Culture refers to growing seed oyster until harvestable. The place where seed oyster grows is an oyster farm. It is important to understand the entire growing process from collection to harvest. Growth of oyster depends on the hanging period, condition of oyster ground, condition of oyster seed and other factors. The general procedure is shown in Table 5-1.
|Process of culture||Preparation for seed collection||Saad collection||Hanging||Culture|
The various culture methods are listed and described below:
1. Traditional Methods
Bottom sowing. The simplest of oyster culture methods. What is basically required is that the sea bottom of the farm be hard. Otherwise, the shells get buried and lost in the mud. This method has been used mainly in the intertidal zones of Korea (Figure 5-1).
Stone culture. In this method, stones are used as collecting materials in an intertidal zone where the bottom is soft. A lot of attention must be paid to selecting the growing site. The bottom must be “combed” or raked to make it smooth and even one month before setting the collecting materials (Figure 5-2, 5-3).
Stick culture. To use this method, the oyster ground must have a smooth bottom and be located in protected coastal areas with little influence of wind and waves. Sticks are 1.2 to 1.8 m long. They can be made out of pine trees, a kind of oak, bamboos or other materials. The advantage of this method is that the facilities for culture are easy to establish.
Fig. 5-1. Bottom culture of Pacific oyster.
Fig. 5-2. Stone culture of Pacific oyster.
Fig. 5-3. Schematic drawing of stone method.
2. Hanging Method
This method promotes the growth of oyster by prolonging the feeding time. However, the cost of the facilities is higher. Operating it also requires more people. Yield is high, though, and because of this, hanging ground is continuously expanded. This method includes raft and long line culture.
Raft culture. Wooden poles are laid parallel to each other about 0.5 m. apart and fastened by wire lashing to lateral beams. Each beam is used to suspend the seed oysters from the raft. The floats for the raft are usually made from styrofoam. A raft is normally 18 m long and 9 m wide and the float materials range from 30 to 40 pieces. About 400 to 500 strings of oysters can be suspended from the raft. One string is 9 m long (Figure 5-5).
Long-line culture. The long-line culture is a modification of raft culture. This method is used for offshore culture. The basic feature of a long-line unit is a series of styrofoam floats arranged in a row. The long-line is secured at each end with two anchors. One long-line is 100 m long and consists of about 51 floats connected by a polyurethane rope 15 mm in diameter. A series of strings of oysters called “rens”, each about 5m long (the exact length varies with the depth of water) is suspended to each rope (Figure 5-6).
3. Other Methods
Rack culture. The rack method is a shallow water adaptation of the hanging culture method. To construct a rack, wooden poles are driven into the bottom, two to four metres apart. These uprights are connected by horizontal poles. The horizontal poles support the suspended strings of cultch, which are placed about 30 cm apart.
Umbrella-type culture. This is a modification of the rack method. A unit consists of one standing pole driven into the bottom to which is tied a series of strings of oysters spread radially as an open umbrella.
Establishing the culture unit is one of the most important tasks in oyster culture. A general description of the culture units has been briefly mentioned; this section describes the long-line and raft methods.
1. Number of units per area
The level of culture units permitted per given area is set by the Ministry of Agriculture, Forestry and Fisheries mainly to prevent an excess of intensive culture. This is shown in Table 5-2. The units are established on 5 to 10 percent of the established oyster ground areas.
Fig. 5-4. Stick culture of Pacific oyster.
Fig. 5-5. Raft culture of Pacific oyster.
Fig. 5-6. Long-line culture of Pacific oyster.
Fig. 5-7. Rack culture of Pacific oyster.
|Long-line culture methoda|
|Raft culture methodb|
a One set of long-line indicates 100m of line.
b One set of raft indicates 9×18m of raft.
Table 5-3. Details of materials for one raft.
|Bamboo pole||ø 10cm, 4.5m long||94 ea.|
|Float||Styrofoam, 450||13 ea.|
|Rope for anchor||Wire rope, ø 24mm||70 m|
|Anchor||Reinforced concrete, 0.6 M/T||4 ea.|
|Hanging string||#13 galvanized wire||3,600 m||9m × 400 strings|
|Pipe||PVC, ø 1cm, 20cm long||16,000 ea.||40 cultches × 400 strings|
|Cultch||Oyster shell||16,000 shells||40 cultches × 400 strings|
Table 5-4. Details of materials for one long-line.
|Rope||PE rope, ø 15mm||100 m|
|Rope for anchor||PE rope, ø 15mm||60 m||30m for a string|
|Float||Styrofoam, 60||51 ea.||At 2m intervals|
|String||ø 5mm, PE coated||142 m||At 70cm intervals|
|Anchor||Stick or iron||2 ea.||Both sides|
|Cultch||Oyster shell||2,850 shells||20 cultches × 142 strings|
|Line for fastening float||PP twist line||255 m||5m × 51 ea.|
2. Methods of establishing a culture unit
Raft culture. Raft is made of buoyant and pliant bamboo. Floats are usually made of styrofoam. Sizes of rafts vary but the most common is 18 m × 9 m. One needs 45 pieces of floats for this raft size. This number would normally weigh about 400 kg. The raft can accommodate 400 hanging strings. A hanging string is 9 m long and is ordinarily reinforced by two pieces of No. 13–14 galvanized wire wound around its length. The number of cultches per hanging string is about 40. The cultches are held about 20 cm apart by a PVC pipe (Figure 5-8 and, Table 5-3).
Long-line culture. This has been described in the preceding section and is indicated in Figure 5-9 and Table 5-4.
Figure 5-8. Structure of a raft for oyster culture.
Figure 5-9. Structure of a long line for oyster culture.
Figure 5-10. Shell growth in terms of height from the start of the hanging culture period.
Figure 5-11. Meat weight increase from the start of the hanging culture period.
3. Oyster growth
Growth of Pacific oysters greatly depends on the conditions of the culture ground and on the hanging period.
Effects of hardening on growth. Oyster spat trained on hardening bed for more than six months after collection are resistant to disease and grow well. Tests showed that the mortality rate of hardened oyster spat was only 18.2 percent in eight months after hanging while that of ordinary oyster spat was 42.6 percent.
Effects of the hanging period on growth. Growth rate varies with hanging period. For example, oyster of 1.5 cm shell height hung in March grew to 9.3 cm by April of the next year while oyster of 2.4 cm in shell height hung in July grew to only 7.6 cm by April of the next year (Figures 5-10, 5-11).
The environment and oyster growth. Gonad development and growth of the Pacific oysters differ according to the habitat, physiology and ecology. In the southern part of Korea, oyster grows rapidly from June when the water temperature is over 20 degrees Celsius, and the rate of fattening becomes faster than the growth of shell from November. Specific gravity and speed of water current also influence the growth of oyster shell; oysters in the outer coast where water current is rapid grow more slowly than oysters in the inner coast (Figure 5-12).
Fig. 5-12. Comparison of growth of oyster in different location of growing area.
Culturing density and oyster growth. Culturing density also influences yield. With the long-line and raft culture methods, suitable size of the unit and number of strings per long-line or raft depend on the condition of the oyster ground. In the southern coast of Korea, hanging strings are set 70 cm apart.
The characteristics of growth according to culturing area. The external shapes of Pacific oysters in Korea differ with habitat (Figure 5-13), small in the western coast and large in the eastern coast. Pacific oysters from the tidal zones are generally small and slow in growth, while those in deeper waters are large. Also, cultured Pacific oysters in the southern and eastern coasts are oblong while those from the tidal zones of the western and southern coasts are oval (Table 5-5). Shell weight also varies with the water depth; Pacific oyster from the southern coast is lighter than those from the eastern coast.
|Rate of meat weight to total weight||Low||Low||High||High|
|Colour of shell||Light gray||Dark gray||Dark purple||Purplish brown|
Figure 5-13. Difference in the external appearance of Pacific oyster (C. gigas) in different regions (A, western; B, southern, C, eastern).