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ผู้เขียน หัวข้อ: Cultured Aquatic Species - Pacific Cupped Oyster  (อ่าน 1968 ครั้ง)
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Cultured Aquatic Species - Pacific Cupped Oyster (part 1)

Information on the culture of the pacific cupped oyster (Crassostrea gigas) from the FAO Cultured Aquatic Species Information Programme.
 
Identity
Crassostrea gigas Thunberg, 1793 [Ostreidae]
FAO Names: En - Pacific cupped oyster, Fr - Huître creuse du Pacifique, Es - Ostión japonés

Biological features
Shell solid, inequivalve, extremely rough, extensively fluted, and laminated; left (lower) valve deeply cupped, its sides sometimes almost vertical, the right (upper) valve flat or slightly convex sitting withing left; inequilateral, beaks and umbones often overgrown; tending to be oblong in outline but often disorted and very irregular. The shape of the shell varies with the environment. Colour usually whitish with many purple streaks and spots radiating away from the umbo. The interior of the shell is white, with a single muscle scar that is sometimes dark, but never purple or black.

View SIDP Species fact sheet

Profile
Historical background

Through its potential for rapid growth and its wide ranging tolerance to environmental conditions, the Pacific cupped oyster has become the oyster of choice for cultivation in many regions of the world. While its origins are in Japan, where it has been cultivated for centuries, it has been the subject of widespread introductions elsewhere, most significantly to the western seaboard of the United States of America from the 1920s and to France beginning in 1966. The Pacific oyster has been introduced either to replace stocks of indigenous oysters severely depleted by over-fishing or disease, or to create an industry where none existed before. The introductions list is likely to be incomplete and may not include accidental introductions made through global shipping activity, i.e. larvae carried in ballast water or adults attached to the hulls of ships. There have also been extensive, small-scale, undocumented secondary introductions from country to country. Historic methods of extensive culture, supported by wild seed capture and relaying in productive areas, have evolved over time to include a wide range of suspended (hanging culture) and off-bottom methodologies utilizing both wild and hatchery cultivated seed. Recent developments include the production of triploid seed in hatcheries and selection programmes that focus on producing faster growing, higher quality seed stock suited to particular conditions.

Main producer countries
Besides the countries shown in the map above, introductions have been recorded also in:

Ecuador, Belize, Costa Rica, Puerto Rico, the United States Virgin Islands, and Brazil.
Israel, Philippines, and Malaysia.
Romania and the Ukraine.
Seychelles.
Fiji, French Polynesia, Guam, Palau, Samoa, and Vanuatu.
Habitat and biology
The Pacific cupped oyster is an estuarine species, preferring firm bottom substrates where it leads a sedentary existence attached to rocks, debris and shells from the lower intertidal zone to depths of 40 m. However, these oysters can also be found on mud and sand-mud bottoms. Optimal salinity range is between 20 and 25 per cent although the species can occur at salinities below 10 per cent and will survive salinities in excess of 35 per cent, where it is unlikely to breed. It also has a broad temperature tolerance, with a range of –1.8 to 35 °C.

Pacific oysters are protandrous hermaphrodites, most commonly maturing first as males. In areas with good food supply the sex ratio in older oysters shows a predominance of females, whereas the reverse is true in areas of low food supply. Females can revert back to male when food supply is limiting as, for example, when they are severely overcrowded. Gametogenesis begins at around 10 °C at salinities of between 15 and 32 per cent and is rarely completed at higher salinities. Spawning generally occurs at temperatures above 20 °C and rarely at 15–18 °C. The species is very fecund with 8–15 cm length females producing between 50–200 million eggs in a single spawning.

Larvae are planktotrophic and are distributed throughout the water column. They measure 70 µm shell length at the prodissoconch I stage – following early embryonic development – and settle out of the water column to crawl, using the larval foot, to seek a suitable settlement location for attachment when 300–340 µm. This may take two to three weeks, depending on water temperature, salinity and food supply, during which time they can be dispersed over a wide area by water currents. As in other oyster species, mature Pacific cupped oyster larvae attach permanently to the chosen substrate by a cement secretion from a gland in the foot. Once settled they metamorphose into the juvenile form. Growth rate is very rapid in good conditions; market size being attained in 18 to 30 months.

Production systems
Various methodologies are used in the production of Pacific cupped oysters, relating to the source of seed supply, the environmental conditions pertaining in different regions and the type of product marketed, whether oysters for the half-shell trade or for meat extraction. Differences in approach are highlighted below with emphasis on seed supply from hatcheries.

Seed supply
Where a supply of natural spat is abundant and reliable, oyster growers set out collectors in the wild to secure their own seed supply, independent of hatcheries. Much of the global supply of spat is obtained from wild seed capture, using a wide variety of settlement materials (cultch) hanging in suspension from longlines and rafts. However, other commercial units operate hatcheries, as described below.

Broodstock

Broodstock for hatchery use is usually obtained from a commercial unit's own sea-based growout facility and is from selected stock maintained in the best possible conditions. Since the sex of adults is not known, groups of adults are obtained at regular intervals from winter through to the natural spawning season. They are held in separate flow-through tanks supplied with cultured algae supplemented seawater at 20–22 °C and salinity within the range 25–32 per cent. During the winter months, when adults are sexually undeveloped, about six weeks of hatchery conditioning is required to initiate gametogenesis and mature the gametes. A progressively shorter period applies as ambient sea temperature increases towards the spawning season.

Adults containing ripe gametes can be spawned by thermal shock treatment but most commonly they are opened and the gametes 'stripped' from the gonads with Pasteur pipettes, a procedure that produces good results. Ripe females of 70–100 g live weight will yield in excess of 50–80 million eggs. For production purposes, the eggs of six or more females are fertilised with small samples of sperm from a similar number of males. Development from the fertilised egg to the fully shelled prodissoconch I (D-larva) stage takes place in large volume tanks filled with finely filtered – and often UV-treated – seawater at 25–28 °C and 25–32 per cent. Tanks are not aerated and no food is added during early development, which takes approximately 24 hours. Much of global hatchery output now focuses on the production of triploids. Ploidy manipulation, generally by temperature shock, takes place shortly after fertilisation although the most recent trend is to cross tetraploids with diploids, which guarantees 100 per cent triploid offspring.

Larval and post-larval culture

Larvae are grown in either static water or flow-through tank systems through to the pelagic veliger stage, which lasts 14 to 18 days at 25–28 °C. Optimum salinity is between 20 and 25 per cent. Initial density is about 20 000 D-larvae/litre and is reduced to about 5 000/litre as development proceeds through natural mortality and selective grading at water changes, which take place three or four times a week for static water tank systems. The object is to retain only the faster growing, healthier larvae as settlement and metamorphosis approaches.

Larvae are fed cultured algae. Early stage veligers (<120 &micro;m shell length) are supplied with daily rations of Isochrysis galbana or Pavlova lutherii, together with a small diatom such as Chaetoceros calcitrans or Thalassiosira pseudonana. The diet of later stage larvae consists of the same species, supplemented with one or other of the various species of the larger green flagellate, Tetraselmis. When larvae are close to settlement, darkly pigmented 'eye-spots' develop, which are clearly visible through the transparent shell valves. A foot also develops at this stage. At this time, settlement substrate is placed in the tanks for the larvae to explore and attach to. Materials such as slightly roughened, black PVC sheets, fluted PVC pipes, shells or shell particles are used as settlement surfaces. Alternatively, mature larvae may be packed and shipped to oyster growers to set themselves and grow to market size at sites often at great distance from the hatchery. The latter is the method of choice on the Pacific coast of North America and the process is referred to as remote setting.

In many other parts of the world, hatcheries set and grow the resulting juveniles (spat) to a few millimetres (between 3–5 mm) in closely controlled conditions within the hatchery facility. This phase of production is usually accomplished in large volume tank systems with partial daily water replacement, in which the juveniles are held as fluidised beds in containers in a process referred to as upwelling culture. Rations of high nutritional value algae from hatchery culture or from algae bloomed in adjacent outdoor tanks or ponds are supplied continuously to promote rapid growth. Once grown to the required size the spat are either transferred to the hatchery's own outdoor nursery systems or are packed and sold to oyster growers.

Nursery

Hatcheries frequently operate either sea-based or land-based nurseries in which spat are grown from as small as 1 mm length to 12–15 mm. Densities may be as high as 100 kg/m&sup3; water flow in highly productive areas. This method is adopted to reduce mortality in small spat, which may occur if they are transferred directly to sea-based grow-out. Sea-based nurseries are generally upwelling systems mounted on barges or rafts and are located in warmer, more productive estuarine environments. Such containment minimises risk of losses through predation. Upwelling methodology is also widely used in land-based nurseries, which may be mounted on barges in salt water ponds or in on-land tank systems supplied with algae-rich water pumped from ponds. The ponds may be naturally productive or be enriched with artificial or natural fertilisers. In the case of remote setting, where spat are set on bagged shell cultch, shell strings or plastic materials, the tanks used to set the larvae are also used to grow the resulting spat for a few days or more. This is prior to the set cultch being deployed in some form of protective culture (either intertidally with mesh or canvas covers or sub-tidally on racks or in suspension) to minimise predation losses. Wild caught spat is handled in a similar way.

Yields of 10–15 mm seed for grow-out can reach 3 tonnes/ha in upwelling nurseries supplied from fertilised ponds.


http://www.thefishsite.com/articles/811/cultured-aquatic-species-pacific-cupped-oyster
January 2010
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