Invasive Alien Species in Northern Ireland

Crassostrea gigas, Pacific oyster

Crassostrea gigas (Thunberg 1793)
The Pacific oyster is a bivalve mollusc cultivated in temperate and Mediterranean regions of the world. It is considered to be the same species as the Portuguese oyster Crassostrea angulata thought to have been introduced from Taiwan to Portugal, from or after, the sixteenth century. Introductions to Europe directly from Japan took place following the demise of native oysters in the 1970s. The species predominantly occurs in shallow bays, inlets and estuaries and was introduced to Northern Ireland for cultivation in the 1970s.
Shells are variable and irregular in shape according to the growing conditions. The upper shell is flatter. Its colour can be off-white to brown and it may bear uneven radiating purple bands or streaks on the shell. They can attain 30cm in length but are marketed at about 6-12cms. As oysters do not burrow many different fouling species often settle on their irregular shell surface.
Country of origin
The Pacific oyster is native to Korea, Japan and China. Stock from these areas was imported to different world regions including British Columbia, Canada to where large consignments of oysters were imported at the beginning of the last century. The stock in Northern Ireland is based on oysters from the north-west coast of America that were introduced to quarantine in Conwy, Wales in 1965. This quarantined stock was distributed to sites about Britain and Ireland in the 1970s. At this time in France a disease caused declines in the native oysters. To overcome the problems of production, large consignments of Pacific oysters were imported by plane from Japan. However, following the European open market in January 1993 Pacific oysters from France were permitted entry to Ireland.
Current distribution
Pacific oysters have been cultivated in Strangford Lough since the 1970s. Hatchery seed is imported and grown within mesh bags on trestles at low water until they are marketable.
Location in Ireland
Oysters are cultured on the western shores of Strangford Lough near to Setrick Island. Oysters are exchanged between here and Carlingford Lough from time to time. It is in culture in many bays on Irish coasts with the main centres of production in Carlingford Lough and Dungarvan Bay. The species is likely to become more widely cultivated.
Life cycle
Oysters have separate sexes with external fertilisation. Spawning takes place at temperatures of c.18°C. An individual may produce c.100 million eggs. Larvae develop successfully at temperatures 18–26°C and salinities 20-35psu and settle after 11–30 days. They attach to hard surfaces once they attain a shell length of c.290 µm and during this time their mortality is high. Once fully settled, growth is rapid and maturation may take place after a year. They may live to ten or more years. In Ireland they have settled in the wild in Donegal Bay, Galway Bay and along the south coast.
Wildlife and habitat impacts
Oysters are normally held in mesh bags attached to trestles. These are covered for most of the tidal cycle, being exposed at low water. However, some oysters may be moved up the shore for short periods when they are being graded or awaiting collection to be marketed. Conditions adjacent to and beneath these trestles can vary with soft sediment and drifting plant accumulations. Birds do not appear to be impacted by the presence of oyster farms but are disturbed when these are tended. Oysters may carry other organisms that may alter habitats and have impacts on native communities. The brown alga Sargassum muticum can be transported as an inconspicuous stage attached to oysters and this plant has recently become widely distributed about Irish coasts, most probably from drifting. The slipper limpet Crepidula fornicata may be imported with half-grown oysters from Britain or continental Europe. Movements of half-grown, or adult oysters, may enable the gut parasite Mytilicola orientalis and the apparently harmless copepod Myicola ostreae, which occurs on the gills, to spread within Ireland from Dungarvan Bay or be introduced with consignments from continental Europe where they are presently established.
Human impacts
As filter feeders they accumulate their food from the plankton. Toxins in some phytoplankton species can accumulate in tissues and if eaten under these conditions can cause a range of symptoms in humans, most usually diarrhoeal shellfish poisoning (DSP). Regular monitoring for these toxins in cultured oysters in Ireland provides a quality assurance. Bacteria may also be accumulated by oysters and it may be necessary to purge these by holding the oysters in clean water before marketing. There are different requirements for shellfish before they are marketed based on the water quality in the area they are grown.
Key vectors
Oysters may arrive in the following ways:
  1. Hatchery produced seed imports or half-grown stages introduced for cultivation.
  2. Introduced alive for human consumption but held on shores.
  3. Imported unintentionally on the hulls of ships or other craft.
  4. Larval stages released from ship ballast water discharges.
  5. Movement of equipment from areas where natural settlements take place.
  6. Larval dispersal from areas where reproduction takes place.

    What you can do as an individual
    There is a requirement for a) a fish culture licence and b) a shellfishery licence issued by the Department of Agriculture and Rural Development, Fisheries Division and consent is needed from the Department of Transport / Marine Safety Agency and the Crown State Commissioners if cultivation is intended. Live imports for human consumption should not be relaid on shores unless such consignments are approved, otherwise these actions may result in the spread of molluscan pests and diseases.
    Advice to specific groups relevant to this species
    Many non-native species have been introduced with half-grown oysters to different world regions. Direct imports from France need to be carefully evaluated as associated species with consignments may cause harm to Irish animals and plants. Imports of half-grown oysters should be examined for species other than oysters and these should be removed. Attached mussels may contain Martelia refringens, a protozoan shellfish disease, and the slipper limpet Crepidula fornicata, a harmful filter-feeding snail that can compete with oysters and other filter-feeders, may become introduced with such movements. Introduction of oysters for cultivation is best undertaken with imports of the late larval stage (eyed-larvae) or settled ‘seed’ stage as this reduces the biomass of other species that might otherwise become introduced with them. A ‘summer mortality’ of oysters is known to occur in Europe and it is unclear whether this results from a disease or is environmentally induced.
    Management measures
    Oysters are grown within confined areas under license and regular inspections of such plots are advised. Inadvertent introductions of non-native species may take place with movements of oyster stock. Past experience has indicated that aquaculture sites are areas where such species may arrive and controls may only be effective should non-native species be found at an early stage.
    Management groups
    The Department of Agriculture and Rural Development, Fisheries Division, Castle Grounds, Stormont, Belfast BT4 3TA manages the licensing of aquaculture.
    Further information
    With the predicted changes in climate, warmer summers may allow Pacific oysters to settle in greater numbers and some settlements may be expected in Northern Ireland. This may not take place for some decades.
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    Cheney, D.P., Macdonald, B.F. and Elston, A. (2000). Summer mortality of Pacific oysters, Crassostrea gigas (Thunberg): Initial findings on multiple environmental stressors in Puget Sound, Washington, 1998. Journal of Shellfish Research 19(1):353-359.
    Grizel, H. (1996). Some examples of the introduction and transfer of mollusc populations. Revue Scientifique et Technique de l'Office International des Epizooties 15: 401-408.
    Menzel, RW. (1974). Portuguese and Japanese oysters are the same species. Journal of the Fisheries Research Board of Canada 31: 453-456.
    Minchin, D. and Rosenthal, H. (2002). Exotics for stocking and aquaculture, making correct descisions. In: Leppäkoski E, Gollasch S, Olenin S (eds). Invasive aquatic species of Europe: Distribution, Impact and Management. Kluwer, Dordrecht, the Netherlands pp. 206-216.
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    Reise, K. (1998). Pacific oysters invade mussel beds in the European Wadden Sea. Sneckenbergiana maritima 28: 167-175
    Shatkin, G., Shumway, S.E., Hawes R. (1997). Considerations regarding the possible introduction of the Pacific oyster (Crassostrea gigas) to the Gulf of Maine: a review of global experience. Journal of Shellfish Research 16 (2): 463-477.
    Utting, S.D., Spencer, B.E. (1992). Introductions of marine bivalve molluscs into the United Kingdom for commercial culture — case histories. In: Introductions and transfers of aquatic species, Selected Papers from a Symposium held in Halifax, Nova Scotia, 12-13 June 1990, (Sindermann, C., Steinmetz, B. and Hershberger, W. Eds) ICES Marine Science Symposia., Copenhagen 194: 84-91.
    Text written by:
    Dan Minchin, MOI, Ballina, Killaloe, Co Clare