NICAboard

Aquaculture and Fisheries News => Aquaculture News => ข้อความที่เริ่มโดย: aranya ที่ มกราคม 16, 2010, 12:22:50 PM



หัวข้อ: Cultured Aquatic Species - Pacific Cupped Oyster (part 2)
เริ่มหัวข้อโดย: aranya ที่ มกราคม 16, 2010, 12:22:50 PM
Ongrowing techniques  

Grow-out is almost entirely sea-based. A variety of bottom, off-bottom and suspended culture methods are used, depending on the environment (e.g. tidal range, shelter, water depth on leases, water exchange rates in bays and estuarine inlets, the nature of substrates, etc.) and tradition.

Growth is rapid between 15–25 °C and at salinities between 25 and 32 per cent. It is dependent on the rate of replenishment of the natural phytoplankton food supply. Pacific cupped oysters will take from 18-30 months to reach a market size of 70–100 g live weight (shell-on). Yields from extensive lease areas (covering thousands of hectares), which are used for all aspects of grow-out, including seed collection, the nursery and growing phases, and for hardening oysters prior to harvest, can yield 25 tonnes/ha/yr. Much greater yields (>70 tonnes/ha/yr) can be obtained from well spatially separated, small area leases.

Bottom culture  

Seed can be sown on suitably firm intertidal or sub-tidal ground, which may be hardened by the pre-application of shell or gravel, at densities of 200–400/m² when 1 to 2 g live weight, with predator-proof protection (fences or net covers). Alternatively, they can be sown without protection at ~200/m² when 10 g live weight. The object is to sow at densities that will require no further husbandry until the oysters reach marketable size.

Off-bottom culture  

Seed are contained in mesh bags or perforated plastic trays of various types attached by rope or rubber bands to wood frame or rebar steel trestles on suitable ground in the low intertidal zone. Such systems are sometimes located sub-tidally but this adds to handling costs. Off-bottom culture may be used for the intermediate nursery phase of growth or as a method to grow product to market size. 10–15 mm seed can be stocked at 1 000–2 000 per 0.25 or 0.5 m² base area trays and need regular maintenance and servicing to transfer at lower density to clean bags/trays of increasing mesh size as they grow. Growth rate slows substantially once the biomass of oysters exceeds 5 kg/m² tray area in reasonably productive areas.

Suspended culture  

Three-dimensional containment units are used in hanging culture, which is from longlines (most commonly) or from rafts. The units can be strings or wires of shells to which spat have attached, or they can be series of nets, mesh bags or plastic trays strung together and suspended vertically from the horizontal lines or rafts. This form of culture is used in deeper waters and the same stock densities per net or tray apply as for off-bottom methods. Care needs to be taken to sink units to water depths where fouling organisms are less prevalent and to avoid them touching the bottom at low water. Regular maintenance and servicing is required, to transfer growing oysters at lower density to clean nets/trays of increasing mesh size as they grow.

Floating Culture  

Wood frame trays with mesh bases or perforated plastic trays provided with buoyancy collars (styrofoam) are used in some locations for earlier stages in the growth of seed. The tops of such units need to be covered with a canvas or tightly woven mesh to exclude light.

Hardening  

Pacific oysters grown in suspended culture are commonly 'hardened' for a period of up to three or four months prior to harvesting. The process of hardening allows daily periods of exposure to air and generally takes place in the intertidal zone or in shallow water where tidal range is sufficient. Aerially exposed oysters have higher meat content and better keeping qualities once harvested.

Harvesting techniques

Oysters are usually harvested at >75 mm shell length when 70–100+ g live weight. Harvesting from bottom culture is by hand raking and picking or by dredges when intertidal beds are submerged, or by dredging sub-tidal beds. Marketable oysters grown in off-bottom rack culture or in trays or nets suspended from longlines are harvested from small boats or self-propelled barges, which are often equipped with mechanical washing and grading machinery when the product is destined for the half shell trade. When grown in areas where Paralytic Shellfish Poison (PSP), Diarrhoeic Shellfish Poison (DSP) and other neurotoxins from harmful algal blooms may be present at certain times of the year, harvesting may be halted during such occurrences. Oysters are not usually harvested for a period of two or three months following spawning, for reasons of poor meat quality.

Handling and processing  

Smaller individuals may be sold as speciality 'cocktail' oysters. They are either sold live in shell or are shucked and processed frozen, smoked and canned in oil, dried, extracted for oyster sauce, or prepared as other value-added products.

Oysters grown in faecal coliform-free waters require minimal processing. They are washed and graded for size and shape and, if from bottom culture, may be held in tanks in a flow of seawater to rid themselves of sand and mud trapped in the mantle cavity. If faecal coliform contaminated, oysters will require a period of depuration in recirculation systems equipped for UV-light or ozone sterilisation. Single oysters destined for half shell markets are packed with the cupped valve down in order to conserve mantle cavity water. Shelf life is seven to ten days when properly packed and stored in cool conditions. Vacuum packed, value-added products in the half shell have been marketed in Europe with some success, although the bulk of trade is for the live product. In many other countries harvested oysters are transported to shucking plants for meat extraction, bottling, canning or freezing, or are used in the preparation of products such as oyster sauces. Processing is strictly controlled in most countries.

Production costs

Information on production costs is difficult to obtain, not only because the information is proprietary but also because of site specific factors, the diversity of methods used and the widely varying levels of technology employed. Seed supply is generally calculated to be in the vicinity of 20–25 per cent of total costs. In developed countries, 3–4 mm hatchery-produced seed is sold at US$10–15/1 000, while mature larvae for remote setting will cost approximately US$2million (ex transport costs). Culturing oysters for the shucked meat trade is less labour and equipment intensive than producing oysters for half shell markets, and bottom culture is less costly than suspended (hanging) culture. Feed does not figure in the cost equation because it is a free resource once seed is transferred to sea-based grow-out. Labour is a major recurring cost and is generally of a seasonal nature.

Diseases and control measures  
In contrast to other oysters of importance in commerce, and despite its widespread distribution around the world, few disease problems of major significance have been reported for the Pacific cupped oyster. Of greater concern have been incidences of 'summer mortality' on the Pacific coast of the US and in France, which appears to be related to post-spawning physiological stress in warm water when oysters are densely crowded. Introductions of Pacific oysters are not thought to have brought pathogens with them that have resulted in catastrophic diseases in indigenous bivalves. However, their transport to some countries for direct relaying in the sea has been inadvertently accompanied by a number of pests and parasites including the Japanese oyster drill, Ceratostoma inornatum, the oyster flatworm, Pseudostylochus ostreophagus, and the copepod parasite, Mytilicola orientalis. Bacterially related diseases of larvae and early juveniles are not uncommon in hatcheries and are most frequently attributed to Vibrio spp. In this context, C. gigas larvae are equally as prone to mass mortalities as are the larvae of other bivalve species.

Production statistics
By 2003, global production of this species had expanded to 4.38 million tonnes, more than any other species of fish, molluscs or crustacea. Nearly 84 per cent of global Pacific cupped oyster production was in China. Two other major producers had a production of over 100 000 tonnes, were Japan (261 000 tonnes), the Republic of Korea (238 000 tonnes) and France (115 000 tonnes). The only other countries producing more than 10 000 tonnes in 2003 were the United States of America (43 000 tonnes) and Taiwan Province of China (23 000 tonnes). The value of the global production of this species in 2003 was US$3.69 billion.

Market and trade  
Much of the production of the major producing countries is absorbed by domestic markets and is supplemented by imports from adjacent countries and trading partners (e.g. trade within the EU, where France imports surplus from other EU countries such as the United Kingdom and Ireland). The relatively short shelf life of this species is an impediment to large-scale global trade for fresh product, and consumer preference is often for live, half shell oysters or freshly shucked meats. Value-added and convenience products, including canned oysters and frozen or vacuum packed oysters prepared with various sauces, appear from time to time and have potential for global distribution. However, they represent only a small proportion of total production. There continues to be international marketing potential for hatchery-produced seed, particularly for triploids.

Status and trends  
Worldwide aquaculture production of the Pacific cupped oyster continues to expand steadily, having expanded from 156 000 tonnes in 1950 to 437 000 tonnes by 1970, and 1.2 million tonnes by 1990. Expansion was very rapid in the 1990s, rising to 3.9 million tonnes by 2000. Expansion is continuing, reaching nearly 4.4 million tonnes by 2003. Production is likely to continue to expand, albeit at a slower rate due to coastal urbanisation and the increasing need to share the common coastal resource with other users.

Main issues  
Unlike many other species in aquaculture, the reliable supply of seed is not a constraint to further development. It is either readily available for capture in the wild or can be produced relatively inexpensively in massive quantity in hatcheries. Of great concern is the potential for environmental degradation, which already exists in some areas of major production, and also the potential for the Pacific oyster to out-compete and take over habitat occupied by indigenous bivalves in countries to which it has been introduced. Pacific oysters have great capacity to filter large volumes of seawater and thereby, in intensive culture, deposit large quantities of bio-wastes. They form dense reefs in areas where they breed naturally and the reefs act as sediment traps by slowing bottom water flow and at the same time altering biodiversity. Hanging culture methodology lessens these environmental impacts.

Introductions, whether deliberate or accidental, into New Zealand and New South Wales, Australia, have highlighted the potential to displace native species. Pacific cupped oysters were first positively identified in the Auckland area of North Island, New Zealand in 1971 By 1977 Crassostrea gigas had become the dominant farmed oyster, having displaced the native Rock oyster (Saccostrea glomerata), through competition for settlement space and by virtue of their greatly superior growth rate. Similarly, the accidental introduction or transfer of C. gigas into bays and estuaries in New South Wales, where an important aquaculture production of the native Sydney Rock oyster, Saccostrea commercialis, exists, has been treated with grave concern by producers, Government and environmentalists alike.

Responsible aquaculture practices  
A number of important issues have been identified above which are being addressed in the development of more responsible and sustainable practices in the production of this species. These are very much in line with the FAO Code of Conduct for Responsible Fisheries (CCRF) and include limiting lease areas within bays and estuaries to keep within the carrying capacity of the waters. With the advent of guaranteed 100 percent triploid production, continually improving fish health awareness and control, and the ability to transport eyed larvae anywhere in the world for remote setting, the potential exists to develop production in areas and countries where the species has not previously been grown before. Triploids pose little threat to native species through reproduction and competition if culture follows environmentally acceptable guidelines.

Pacific oyster culture is well-suited to small family businesses, cooperatives or regional industry, and the grow-out production phase can be practised by relatively unskilled labour with minor investment in equipment and infrastructure, compared to shrimp or fish culture.

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