The Pacific cupped oyster is native to Japan and was introduced into Europe in the 1970s after the depletion of the Portuguese oyster ( Crassostrea angulata ), decimated by several successive diseases. With its fast growth and adaptability to different settings, the Pacific cupped oyster is now the most widely reared oyster worldwide, including in Europe. The other species of oyster farmed in Europe – the flat oyster ( Ostrea edulis ) – is still well below its earlier level of production after falling victim to two outbreaks of epizootic disease in the 1920s and 1980s.
Latin name – Crassostrea gigasProduction (EU-27) – 142 000 t (2007), fourth highest production level worldwide.Value (UE-27) – EUR 295 million (2007).Main EU producer countries – France (Europe’s top producer and fourth worldwide), Ireland, Spain, Portugal.Main producer countries worldwide (outside Europe) – China, South Korea, Japan.Fact sheet
A large part of global production originates with the collection of spat (oyster larvae) in the natural setting. Some spat comes from hatcheries, however, and in this case the broodstock is maintained in sea-based facilities. Groups of adults are collected at regular intervals throughout the winter and placed in tanks. This sampling is random because the oyster’s sex is not known (oysters are hermaphroditic, being male or female successively depending on the year).
The oysters are stripped of their gametes in spring using thermal shock treatment or a laceration procedure. The eggs of six or more females are fertilised with sperm from an identical number of males. To ensure proper hatching, the water must be kept at around 21°C and not be too salty. The larvae are then placed in closed-circuit tanks and fed cultured algae. Nowadays, most hatcheries focus on the production of triploids, i.e. oysters made sterile after undergoing thermal shock treatment shortly after fertilisation, which keeps them from becoming milky later.
The oyster produces large quantities of larvae in the summer. The larvae drift with the currents in search of a suitable attachment site. To collect them, the oyster farmer uses supports called collectors placed at strategic locations: plastic supports (tubes, small dishes, small strips, etc.) or curved tiles, slate stakes or shells. When the spat are formed, they are removed from the collector and are ready for rearing.
In hatcheries, when the larvae are about to settle on a support, they develop darkly pigmented spots and become more visible through the shell valves. This is when they are ‘collected’ by placing a clean solid settlement support in the tank.
Four main methods of oyster rearing are used depending on the environment (tidal range, water depth, etc.) and traditions.
• Off-bottom culture: the oysters are placed at sea in mesh bags attached to trestles set up on the intertidal ground.
• Bottom culture: the oysters are placed directly on intertidal ground.
• Deep-water or container culture: the oysters are sown in parks that can be located in depths of up to 10 metres.
• Suspended culture: the oysters are reared on ropes, like mussels, making it possible to rear them offshore. Since they are constantly submerged, they fatten more quickly. This method is suitable for rearing in waters without tides or offshore.
Pacific cupped oysters naturally feed on plankton found in sea water, which they filter constantly. They consequently can only be cultured in areas that meet certain criteria in terms of current, depth and availability of plankton, i.e. generally either close to estuaries or in lagoons or coastal ponds. The number of concessions available is determined scientifically based on the quantity of plankton available. Oysters reach commercial weight after 18 to 30 months. Harvesting methods vary depending on the type of culture: oysters grown in off-bottom culture are harvested by detaching the oyster bags from the trestles; oysters reared in bottom culture are collected at low tide using rakes or by dredging if the water level allows; oysters reared in deep water are collected by dredges that can take up to 500 kg.
In some production areas, adult oysters can be matured to give specific qualities to their meat. The oysters are placed in claires, shallow clay basins supplied naturally with sea water, where they develop a green flesh due to the presence of a specific type of algae, blue navicula. They can also be placed in maturing parks on intertidal ground where they develop firm white meat. For oysters grown in deep water, a finishing technique is used: hardening, which consists of placing them in parks where the oysters are regularly out of the water, causing them to keep their valves shut.
Most oysters are produced and consumed in France, where they are eaten a very specific way (live).
The distinctive characteristic of the oyster market is its seasonal nature: in France, half the oysters produced are consumed during the end-of-year period, between November and January. The grading scale for cupped oysters ranges from 0 to 5. The smaller the number, the larger the oyster. Fine oysters have average meat content and special oysters are meatier.
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