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Appendix 1
PLANS OF OPERATION AND TIME SCHEDULES

(1) Plan of Operation (Method A) - Collection and Culture of Native Oysters (Crassostrea cucullata)

(a) Biological notes

During the spawning season which usually coincides with periods of warm water, adult oysters release young oyster larvae (spat) into the water; here they pass through a free-swimming stage, which lasts usually for about two weeks. At the end of this period these larvae attach themselves to a suitable hard surface or collector where they remain stationary the rest of their lives.

(b) Collection of spat

When the time for attachment arrives, collectors must be already in the water to attract the larvae. The collectors must be placed in the correct water level where the tidal current carries the larvae back and forth, and at the correct time. If too early, the collectors will be covered by barnacles, mussels, other animals or seaweeds and the oyster spat will not be able to settle and will be lost. The time of placing the collectors must be carefully correlated with the beginning of the major setting period.

To determine this period it will be necessary for the oyster expert (1) to take daily record of water temperatures, and (2) to make counts of samples of swimming larvae taken by plankton net. The collectors can then be placed at the most appropriate time to ensure a maximum set.

Oyster larvae are free swimming, preferring the near surface waters and the collectors are placed in this rather narrow water layer which contains the maximum number of larvae. In the past, bamboo, tree branches, roof tiles and stones have been used as collectors but now old oyster shells strung on wires, plastic netting, ceramic tiles and plastic plates strung on wires are used. The expert will determine which types or kinds of material make the best collectors by experiment but for the initial trial collection it is suggested that if readily available, old oyster shells pierced and strung on wires be used.

Large scale collection is accomplished by suspending these strings of oyster shells 1 to 2 m in length from floating rafts (Fig. 2). The rafts may be any convenient size but are usually 15 × 30 ft (4.6 × 9.1 m) and carry 250 to 300 strings of collectors. The rafts are constructed of bamboo or wooden poles 3 to 4 in (76 to 102 mm) in diameter lashed in two layers at right angles to each other with the poles 2 to 3 ft (61 to 91 cm) apart. The rafts are buoyed by 50 gal steel petrol drums, which have been treated with tar or other preservatives. Structure of a raft larger than described here but otherwise representative is shown in Fig. 2.

(c) Culture and growing

Raft culture method

After the spat have settled on the collectors, the collectors are removed from the wires, washed and placed into cages or trays which are then suspended beneath the rafts. Each cage contains up to 100–150 oyster shell collectors, which remain in the cages where the spat continues to grow until market size is reached. During this period the cages are lifted periodically, the oysters cleaned of any harmful marine growths, and dead or stunted oysters are removed. As the oysters increase in size, they are transferred to additional cages to avoid crowding, and additional floats are added to the rafts as necessary. Representative trays are illustrated in Figs. 6 and 7.

Longline method

The longline method is a modification of the raft-culture method and is suitable to more open or relatively unprotected areas. It is primarily a technique designed to eliminate the raft; and, in principle has all the advantages of a raft but avoids the rigidity of the raft and is therefore less susceptible to damage in rough weather.

The basic structure of a unit of this type consists of a series of wooden barrels or metal drums used as floats and arranged in rows. Each end of the line is anchored with two or three anchors; on occasion the middle portion of the line is anchored. The lines are usually 200–250 ft (61–76 m) in length with 10–12 floats per line. Each line is parallelled by two nylon ropes 1½ in (38 mm) diameter about 2 ft (61 cm) apart. At each end of the parallel longlines a series of vertical ropes are suspended (nylon at ½ in (13 mm) diameter) and from 20–30 ft (6.1–9.1 m) in length depending upon depth of water. This is done after the collector shells with their attached seed oysters have been placed on the ropes for further growth. The collectors are separated by a piece of bamboo or plastic pipe about 6–8 in long (15.2–20.3 cm). The vertical ropes do not reach the sea bottom at any time and are hung about 3 ft (91 cm) apart along the supporting lines between the barrels. Representative longline construction is illustrated in Fig. 3. Dimensions are larger than described here but basic structure is similar.

Rack method

This is a shallow water modification of the hanging method. A framework or rack is constructed by driving poles 6–9 ft long (1.8–2.7 m) into the ground in a line 3–6 ft (0.9–1.8 m) apart and connecting them by horizontal poles spaced at 2 ft (61 cm) apart. The cages are then suspended from the horizontal poles at the depth desired. Fig. 4 illustrates basic structural dimensions.

While these methods are among the most widely used and give highest yields the expert may wish to try other methods for collecting and rearing the native oysters depending upon local conditions and his previous experience.

(2) Time Schedule of Operation for Culture of the Native Oysters, Crassostrea cucullata Method A

The expert should be engaged in time to arrive during the latter part of the year which is assumed to be at least two months prior to the beginning of the spawning season of the native oyster. While this period of spawning has not yet been definitely established, from observations made during the survey it is believed that spawning occurs during the later summer months, i.e. January, February, March, when water temperatures are presumed highest.

It is recommended that operations be carried out initially at Anse à la Mouche.

The expert by making further observations and examining the gonads of the local oysters, analysing plankton samples and taking periodic water temperatures, etc. will be able to establish this period with certainty and to determine the time and placing of the collectors.

These collectors can be fabricated while the expert is conducting his pre-spawning observations and must be completed and ready to place in the water at the beginning of the major setting period.

When the oysters which have settled on the collectors reach seed size (1–1.5 cm; anticipated within about a month) the collectors are removed from the wires, washed and cleaned of excessive marine growths and placed in the trays which are suspended from rafts. The correct depth at which the trays are suspended from the rafts must be determined by the expert. When the optimum depth has been located, then all of the trays will be held to this depth. As the young oysters grow in size they are separated from the collectors and distributed among additional trays where they will remain until reaching marketable size. Some of the collectors may be restrung and placed on the longline, others can be suspended from the racks.

The collectors that are suspended from longlines and racks are strung on nylon ropes, each collector separated from the next by a piece of plastic pipe or hollow bamboo 6 inches to 8 inches in length (15.2–20.3 cm) threaded on the nylon rope. These lengths of nylon are then attached to the longlines or suspended from the cross-bracing of the stationary racks.

All rafts, longlines, racks, etc. must be monitored periodically according to a regular maintenance schedule determined by the expert and records of growth rates, mortalities, etc. should be kept.

Constant surveillance will be required to ensure the rafts are not damaged or break loose during periods of rough or stormy weather and provision must be made to remove the rafts and oysters from the water or to move them to protected areas in event of violent storms.

It is reported that the native oyster takes about five years to reach marketable size under present natural growing conditions. The methods as recommended in this project, however are expected to reduce this time and it is hoped that the first crop could be harvested in 2½ to 3 years.

Collectors must be set out each year during spawning season so that after the initial growing period which it is anticipated will be in 2½ years for the first crop, oysters would then be available on a regular yearly basis, provided of course that the native oysters continue to reproduce in adequate numbers during the spawning season.

While initial experimental work would be concentrated at Anse à la Mouche, additional collecting and growing experiments would be conducted on a smaller scale according to time and facilities available at the other areas recommended.

(3) Plan of Operation (Method B) - Culture of imported oyster seed: the Japanese oyster, Crassostrea gigas

(a) Biological notes

The most recent and important advance in oyster culture has been the development of “cultch-free” seed oysters on a large scale commercial basis by a hatchery in California. The most significant feature of the process is that the young seed oysters are prevented during their development from attaching to a substrate. During larvae development young oysters undergo a swimming stage which usually lasts about two weeks and, nearing the end of this phase, settle down on a solid object, become attached and remain there for the rest of their life. However if the young oyster is prevented from becoming attached during this period (which lasts but a short time), it never will. In nature the young unattached oyster usually sinks to the bottom and dies in the mud. In the hatchery however the young oysters do not become permanently attached to a substrate but continue to grow and develop as “free” oysters. At this stage they are complete but only miniature oysters about the size of a grain of wheat. They can be shipped conveniently and inexpensively by air in large numbers and transplanted in suitable areas.

This simplified but effective method of oyster culture has a number of advantages.

  1. The initial cost of the seed oyster in large and small numbers can be shipped via air at low cost.

  2. Growers are not dependent upon the uncertainties of natural spawning of the local adult oyster population for supply of young seed oysters.

  3. The most suitable species of oyster for local growing and market conditions can be obtained.

  4. The necessity of setting out spat collectors is eliminated.

  5. Hatchery produced oyster seed can be obtained at any time throughout the year and growers can regulate their harvesting to coincide with most favourable marketing conditions.

  6. Growers by importing seed oysters over an extended period can adjust their production so that harvesting of marketable size oysters can be obtained on a year round basis.

  7. Mortalities are practically eliminated as the greatest losses occur during the free-swimming larvae stage - enabling the grower to harvest up to 95 percent of the original seed oyster shipment.

  8. The method allows small scale industries or cooperatives to engage in large scale oyster culture with a low investment and high return.

  9. Operation and maintenance are simplified and can be performed by unskilled labour with only minimal supervision.

Since the Japanese oyster (C. gigas) is a “cold” water oyster it is not expected that it will reproduce to any extent in the constantly warm waters and high salinities of the Seychelles. It will in all probability be necessary to import additional seed for each annual crop. However as already mentioned seed can be obtained in whatever amounts required and at any time of the year.

(b) Outline of culture methods

Seed oysters may be imported from Pacific Mariculture Oyster Hatchery1, California, U.S.A. via air to Seychelles. These should be ordered as soon as facilities are ready to receive them.

On arrival seed oysters are placed in the specially constructed containers or trays, (approximately 5 000 per tray) which are then suspended from rafts.

Some of the oyster seed may be placed on trays or tables in other protected areas for survival and growth rates to be compared with those reared by raft culture.

Trays containing oysters will be monitored daily for the first two weeks, three times per week for the following two weeks, twice per week for the next two weeks and once per week thereafter (or whatever interval is found necessary).

1 This is the only source of “cultch-free” seed at this time, but another hatchery has been established in France.

Trays are checked to see that:

  1. Seed oysters are not being eaten by pests such as snails, starfish, crabs, eels, etc.

  2. Algael growths are kept at a minimum. This will require periodic cleaning of the containers; time and frequency of cleaning operations to be determined from field examinations. The cleaning of the trays is very important and must be faithfully performed.

  3. Rafts, containers and trays are not broken or damaged by adverse weather or sea conditions.

  4. Precautions and vigilance are maintained against loss by theft and poaching.

Records will be kept of all expenses and cost analysis will be made of the entire operation.

Records will be kept of the results including rates of growth, survival rates, frequency of transfers, occurrence and effects of pests, algael growths, etc., and comparisons made among the several experimental methods and areas to determine which produce the best results.

(4) Time Schedule of Operation for Imported Seed Culture Experiment (Method B)

Seed oyster should be ordered as soon as possible. They are available at any time of the year but should not be delivered until all preparations have been completed to receive them (i.e. adequate numbers of trays or cages are available and rafts have been constructed and are in position).

At least 30 000 of the initial order for 50 000 seed should be placed in Anse à la Mouche. The remainder may be placed in some of the other areas to enable the expert to compare survival and growth rates.

To accommodate 30 000 oyster seed seven to ten trays of approximately 2 ft × 2 ft × 3 in (61 × 61 cm × 76 mm) and one raft 15 ft × 30 ft (4.6 × 9.1 m) will be needed. Within one or two months it is estimated that the size of the 30 000 seed oysters will probably have doubled and will require transfer to additional trays. Double the number of trays are generally required for each transplant. Although there are no data available on the growth rate of C. gigas in this area it is anticipated that three to five additional transfers will be necessary and that growth will be relatively fast. These oysters could reach a marketable size of 3–3½ in (76–89 mm) in 10–15 months. In Californian waters which are considerably cooler than those of the Seychelles, this species reaches 3 in (76 mm) in approximately 18 months when grown on the bottom of bays and estuaries; in Japan the usual period is three to four years.

For the final transfer a total of 300–400 trays and 30–40 rafts 15 × 30 ft (4.6 × 9.1 m) will be required. Larger rafts can be constructed if weather and sea conditions allow, based on dimensions provided in Fig. 2.

When ready for market the oysters can be sold and from the returns cost estimates can be calculated. From the resulting cost analysis a feasibility report can be issued and a project evaluation made.

If the imported seed appears to be growing well and no major difficulties are encountered during this initial trial period, another order should be placed for as much seed as can be conveniently accommodated. The second order could be placed about six months after the first seed had been received, although if growth was very rapid it could be ordered sooner. There would be then the possibility of having the first crop of marketable size oysters in approximately one year with another coming up six months or so later. However these decisions can be left to the expert as the appropriate time based on the results of his experiments.

Figure 1

Figure 1

AREAS OF NATIVE OYSTERS (Crassostrea cucullata)
Reported by survey - Drawn by A. Hosanee

TOP VIEW

Figure 2

SIDE VIEW

Figure 2

ARRANGEMENT OF RAFTS IN SERIES

Figure 2

Figure 2

RAFT CULTURE

Dimensions of rafts used for oyster culture in Hiroshima, Japan. Specific details of construction can be varied according to local conditions both as to structure, size and materials.

For this project the initial experimental rafts of 15 × 30 ft2 (1.4 × 2.8 m2) should be constructed. These are easier to manage than the size illustrated here as well as simpler to construct.

The following are specifications for construction of a raft of approximately 50 m2 in area. From Cahn, U.S.F.W.S. LEAFLET 383.

MaterialSpecificationQuantity
PolesBamboo 7-5 m long. Pine etc. 30–40 cm diameter. 30
 Eucalyptus 
Nails12 cm long. 15 cm long       2 kg
AnchorsCement blocks 1 × 1 × 2 ft (30 × 30 × 61 cm)  4
Rope (Nylon)2 cm diameter     60 kg
Rope (Straw)1 cm diameter     20 kg
FloatsMetal petrol drums 50 gal each10
Preservative for floats         5 gal

Figure 3

Figure 3

FIVE RAFTS of approximately 15 × 30 ft (4.6 × 9.1 m) made into single unit for culturing pearl oysters. (Floats are wooden barrels. Toba, Japan - Photograph K.W. Cox - May 1971)

Figure 4

Figure 4

THE LONGLINE METHOD

The longline dimensions as illustrated may be adapted according to local conditions. The significant factor is the proportion of the unit. Nylon rope may be substituted for straw rope, and 50 gal petrol drums for wooden barrels.

If used as a collector, the shells can be strung on nylon line (¼ in diameter) and separated by a spacer of bamboo or plastic. The technique is the same for growing seed oysters collected by the hanging cultch (Method A). The collectors are strung on nylon line separated by spacers, the length of the line depending upon the depth of the water.

A KUSATSU, HIROSHIMA PREFECTURERACK METHOD
OF
OYSTER CULTURE
Figure 5Figure 5
B MATSUSHIMA-WAN, MIYAGI PREFECTURE
Figure 5

Figure 5
JAPANESE METHODS OF RACK CULTURE

Size of the rack to be constructed will depend upon local conditions and requirements. The list of materials provides for a rack covering an area of approximately ¼ acre (0.10 ha).

SPECIFICATIONS FOR RACK

MaterialSpecificationQuantity
PolesDiameter 6–7 cm     250
Bamboo, Pine, Eucalyptus
Length 2 m     100
Rope (Nylon)1 m lengths. 5 cm diameter  1 200
Rope (Straw)3 rope twist 
Spacers6–8 in long (15 × 20 cm) 
Bamboo or plastic
  
CollectorsShells20 000
WireNo. 10 – 40 cm length  1 200
NailsNo. 12 – 5 cm length            32 kg

Figure 6

Figure 6

OYSTER CULTURE TRAY WITH WOODEN FRAME, DIVIDER AND NYLON ROPES MESH SIZE ½ INCH

Figure 7

Figure 7

SPECIFICATIONS FOR TRAYS


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