Oysters are bivalve molluscs, and have a lot going for them – they are delicious to eat, packed full of essential vitamins and minerals, and as super filter feeders, they are also very good for the environment.
Around the world, there are more than 12 farmed species of oyster and over 200 different varieties. The two main types of oyster used in aquaculture are cupped oysters, which are Crassostrea or Saccostrea species and flat Ostreacea oysters.
The main farmed species are:
- European flat or native oyster, Ostrea edulis;
- Pacific, Japanese cupped or rock oyster, Crassostrea gigas, recently renamed Magallana gigas;
- Kumamoto oyster, Crassostrea sikamea;
- Atlantic oyster, Crassostrea virginicas;
- Sydney rock and New Zealand rock oyster, Saccostrea glomerata;
- Olympia oyster, Ostrea lurida
Global oyster production of all species in 2018 was 6 million tonnes, with China producing around 85% of the total. Pacific oysters are the most important species.
Oysters have been cultivated commercially for centuries, and were once a staple food of the poor, rather than the more expensive treat they have become today. They were produced by both the ancient Greeks and the Romans, and in the modern world, oyster cultivation originated in Japan. It is from here that the Pacific oyster became introduced to North America, Australia, Europe and New Zealand.
In the wild, Pacific oysters are typically found in warm temperate regions and they are native to the NW Pacific and the Sea of Japan. They are mainly found in shallow brackish estuaries but can live down to a depth of around 40m. The oysters attach naturally to firm substrates on the sea floor, to rocks, other shells and harbour walls.
Pacific oyster cultivation was introduced to Western America in the 1920s, France in 1966 and the UK in the late 1960s. This species was introduced predominantly to replace depleted wild stocks of native oysters, which were dying out through overfishing and disease.
The adaptability of oysters makes them a good species for aquaculture, and because they are filter feeders, they rely totally on natural plankton in the sea for feed.
Farmed oysters start life in a hatchery, where they are spawned and hatched in a controlled environment. Once they are big enough, they are moved to nursery tanks or ponds until they are large enough to be transferred to sea.
A variety of different methods are used to ongrow oysters. Some oyster farmers use mesh bags laid on low trestles staked to the beach in the intertidal zone, some use bags, trays or cages suspended beneath longlines, and others place them directly on the seabed.
Impacts of farming Oysters
Oysters are generally considered to be good for the environment and have been shown to have a positive impact on the surrounding marine environment by providing valuable habitat ecosystem services to other species. The environmental footprint of bivalve aquaculture is lower than many arable crops in terms of greenhouse gas emissions, land and freshwater use, and just 11 tonnes of greenhouse gassed are emitted per tonne of bivalve protein, compared with 3340 tonnes of emissions per tonne of edible beef.
As filter feeders, they remove algae and excess nutrients from the water, but when too densely stocked, there is a danger that too many nutrients may be removed.
There is also concern that when farmed oysters start to breed naturally in an area, they can significantly alter diversity, community structure and ecosystem processes.
ASC certification demands that farms adhere to strict guidelines to ensure they produce a healthy product, while minimising any negative environmental and social impacts associated with their operations.
ASC certified oyster farmers work hard to improve standards in their industry, work with their local communities, and respect their workers and the environment.
ASC certified oyster farms must minimise their impact on their local ecosystems in a number of ways, such as ensuring that farms are not sited in areas with key biological or ecological functions. Farms must implement environmental management plans to ensure they are not adversely affecting the ecological integrity of the area in which they are located. No harm to threatened or endangered species or their habitats is allowed.
ASC certified oyster farms that use wild seed must procure the seed from well-regulated natural sources.
ASC certified oyster farms are required to manage the organic deposits on the sediment beneath the farm in a responsible manner, although no feed input is needed, so this is minimal. Sulphide levels in the sediment need to be measured at regular intervals and remain within set limits.
ASC certified oyster farms must adhere to rigorous requirements to minimise disease outbreaks. Certified farms are not allowed to use harmful pesticides. If chemicals are used, only those that do not harm the marine environment are permitted. Farms must also ensure that actions to prevent diseases do not cause harm to endangered species or have a permanent impact on critical habitats.
Any form of forced or child labour is strictly prohibited under ASC certification, based on the core principles of the International Labour Organisation (ILO). All ASC certified farms are safe and equitable working environments where employees earn a decent wage and have regulated working hours. Certified farms must engage with local their local communities and have a community stakeholder complaints process in place. They must also inform communities about potential health and safety risks or changes in access to resources.
Cooking with oysters
Oysters are traditionally eaten raw straight from their half shell, often accompanied by Tabasco sauce, a shallot and vinegar dressing, or a simple squeeze of lemon juice. Larger oysters lend themselves well to cooking and are very good topped with garlic butter breadcrumbs then grilled, or added to a steak pie.