have secreted new byssus and have attached themselves to the ropes. Farmers attach from 1.5-1.75 kg of seed/per metre of rope, and the average weight of seed for each rope is 14 kg. The ropes, usually 3 cm thick and made of nylon, polyethylene, or esparto grass (S. tenacissima), vary in length from 6-10 m. Their rough surfaces facilitate the attachment of the mussels. Each rope with attached mussels has a loop at one end, which is fastened to a thinner polyester rope called a 'rabiza' (12-14 mm thick), which in turn is lashed to the girders of the rafts. The rabiza usually lasts only 3-4 years because it is exposed to air and sunlight, while the major ropes last an average of 5.8 years. Each raft has from 200 to 700 ropes. Every 30-40 cm, wooden or plastic pegs 20-30 cm long are inserted between the strands of the ropes to prevent the clumps of mussels from sliding down. Farmers attach from 1-3 ropes/m² of raft. This distribution allows an adequate flow of water rich in food for the mussels, and prevents the mussel ropes from touching each other. Farmers install the ropes mainly from November to March.

Thinning

The third step (after obtaining the seed and attaching it) is thinning, which has to be done to prevent the mussels from falling off in rough weather; thinning also encourages rapid and uniform growth. Farmers do this when the mussels are half grown (shell length 4-5 cm) after 5-6 months of growth, usually from June to October. They lift the ropes into their boats using a crane and rub off the clusters of mussels by hand into a steel screen which separates them into different sizes. A mechanical cylindrical screen may also be used. The mussels from each original rope are attached to two to four new ropes with cotton or rayon netting. The average weight of the ropes is 46 kg. Those farmers who automate this operation spend 5-15 seconds per metre of rope, or less than 14 hours for 500 ropes of 10 m length. This work is repeated once again before harvesting if the mussels grow rapidly (in which case their weight and density increases the risk that the mussel clusters will fall off). It is also necessary to repeat this operation in order to ensure that all mussels reach a similar size at harvest time.

Rearing

The rearing of mussels constitutes the fourth culture step. In the Galician region where growth is rapid, mussels can attain market size (8-10 cm) in 8-9 months, especially in the areas closest to the ocean side of the rias. The usual time required in some bays is around 13 months. However, a high raft density can retard mussel growth. Growth is minimal in summer and highest in winter. Slow summer growth is related to the relative abundance and availability of food (phytoplankton) in the water column then; this is more important than high temperature and causes the seed placed on the ropes in the spring and the fall to reach the same size at the end of the first winter.

Each raft normally holds three types of ropes: those for collecting seed, those with growing mussels, and those with marketable mussels; in this way, growers maintain continuous production. Since the mussels grow faster near the water surface, some growers periodically invert the ropes to produce mussels of about equal size. In rafts with only one central float, the equilibrium of the raft is altered when farmers raise ropes for thinning or harvesting, and they have to put containers filled with water on the appropriate area of the framework to avoid tilting them. A large number of mussel seed and fouling organisms attach to the floats and, as they grow, the weight of the raft increases; therefore farmers have to clean the floats occasionally. This process is easiest when the raft is nearly empty and it floats higher, leaving many mussels and fouling organisms exposed to air where they die and are easy to remove. For major repairs of the framework or floats, farmers take the rafts to shipyards or factories. A medium-sized wooden raft has a life span of 10-15 years, while modern fibreglass rafts last considerably longer. Rafts range in age up to 30 years, with an average of about 8 years.

Harvesting techniques

Mussels of commercial size are available throughout the year and can be harvested at any time but the main harvest is from October to March, when market demand is high and their condition is best. Meat weights can approach 50 per cent of total wet weight when the mussels are in best condition. When a large per centage of mussels is close to spawning or just past spawning, harvesting should wait until they are in better condition. The mean production averages 130 kg/m² of raft area; for an entire raft this equates to 20-100 tonnes, with a mean value of around 47 tonnes. Such values are highly variable and depend on size of the rafts. Production can also be defined as about 10 kg of mussels per metre of rope. Annual losses (natural mortality and handling) have been estimated at 15 per cent. Recent experimental results show that natural mortality in mussels is around of 5 per cent.

For harvesting, farmers use a crane to raise the ropes to their boat, where the mussels are separated and graded by rubbing them over a grid of iron bars. They are then washed clear of small mussels, silt, empty shells, ascidians, and other unwanted organisms. Any mussels too small for the market are wrapped onto new ropes for further growing.

Handling and processing

The marketable mussels are packed by women in nylon bags and taken in boats directly to the depuration plants, or to canning factories. Women usually do all of this work. Each handles about 200 kg of mussels every 8 hours. The mechanization of handling is minimal to reduce damage to mussel shells, and thereby enhance the shelf life of the mussels during transportation. In the warm season, refrigerated trucks are used to transport the mussels. Sometimes, mussels for the Spanish market are transported by train. The mussels that go directly to the canneries are those having the poorest quality and size. They are prepared by frying or boiling and then covered with various sauces; they can be served in a large variety of ways.

Production costs

The cost of a floating raft is governed by its size and the materials used. In 1948 a single raft with all the equipment, including 800 ropes, cost around 83 000 pesetas. In 1958 its cost had reached 250 000 pesetas; of this, 150 000 pesetas were for carpentry, 21 000 for the chain and anchorage, 65 000 for the esparto grass ropes (S. tenacissima), and 14 000 for the boat and incidentals. In 1976, the cost was from 1 500 000 to 2 000 000 pesetas; in 2000, their cost was about 15 000 000 pesetas.

The price of fresh mussels in 1951 ranged from 2.0 to 2.5 pesetas/kg, and in 1958 from 3.5 to 3.75 pesetas/kg. In 1976, the price directly from the rafts was from 7.5 to 9.1 pesetas/kg, and at the first sale the price reached 15-20 pesetas/kg. The price to the consumer was about 30 pesetas/kg. By 2000, the price for the consumer is around 180 pesetas/kg. These prices are very low when compared with prices for other kinds of shellfish or meat. The profits for a family have been calculated as about 25 per cent of the total value of the production sold.

May 2009