MUSSEL CULTURE

INTRODUCTION

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. In the Philippines, approximately 12,000 MT of mussels were produced in 1987. This amount consisted only of farmed green mussel, Perna viridis, and not the brown mussels which are exclusively gathered from natural beds.

In the wild, mussels are mostly found in the littoral zone, attached in clusters on various substrates. Being a filter-feeder of phytoplankton and detritus, it is considered the most efficient converter of nutrients and organic matter, produced by marine organisms in the aquatic environment, into palatable and nutritious animal protein. Its very short food chain (one link only), sturdy nature, fast growth rate and rare occurrence of catastrophic mass mortalities caused by parasitic micro-organisms, makes it possible to produce large quantities at a very reasonable price (Korringa, 1976). Likewise, its ability to attach to substrates with the byssus, makes it an ideal aquaculture species using different culture systems. According to Bardach et al. (1972), mussel culture is the most productive form of saltwater aquaculture and its proliferation is virtually a certainty.

2.0 BIOLOGY OF THE MUSSEL

The green mussel, Perna viridis has separate sexes, although hermaphrodism usually occurs. Externally, it would be difficult to determine the sex, however, internally, the gonad tissue of a sexually matured male appears creamy-white in color, while that of the female is reddish-apricot. Sometimes young sexually immature females cannot be distinguishable by color from male specimens.

This bivalve species reaches sexual maturity within the first year and spawns with the rising of seawater temperature. In the Philippines, mussels spawns year-round, however the peak period of spawning and setting is in April and May and again in September to October. Eggs and sperms are shed separately and fertilization occurs in the water (Jenkins, 1976).

Mussels have two relatively distinct phases in their life-cycle. A free swimming planktonic or larval stage and a sessile adult stage. The free swimming larvae remain planktonic for 7–15 days depending upon the water temperature, food supply and availability of settling materials. At about 2–5 weeks old, the larvae (0.25–0.3 mm) seek a suitable substrate to settle on and final metamorphosis takes place, changing its internal organ structure to the adult form. The young spat then grow rapidly and within 4–8 weeks, after settlement, they measure 3–4 mm in shell length.

Subsequent growth of the bivalve can be distinguished into shell and body growth. The shell length does not necessarily reflect the meat content. During spawning or food shortage, internal energy reserves are consumed while the shell may continue to grow. Overall growth of the mussel, as far as shell measurement is concerned is influenced by factors like temperature, salinity, food availability, disturbances and competition for space. On the other hand, body growth is affected by the season which primarily relate to sexual cycle and over-crowding to a certain extent.

3.0 CULTURE ASPECTS

The cultivation of mussels has taken various forms in different countries of the world. However, as in all farming procedures, it requires careful consideration of environmental, ecological and seasonal factors, in order to ensure proper growth and survival of the stock through harvest.

3.1 Criteria for site selection

3.1.1 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. Another important consideration is the presence of natural mussel spatfall. Areas serving as catchment basins for excessive flood waters, during heavy rains, should not be selected. 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.

3.1.2. 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.

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. 3.3.1. Bottom culture

Bottom culture as the name implies is growing mussels directly on the bottom (Fig. 1). 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.

Figure 1. Mussel bottom culture.

3.3.2. 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.

Figure 2. Mussel hanging (bitin) culture method on bamboo plots as practiced in the Philippines.

Figure 3. Mussel stick (tulus) culture method as practiced in the Philippines.

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 (Fig. 5). 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 (Fig. 6). 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.

Figure 4. Mussel tray culture method as practiced in the Philippines.

Figure 5. Mussel wig-wam culture as practiced in the Philippines (Source: A. Lovatelli).

Figure 6. Mussel rope-web culture method as practiced in the Philippines.

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.