Sources and Quantities
Across the world farmed oysters include over twelve different species (native and introduced)1 as shown in the table opposite. Oysters are bivalve molluscs and feed by filtering mainly microscopic algae (phytoplankton), but also some organic detritus from sea water. Oysters grow naturally on the seabed or by attaching to a hard substrate.
Most species of oyster used in aquaculture can be found naturally in marine and brackish areas close to the coast in shallow depths. Their ability to close their shells when environmental conditions are unfavourable allows them to survive when salinity drops or tidal levels leave them exposed. Their adaptability makes the oyster a good species for aquaculture. Oysters can be farmed in locations which are accessible at low tide which often makes expensive farming equipment or service vessels unnecessary.
There are two distinct types of oyster used in aquaculture; the ‘cupped’ oysters (Crassostrea/Saccostrea species) and the ‘flat’ oysters (Ostreacea):
As the map shows, oysters are farmed across the globe. Aquaculture production of oysters has increased substantially since 1990; from 1.2 million tonnes to 5.59 million tonnes in 2016 with a value of US$6.6 billion. China accounts for around 86% of globally farmed oysters and consumes almost all of its own production1, 5.
Annually only around 50,000 tonnes of oysters are internationally traded. The majority of oyster exports originate from Korea, France and China. In the EU, France, Spain and Italy are the main importers of oysters, while France is also among the top exporting countries5.
Less than 5% of total world bivalve production enters international markets; one of the lowest proportions in seafood trade. This is due to the very nature of bivalves, which are highly perishable and potentially risky for human health if not properly handled. The relatively short shelf life of oysters is an impediment to large-scale global trade as 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, do appear and have potential for global distribution; however, they represent only a small proportion of total production5.
Domestic Market Information6, 7
From 2008 to 2018 Great British retail sales (i.e. in England, Scotland and Wales) of oysters decreased in value and volume by -7.8% and -19.6% respectively from a base of £1.37 million and 69.6 tonnes in 2008.
In 2018, UK retail sales of oysters were worth £1.3m (-4.4% compared to 2017) with a volume of 57 tonnes (-12.1%), average price £22.53 per kg; ranking as the 39th most popular species by value in the 52 weeks to 16/06/2018 (including discounters).
In 2018, the UK imported 394 tonnes of live oysters.
Note: the difference between the volume of oysters sold in UK retail and that which is imported is due to its use in the foodservice industry (e.g. restaurants) (no data available) and that which is re-exported.
The Japanese have been cultivating Pacific oyster for centuries. The species has been introduced elsewhere, most significantly to the western seaboard of the US in the 1920s, France in 1966, and to the UK at various times in 1960’s and 70’s4, 8. 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 before9, 10, 11.
Introductions of the Pacific oysters, either intentionally or not (e.g. carried in ballast water or attached to ship hulls) have seen the development of small fisheries with the naturalisation of the species12. There are several incidences globally where introductions of new species of oyster for aquaculture are out competing native oyster species (e.g. Australia, UK, Netherlands and New Zealand)4, 13. Although perceived as a nuisance in some environments, it is also considered a valuable resource by others.
Oyster aquaculture includes a wide range of grow-out techniques in inter-tidal or sub-tidal waters: suspension of oysters in the water column via rafts, floats, racks or trestles, or bottom culture where oysters are grown directly on the seabed14, 15. Methodologies, utilising both wild and hatchery cultivated seed, are shown in the schematic opposite.
Wild oyster spat (seed) can be collected where it is abundant and reliable. A common method is to use ‘cultch’ (e.g. oyster shells) to act as a surface for the oyster larvae to settle on. When they have grown to a few millimetres, they are removed from the collectors and are ready for on-growing. Much of the global supply of Pacific oyster seed is obtained from wild seed capture14.
European oyster production however is commonly derived from hatchery-reared seed. Oyster broodstock is maintained in marine facilities, where their fertilised eggs are developed to larvae in large tanks filled with filtered (and often UV-treated) seawater, and fed cultured algae. Generally the larvae will be encouraged to settle and then grown until they are large enough to be transferred for on-growing.
On-growing of oysters is almost entirely sea-based; the type of rearing method depends on both the environment (tidal range, water depth, etc.) and tradition. No feed is supplied and no chemicals or medicines are administered.
In Europe, along France’s Atlantic coastline for instance, oysters are mainly produced by ‘off-bottom culture’, i.e. placed in plastic mesh bags attached to trestles or in rigid, perforated containers such as the Ortac system16. ‘Suspended culture’ is where oysters are reared on ropes; a technique suitable for rearing in sheltered waters. Countries such as Spain use this technique.
Growth of farmed oysters varies depending on temperature and natural food supplies, but Pacific oysters will take 18-30 months to reach a market size of 70-100 g live weight (shell-on).
Governance systems play an important part in ensuring environmental sustainability, and whilst these have evolved rapidly with the growth of the industry, there are differences between regions and countries. Poor governance can result in industry stagnation, the spread of preventable diseases, environmental damage and opposition to aquaculture activities by local communities and groups such as non-governmental organisations (NGOs). Key governance responsibilities are ensuring environmental assessment and decision making processes are in place for sensitive and coastal ecosystems, which help deliver sustainable aquaculture whilst managing possible adverse impacts. Other regulatory and governance aspects should cover aspects such as water abstraction and discharge, health monitoring, and so forth.
Four principles – accountability, effectiveness and efficiency of governments, equity, and predictability of the rule of law – are necessary for effective aquaculture governance. These principles should guide the administration, legislative and regulatory framework of aquaculture. In addition to governments, other stakeholders such as communities, non-governmental organisations (NGOs) and producers should also be involved in the governance of the industry1.
The vast majority of oyster production is undertaken in three global regions: Asia-Pacific, North America, and Europe.
While many countries in Asia-Pacific have made commendable efforts to set up policies, administrative, legal and regulatory frameworks to properly develop and manage aquaculture, some countries in the region are still lagging behind. However, many Asia-Pacific regional countries (e.g. Australia, NZ) enjoy established strong aquaculture governance structures (policies, institutions, regulations, etc.) in support of sustainable development and management of aquaculture at all levels.
National and provincial/state governments in both Canada and the US have strategies for the development of aquaculture, and governance systems are highly evolved. The thrust of aquaculture development in Canada is focused on environmental sustainability. In the US, development is also geared toward sustainability with offshore expansion.
With the notable exceptions of the major European aquaculture producers Norway, Russia and Turkey, the shaping of regulations and the instruments for development and investment in European aquaculture falls under the European Union (EU), and are highly evolved. The principal frameworks for EU aquaculture is the Common Fisheries Policy (CFP) as well as the EU Blue Growth Strategy, intended to stimulate and guide aquaculture development in Europe which is environmentally, socially and economically sustainable. In non-EU member states there are largely equivalent policies.
Although there has been criticism surrounding bivalve/oyster aquaculture5, generally the impacts of oyster farming are seen as low and relatively benign6, 7, 8. Impacts can include: seabed deposition of solid wastes such as pseudofeces; changes to biodiversity; the depletion of phytoplankton for other species to eat; and the reduction of light reaching the sea bed. The positive impacts and benefits that bivalve/oyster aquaculture can have on marine ecosystems, include:
Feed is generally perceived to be one of the major risk factors in aquaculture production of fish and crustacea. However oysters consume food that occurs naturally in the environment and are not supplied with commercial aquafeeds; also they are not treated with chemicals or veterinary medicines unlike in other forms of aquaculture. Excluding seaweeds, one-third of all farmed seafood, some 20 million tonnes annually, is produced without additional feeding. The most important non-fed animal species, apart from bivalve molluscs (mainly clams, oysters, mussels and scallops), include two finfish species (silver carp and bighead carp), as well as other filter feeding animals such as sea squirts11.
These positive, natural and ‘ecosystem service’ aspects of bivalve aquaculture in general are increasingly being seen as major factors to promote their culture as the most environmentally sustainable type of seafood production11. As such, increased mussel aquaculture is anticipated, including the potential for increased production in the UK12. The Seafood 2040 Strategy for instance, highlights bivalve aquaculture as an opportunity to generate sustainable protein for domestic consumption or export, provide employment in fragile coastal communities, whilst offering significant ecosystem services13.
As bivalves are farmed in open marine environments, and because there are no treatment or vaccination options, disease prevention is essential. Work continues to improve understanding of bivalve diseases and develop innovative solutions and tools for their management and prevention14, 15.
Site selection for cultivating oysters is extremely important and factors include substrate and/or depth of water, salinity, temperature, exposure to air, wind and currents, sedimentation rates; and food availability.
Buyers should seek assurances that all national and local laws are adhered to. All farms should have the required licences, permits and registrations in regards to their site and its operations accompanied by documentary evidence to demonstrate this compliance. Oyster operations may be managed to minimise site operations during peak sensitive periods or in times of low water quality episodes. Farm leases and permits can stipulate sustainable management practises.
In considering the environmental impacts of oyster aquaculture, it is important to view both the scale of the sector as well as the diversity of production systems1. The impacts of oyster aquaculture on the environment are often considered less than those of finfish and warm water prawn culture.
Bivalves are considered keystone species in the ecosystem and therefore they can affect the surrounding environment in various ways2. At all scales of bivalve production, their culture and the physical on-growing structures to support them may produce changes in water movement and sediment dynamics that can affect both planktonic and seabed communities. However, the on-growing structures may also act as new habitat, and nursery areas for fish crustaceans and molluscs3.
Operations associated with the growing and harvesting of oysters are relatively low impact in terms of activities that might lead to amenity and wildlife disturbance. One of the greatest potential impacts of cultivating filter feeders such as oysters is the net loss of energy (i.e. phytoplankton) from the ecosystem. Large monocultures, particularly in enclosed bays with limited water exchange, may exceed the carrying capacity in that area (e.g. the supply of planktonic food) and thus affect all aquatic organisms including the farmed oysters themselves. Conversely, as oysters are primary consumers they can potentially mitigate impacts of nutrient enrichment (e.g. from land-based discharges and run-off), which can lead to eutrophication of coastal waters.
Coastal areas and estuaries where oysters are farmed are often sites of ecological and high amenity use, and therefore any large-scale cultivation may have impacts such as disturbing shore bird feeding sites, or on local navigation. Marine aquaculture operations may also have an aesthetic impact.
In many countries, siting an oyster farm would be restricted in areas with key biological or ecological functions. In the absence of such restrictions, the farmer should implement an environmental management plan to ensure no adverse effects on the ecological integrity of the area, and demonstrate there is no harm to threatened or endangered species and/or habitats. In relation to the carrying capacity of the ecosystem and phytoplankton availability for other aquatic animals, farms should consider stocking at appropriate densities.
Best Management Plans (BMP’s), Codes of Good Practice (often developed by industry groups)4, and certification has been used as a means of prevention for unacceptable environmental interactions.
Different countries regulate aquaculture and enforce policies differently, but often with the same goal of minimising environmental impact. Overall, the content of habitat regulations surrounding oyster culture takes into account environmental impacts and ecosystem services. Similarly, enforcement organisations should be identifiable, permitting and licensing process transparent and based on zoning or planning5. It is not clear if such regulations are as effective or well-enforced in all locations6.
The consideration of water quality in oyster farming is important. Oysters feed by filtering phytoplankton and in doing so they can accumulate and concentrate bacteria or viruses, some of which can be a risk to human health. These are often derived from land-based activities such as water treatment, storm drainage and diffuse agricultural run-off. Bio-toxins may also be contained in seasonally occurring marine algae (Harmful Algal Blooms or HABs)1, 2 which the oysters can ingest.
Rigorous controls need to be in place in regards to harvesting shellfish such as mussels in order to protect consumers; ensuring that those sold into the supply chain meet strict food safety (health and hygiene) standards3. The UN Food and Agriculture Organisation (FAO) has developed the Codex Alimentarius or “Food Code” which is a set of voluntary standards, codes of practice and guidelines covering food and its production, including those for bivalve molluscs4, 5. The aim of the Codex is to protect public health and to support balanced trade relationships in food, and all World Trade Organisation (WTO) signatories are obliged to observe them. In practice, most countries have laws that are very broadly equivalent to Codex guidelines but differ from them in the detail.
European Hygiene Regulations6 state that shellfish business operators are responsible in ensuring that bivalve molluscs meet strict standards. The UKs Food Standards Agency, in compliance with European regulations, has classified shellfish harvesting areas and beds on the basis of the level of the bacterium E.coli in mollusc flesh. Depending on shellfish production area classification (A, B or C) certain procedures must be followed to enable harvested mussels to enter the market. Molluscs harvested from Class A areas can go straight to market, but they are often purified (depurated) to provide additional assurance of quality; those harvested from Class B areas must be purified before being sold to consumers. All molluscs sold on the UK market must contain less than 230 cfu (colony forming units) of E.coli per 100 g of flesh7, 8.
Depuration is a technique whereby live shellfish and bivalves, that may contain undesirable substances (e.g. sand, silt), pollutants, parasites or organisms of possible harm to human beings (e.g. pathogenic bacteria), are placed in systems with continually circulated and sterilised seawater for a specific period of time in order to clean themselves9. Depuration is effective in removing many undesirables, but it is not in regards to removing viral contamination (e.g. norovirus)10. As oysters are generally eaten raw, norovirus viability can remain an issue, however appropriate cooking can eliminate the norovirus risk. End product testing of live shellfish is also undertaken in countries such as the UK, to ensure shellfish products that enter the market are safe to eat11, 12.
For many reasons there is an on-going societal need to reduce coastal water pollution, and this is of particular importance to protect and enable the growth of bivalve aquaculture. Legislation exists to control water pollution, for instance that in the UK13, but pollution incidents do occur, and efforts continue to improve water quality14, 15.
Active Management is continuous, real time decision making, which takes into account of all the available information enabling shellfish producers to decide what action to take in regards to when and where to harvest. It is an increasingly important tool, especially in inshore waters, to reduce the risks from poor water quality and HAB episodes16.
Most global oyster aquaculture does not rely on hatchery seed but on wild spat collection and this should be from abundant, well-regulated natural sources. In countries where natural spatfall is poor, or in the case of introduced oysters, spat can be hatchery reared.
Pacific oyster is the most widespread farmed oyster species globally. Through intentional and accidental introduction the species has become established in the wild in many regions outside of its natural range. They may then impact or compete with other bivalves, but relatively little is known about the ecological impact (either positive or negative) of Pacific oysters on native communities.
Pacific oysters are not believed to impact on native oyster populations1; generally the lack of native European oysters seems to be due to a combination of over-exploitation, environmental conditions, and disease events, rather than competition from Pacific oysters2, 3. In addition, Pacific oysters in the Wadden Sea (part of the Dutch, German and Danish North Sea) co-exist with native mussels and form intertidal beds and “oyssel” reefs which are considered important for the Wadden’s community composition and ecological functioning4. Although eradication of naturalised Pacific oyster beds is still advocated by some5, this has proven unsuccessful6 and it is not considered practical to restrict farming of Pacific oysters once established.
The use of hatchery reared triploid (sterile) oysters for large aquaculture production of Pacific oysters has been seen as a method of preventing the release of spat from farms7. However, there is some doubt on the effectiveness of this as an approach8,9. Triploid oyster seed may also be difficult to obtain from hatcheries and more costly for on-growers to purchase.
Oyster relocation has in the past been the source of invasive non-native species (transported along with the oysters as ‘hitchhikers’)10, 11, and the movement of oyster seed, stock and equipment could potentially introduce or transfer diseases and parasites. This could potentially promote their incidence and spread and affect both farmed and native oyster species.
The introduction of non-native bivalve species for aquaculture purposes is now highly regulated helping to reduce the introduction of diseases and pests. Internationally the “Code of Practice on the Introductions and Transfers of Marine Organisms 2005”12 has been adopted by many countries, whilst in the EU international shellfish trade has been regulated for many years13. Upcoming European legislation will further prioritise the prevention and control of biological invasions14.
Biosecurity measures are important to mitigate oyster diseases that can affect oysters and could lead to high levels of mortality (e.g. Bonamiosis and Oyster herpesvirus)15, and regulatory measures aim to limit imports only from countries where no outbreak of disease occurs16. Key elements of biosecurity include; practical and appropriate legislative controls, adequate diagnostic and detection methods for infectious diseases, disinfection and pathogen eradication methods, reliable high quality sources of stock, and best management practices17, 18. It is also critical that oyster hatcheries implement strict biosecurity plans to help prevent transfer of disease into, within and from their facilities.
Transfers of spat from hatcheries to on-growing areas, and the relaying of oysters between sites, must be carried out in ways that minimise the risk of disease transfer15, and the monitoring of oyster populations as well as parasite occurrence/levels is important. Management measures include regulation (e.g. lease conditions and permit requirements) but also the use of voluntary agreements, Codes of Good Practice and certification.
Also important are designations to protect sensitive marine habitats. For example in the UK these include Marine Protected Areas (MPAs), Special Areas of Conservation (SACs), Special Protected Areas (SPAs), and intertidal areas identified as Sites of Special Scientific Interest (SSSIs)19.
Aquaculture (and fisheries) certification and labelling programmes have become a primary tool to address sustainability issues of farmed seafood, and the development of third party assessment and certification has provided new forms of governance traditionally dominated by state-based regulation1, 2. The growth in the number of certification schemes has led to confusion surrounding the myriad of them out there. To try and combat this, the Global Sustainable Seafood Initiative (GSSI)1 has developed its global benchmarking tool to measure and compare certification schemes and standards performance across seafood production.
Given the prominence of environmental issues as the driver for the development of aquaculture standards, there is an understandably strong emphasis on environmental criteria within them3. Certification enables aquaculture producers to voluntarily demonstrate their responsible farming practices by: complying with national legislation; minimising impact on habitats and wildlife; making the best use of locally available resources; and ensuring the best use of feed and therapeutic products.
Aquaculture certification currently has moderate to high coverage of labour standards (e.g. minimum wage)3, however, increased social and economic requirements related to human rights, gender and sustainable livelihoods are being developed.
Since 2011, a partnership of UK businesses called the Sustainable Seafood Coalition (SSC)4 have been working to ensure all fish and seafood sold in the UK comes from sustainable sources, and aquaculture certification plays a pivotal role. All members need to ensure that the aquaculture source (considering feed mills, hatcheries, and farm sites) is certified under a third party standard, or audited to a members own good aquaculture standard or code of practice5.
The table below looks at some of the major aquaculture certification schemes, including those for bivalves, and if they address the Key Considerations highlighted throughout the profiles. It also highlights which scheme has a standard/s that have been successfully benchmarked by the Global Sustainable Seafood Initiative (GSSI)1.
Read more on aquaculture certification, including that of oysters, after the table.
|Certification Scheme: Description and Links||Governance||Farm Siting||Nutrient Pollution||Feed||Disease, Medicines & Chemicals||Escapes & Introductions||Wild Seed||GSSI Benchmarked|
Founded in 2010 by WWF and IDH (Dutch Sustainable Trade Initiative) the ASC is an independent not for profit organisation with global influence. ASC aims to be the world’s leading certification and labelling programme for responsibly farmed seafood.
The ASC consumer label demonstrates the integrity of the seafood product.
GAA BAP has been certifying aquaculture since 2004, and is administered by the GAA, a nonprofit organization dedicated to advocacy, education and leadership in responsible aquaculture. Aquaculture Facility Certification:
The BAP program employs a star system to signify the integration levels of BAP certification along the aquaculture production chain. These stars are displayed on the BAP logo and appear on packaging for a variety of farmed seafood products worldwide.
GG offers 16 standards for 3 scopes: Crops, Livestock, and Aquaculture. GG has been one of the most widely accepted private sector food safety certification in the world since 2007.
GG Aquaculture certified producers have now the option to use the consumer facing GGN Certified Aquaculture label.
Friend of the Sea (FoS)
FoS is a non-profit NGO, whose mission is the conservation of the marine habitat. FoS is now a leading international certification project for products originated from both sustainable fisheries and aquaculture:
All aquaculture products sold as “organic” in the European Union (EU) have to fulfil the requirements that are laid down by the EU regulations:
This includes products certified according to private organic standards. The EU Organic logo makes organic products easily identifiable by consumers
Naturland was founded in 1982 when organic agriculture was regarded as a marginal issue. Since the mid-1990s, Naturland has developed standards for different species and production systems in aquaculture:
The Naturland label is intended as a guide for consumers.
The Soil Association was formed in 1946 and champions organic principles and practice.
The Soil Association logo sends a message to consumers that the product is organically produced.
• – Indicates the Certification Scheme addresses the Key Consideration
✔ – Indicates the Certification Scheme has had one or more of its Standards benchmarked against the Global Benchmark Tool Version 1 and recognised by the GSSI Steering Board
Certified Aquaculture Production
Production of certified seafood, both aquaculture and wild catch, has grown rapidly over the past decade and now represents a significant portion of global seafood production. Certified sustainable seafood in 2003 equated to some 500,000 tonnes (0.5% of global production); in 2015 this figure had risen to 23 million tonnes (14% of global production). Some 80% of certified seafood is wild catch, but certified aquaculture is growing twice as fast and is set to dominate growth in certified seafood for the foreseeable future3.
In 2015, certified aquaculture accounted for 6.3% of world aquaculture production. Of this 6.3%, seven species groups were dominant (i.e. salmon, pangasius, mussels, tilapia, prawns, trout and sea bream) and accounted for 97%3. This relatively low global level of certified aquaculture and the narrow range of species groups, is largely due to:
Certification of small-scale aquaculture continues to be an issue, mainly due to the cost and difficulties in complying with standards; key challenges include finance, technical knowledge and organisational capacity. Educating small-scale farmers on how to comply, as well as identifying national policy and regulatory gaps supporting small-scale aquaculture certification, is becoming ever more necessary6. Multiple-farm or ‘cluster’ certification may be a way forward for small-scale producers.
Four schemes are responsible for the majority of certified aquaculture production, namely the Aquaculture Stewardship Council (ASC), Global Aquaculture Alliance Best Aquaculture Practice (GAA BAP), GlobalG.A.P. (GG), and Friend of the Sea (FoS)3.
Certified Oyster Production
In 2015, farmed bivalves accounted for 8% of certified aquaculture globally3.
Figures provided by the certification schemes themselves and relating to their totals of certified farmed oysters/bivalves break down into:
(It is important to note that certification under one scheme does not preclude certification under another, and gathering accurate data on rates of multiple certification is very difficult. As a result simply adding the production volumes of individual schemes can result in double counting and overestimation. The authors of the aggregate data referred to above make no adjustment for multiple certification.)