Standing advice for Pacific oysters

https://www.nature.scot/doc/standing-advice-pacific-oysters

Pacific oyster (Magallana gigas) cultivation in Scotland has grown substantially in recent years, with an estimated value in 2023 of £2.4 million, representing an important value to the rural economy. They are grown on trestles on the foreshore from juvenile oysters known as “seed stock” sourced from dedicated hatcheries. 

Although a widely cultivated species in Scotland, Pacific oysters are classified as a non-native species, which if permitted to establish in the wild can form reefs, outcompeting and displacing native species potentially threatening marine biodiversity. Their distribution is limited by seawater temperatures. Scotland does not currently provide suitable conditions for the establishment of wild populations. However, over the last decade there have been several occurrences of natural settlement of isolated individuals in Scotland. 

The establishment of feral Pacific oysters may be caused by spawning, under appropriate conditions within farms or the accidental release of Pacific oysters through infrastructure failure or operational activities. 

It is an offense under the Wildlife and Natural Environment (Scotland) Act 2011 to release or allow to escape from captivity any animal to a location out with its native range.  On this basis any release into the wild of Pacific oyster would constitute an offence, including individuals that are inadvertently released out with their containment as well as any feral populations which become established following a natural spat fall.

As Pacific oysters do not normally reproduce with current Scottish seawater temperatures, production is dependent on imported stock from sources in England and the Channel Islands. The movement of live oysters represents a biosecurity risk for the introduction of non-native species and the spread of pathogens. 

NatureScot supports the sustainable growth of the Shellfish sector. However, given the biosecurity risks, we advise that any Pacific oyster cultivation should develop a robust biosecurity plan.  This should include the following measures:

  • Oyster baskets/bags will be filled and graded off-site or on-site using secondary containment to ensure that no individuals are inadvertently released.
  • Containment: Oysters should be held in appropriate containers for the local conditions and inspected regularly to manage the risk of accidental release.
  • No discarded shells, dead or alive, should be deposited on the seabed / intertidal zone
  • Any individuals identified during operational management which are out with containment will either be returned to the baskets or removed from site and destroyed.
  • Early detection is crucial of any feral Pacific oysters, and it may be possible to contain or manage populations found only in patchy locations. Undertaking inspections of the production area will aid in identifying the presence and if required management of any feral populations/ individuals.
  • Reporting: Report sightings of non-native species (Including feral Pacific oysters) to iRecord Or alternatively report to [email protected]
  • Have regard for the Association of Scottish Shellfish Growers Code of Good Practice

The following guidance documents can be used to aid the development of a robust biosecurity plan: