Seed supply
Juveniles are obtained either through wild spat collection or by hatchery production (cultchless seed). Presently, most is wild spat from natural reproductive areas, whereas hatchery production is mostly focused in areas without natural supply. However, depleted beds and the impact of disease (
Bonamia ostreae) have prompted the hatchery industry to try developing a disease resistant strain.
Broodstock
The sexual maturation and reproduction of
Ostrea edulis is obtained by increasing seawater temperature and providing additional (
ad libitum) food (phytoplankton). Conditioning is based on mimicking the natural reproductive cycle and environmental conditions. Compared to other traditional hatchery reared species (e.g.
Crassostrea gigas or
Ruditapes philippinarum) in-vitro fertilization remains difficult for this species, resulting in an extremely low survival rate. So far, therefore, an incubation phase is still required. Synchronized maturation and spawning induction are also difficult tasks from a technical point of view. Thermal stress to induce spawning does not allow full sexual identification. Although the use of chemicals (serotonin) has facilitated reproduction control, this technique cannot be used on a routine basis. Therefore, two techniques have been developed. The first is mass spawning, based on the maturation of a whole oyster batch. This neither permits any assessment of the contribution of individual breeders to the generation nor any controlled cross-breeding. The second technique does permit controlled breeding (full siblings) by maturing only two oysters per tank and then collecting the resulting seed. All these constraints demonstrate that the entire reproductive process is still not entirely under control at the hatchery level.
Hatchery production
Hatchery rearing requires the production of suitable microalgal species as food. Usually, the phytoplankton species used are flagellates combined with diatoms to provide a well balanced diet, therefore facilitating gametogenesis and larval development. Food quantity depends on larval density. Hatchery production has remained low because no disease-resistant strain has yet been developed.
Natural spat collection
Most European flat oyster culture remains based upon the use of spat collectors to obtain wild juveniles. Spat collectors are either mussel shells, sown in June-July (18 °C), mostly in densities of 30-60 m³/ha (Netherlands) or tubular nets filled with mussel shell (each containing about 600 mussel shells) and deployed off-bottom (France). In the latter case, nets are suspended under steel frames in shallow waters (3-6 m deep). Since farmers use cooked mussel shells as spat collectors, there is no need for further work as these will naturally disaggregate. Usually, one steel frame supports around 120 tubular nets and yields between 70 to 75 kg of spat/yr. More recently, PVC dishes covered by lime have been used intertidally for this process. The main interest in using this technique is the capacity of automatic equipment to remove spat from the collectors, therefore reducing operational costs.
Nursery
Although clams have a protective shell, these will break if not handled with care during the screening and sorting processes. It is therefore necessary to collect the oysters post-setting by filtering the seawater outflow from the conditioning tank. Then the spat is cultured using traditional techniques for the micronursery and nursery stages. These include the use of micronursery trays built with nylon screens and a flow-through recirculating system with frequent water exchange (e.g. water tables, downwellers, and upwellers). Whenever the spat are removed from the water, care must be taken to ensure they do not dry out or become too warm. Once reaching 5-6 mm size (kept by a 4 mm mesh sieve), spat can be transferred using baskets to open waters for ongrowing.
Ongrowing techniques
Two main types of production techniques have been used to produce flat oysters: off-bottom and on-bottom.
Off-bottom techniques
These consist of floating trays or rafts, longlines, suspended rope, lanterns or plastic baskets hung from the rafts, intertidal trestles and oyster bags. In Spain, floating rafts like those used for mussel culture are also used for oyster culture. The technique involves manual attachment of medium size oysters (5 cm) to ropes with cement. Workers hang the ropes from rafts and periodically clean them by removing algae and mussel seed. More recently, plastic baskets have been hung from the rafts. Oysters are thinned out as they grow.
On-bottom techniques
Oyster spat are directly re-laid by boat on-bottom in subtidal grounds at a density of 50-100 kg/ha, 5 to 10 times less than for cupped oysters (
Crassostrea gigas) in France. The common size at seeding, which is carried out in May-June, is 1 cm at about 1 year-old. Oyster deployment, which used to be done by manual shovelling, is now carried out either by conveyor belts or by washing out spat using seawater. Traditional management practices include the use of cotton nets to collect starfish predators, as well as teethed steel frames, being dragged in tow on the leasing grounds. Deep water culture maximizes growth rate since
Bonamia disease drastically reduces the survival rate of 3-4 year-old oysters. Thus, 2 year-old oysters (60-80 g) are harvested by dredging before disease-induced mortality occurs. In spite of this early harvesting, survival rates remain low, about 5 percent after a 3 year rearing cycle.
In Maine (USA), oysters are cultured in floating trays until they are in the 35-50 mm range in the late fall. Fouling is controlled biologically by the browsing periwinkle
Littorina littorea. Oysters are then transferred to on-bottom leasing grounds for growth to market size. Developing effective predator control methods against starfish and crabs is the main issue for on-bottom culture.
Harvesting techniques
Since most flat oyster culture is developed in subtidal areas and in an extensive manner to avoid disease problems, oysters are usually harvested with two steel dredges about 3.5-4 m wide and 2 m deep with 3-5 cm teeth blades, operated by hydraulic or pneumatic winch.
Where intertidal culture on trestles is carried out, farmers bring the bags back to the packing houses for sorting, grading and restocking.
Handling and processing
Once oysters have reached their marketable size, farmers grade and store them temporarily in clean waters before marketing. In France, oysters are marketed according to their size (grade 0 to 5). Refrigerated trucks are commonly used to transport oysters in accordance to EU regulations.
In Spain, like other shellfish destined for the fresh market, oysters must be depurated. They are deployed into cases, into depurating tanks filled with chlorinated water. The maximum authorised weight is 30 kg/m², while the depuration time lasts 42 hours. Oysters chilled on ice are trucked to market. During sale, they are maintained at 3-10 °C.
Production costs
Although highly variable, depending on rearing site characteristics and mainly survival rate, production costs are likely to be high. This is mainly due to disease problems with this species and the very low survival rates which have prompted farmers to reduce production, but also to the length of the rearing cycle. European flat oysters are cultured extensively, and are sometimes mixed with cupped oysters (
Crassostrea gigas) to reduce disease effects; this increases sorting activities. Investment is required to operate in deep waters with specific gears. Raft culture is labour intensive and requires considerable time to prepare the units as well as to clean them during the production cycle. Compared to deep water culture, which is highly mechanized using hydraulic gears and dredges, raft culture requires regular manual handling for various operations.
Herpes-like infection |
Herpes virus like (OHSV-1) |
Virus |
No direct impact at the population level observed |
No curative measures available; ensure good site selection & management practices to limit impact & compulsory oyster monitoring for transfers; zoning system |
Shell disease |
Ostracoblabe implexa |
Fungus |
Limited impact but black malformations on inside of shells may impede development & cause eventual mortality |
|
Bonamiasis |
Bonamia ostreae |
Protozoan |
Most infected oysters show no obvious macroscopic symptoms; however, infections sometimes accompanied by yellow discoloration & extensive gill & mantle lesions; becomes systemic & usually lethal when oysters 2-3 years old |
|
Marteiliasis |
Marteilia refringens |
Protozoan |
Discoloration of the digestive gland epithelia; cessation of growth; loss of tissue condition; decrease of glycogen reserves; may cause recurring mortalities |
|
Denman Island disease |
Microcytos mackini |
Protozoan |
Green to yellow pustules or abscess-like lesions (<5 mm), resulting from haemocyte infiltration & necrosis, occur within the body wall, on the surface of labial palps or mantle, or adductor muscle; often induces brown scar adjacent to mantle surface; parasite may cause recurrent abnormal mortality rates in older oysters |
|
Preventive measures
With regard to shellfish regulations, preventive measures aim to limit imports only from countries where no outbreak of disease occurs, according to the list specified by OIE International Aquatic Animal Health Code (notifiable pathogens). The three parasites mentioned in the table above are notifiable pathogens, therefore limiting any flat oyster transfer from enzootic to disease-free areas. Imports of molluscs should occur only from countries where no outbreak of disease caused by those agents has occurred for at least the two previous years and no parasite has been detected in any mollusc tested during an official health surveillance programme using the procedures described by the OIE, for at least two years. Moreover, European zoosanitary directives have enforced a zoning system to limit the expansion of these diseases. Therefore, monitoring
O. edulis populations and parasites is critical to prevent and limit associated risks. Without cure, preventative measures have been adopted to limit disease effects, mostly concerning management practices, including site selection and density reduction.
Suppliers of pathology expertise
Assistance can be obtained from the following sources:
- European Shellfish Zoosanitary Reference Laboratory (OIE Reference laboratory for Marteilia and Bonamia diseases), IFREMER La Tremblade, BP 133, 17390 La Tremblade, France.
- Department of Fisheries & Oceans, Pacific Biological Station, Nanaimo, BC, Canada V9R 5K6.
- Instituto de Investigaciones Marinas Consejo Sup. de Invest. Cie., Eduardo Capelo 6, 36208 Vigo, Spain.
- CEFAS, Weymouth Laboratory, Barrack Road, The Nothe, Weymouth Dorset, DT4 8UB, UK.
- CIDC Lelystad, Institute for Animal Disease Control, PO Box 2004, 8203 AA Lelystad, The Netherlands.