1.0 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.
France can probably be credited to have the longest history of mussel culture which dates as far back as 1235 (Bardach et al., 1972), while Spain has been reported to be the top world producer of farmed mussels.
In the Philippines, mussel culture started only in 1962 at the Binakayan Demonstration Oyster Farm, in Binakayan, Cavite by the biologists of the then Philippine Fisheries Commission, now Bureau of Fisheries and Aquatic Resources (BFAR). Mussels were initially considered as a fouling organism by oyster growers. The impetus for mussel culture in Manila Bay came about when oyster growers, attempting to collect oyster spats in less silty offshore waters, obtained instead exceptional heavy and almost pure mussel seedlings.
Mussel farming does not require highly sophisticated techniques compared to other aquaculture technologies. Even un-skilled laborers, men, women, and minors can be employed in the preparation of spat collectors as well as harvesting. Locally available materials can be used, hence minimum capital investment is required. The mussel harvest can be marketed locally and with good prospects for export.
Success in mussel farming, however, depends in providing some basic requirements to the bivalve such as: reasonable amount of sheltering of the culture areas, good seawater quality, and sufficient food in the form of planktonic organisms. These pre-requisites are found in some coastal waters, hence locating ideal sites for mussel cultivation is essential.
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 can not 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 remains 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.
3.3.3. 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.
3.2 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.
3.3 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.
- “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.
3.3.3. 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 (Fig.
7). 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. From a catamaran-type raft with 1,000
rope, 6–9 m in length, about 4,666–5,333 MT of
marketable mussel can be produced (Korringa,
1976).
Figure 7. Mussel raft culture method.
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.
4.0 Mussel transplantation to new site
Transplantation of young mussels from natural spawning grounds to sites with favourable conditions for growth is practiced in numerous countries as mentioned earlier. In the Philippines, however, mussel transplantation to new sites is being encouraged to develop new areas for mussel culture, due to various reasons. Major reasons are: rampant pollution of some existing mussel areas, urbanization growth near mussel farms and competitive use of lands.
Mussels to be transplanted could be breeders or young adults. Important points to be considered are: Conditions from natural spawning areas must be almost similar to the new area, mussels on original collectors showed better survival than those detached, and in transporting the mussel avoid being exposed to heat and freshwater.
5.0 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.
6.0 Depuration of mussels
To date, depuration of mussels in the Philippines is not yet undertaken due to its prohibitive cost. Mussel farmers cleansed their harvest by relaying them in clean water. This procedure, however, is unlikely to reduce heavy contamination by toxic wastes, accumulated during growing period.
7.0 Economic aspects
See Table 1 and Table 2.
TABLE 1. Project costs, income from mussel farming at Samar by long-line, rope-web and stake culture systems.
| Item | Unit | Culture System | ||
|---|---|---|---|---|
| Long-line | Rope-Web | Stake | ||
| Investment cost |  /ha | 64,6631 | 31,618 | 1,617 |
| Operating cost | /ha/yr | 28,560 | 37,808 | 102,086 |
| Interest2 | /ha/yr | 33,948 | 16,600 | 849 |
| Total expenses | /ha/yr | 127,171 | 86,026 | 104,552 |
| Total receipts | /ha/yr | 528,8403 | 333,4504 | 222,3005 |
| Net earnings | /ha/yr | 401,669 | 247,424 | 117,748 |
| Earnings on sales | % | 76 | 74 | 53 |
1 Amortized over 5 years.
2 Based on 14% of 74% of the total investment.
3 476 t/ha/yr less 5% losses × 1.17/kg.
4 300 t/ha/yr less 5% losses × 1.17/kg.
5 200 t/ha/yr less 5% losses × 1.17/kg.
Source: Bivalve Culture in Asia and the Pacific.
TABLE 2. Cost-benefit ratio for a 300 sq. m. area of mussel stake culture.
| A. | Capital outlay | 15,270.00 | |
| 360 bamboos (tulos) 8–10 m long at 20 each | 7,200.00 | ||
| Caretaker's hut | 3,000.00 | ||
| One (1) banca with outrigger | 3,000.00 | ||
| One (1) bolo | 90.00 | ||
| One (1) hammer | 80.00 | ||
| One (1) cross-cut saw | 100.00 | ||
| Labor for sharpening of 360 tulos at 2 each | 720.00 | ||
| Labor for staking the 360 tulos at 3 each | 1,080.00 | ||
| B. | Operating cost/year | 15,735.00 | |
| Five (5) empty sacks at 5 each | 25.00 | ||
| Labor for harvesting 3,600 gal. at 3 each | 10,800.00 | ||
| Municipal permit | 50.00 | ||
| Sales charges at 5% of sales | 3,240.00 | ||
| Bad debts at 2.5% of sales | 1,620.00 | ||
| C. | Annual/amortization (14%) | 2,800.00 | |
| In the capital outlay the amount of 20,000 will be borrowed from any financing institution with an interest of 14% p.a. payable in 2 years. | |||
| D. | Gross income/year | 64,800.00 | |
| Sales of 3,600 gal. of mussel at 18/gal. | 64,800.00 | ||
| E. | Total expenses & annual amortization | 33,805.00 | |
| F. | Net Income | 30,995.00 | |
| (Income less operating cost and annual amortization) |
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