Evaluating the net impacts of a naturalised non-native species and attempts to control its spread in the UK: Addressing the oyster in the room

Alexander W Shakspeare, Tom C Cameron, Michael Steinke

The Pacific oyster (Magallana (Crassostrea) gigas) was introduced to UK waters in the mid-20th century and accounts for >95% of UK oyster production. Recently, its nonnative origin has led landowners and policymakers to consider limits on UK oyster aquaculture operations. M. gigas is ecologically naturalised in the UK, with multiple records of populations originating from wild sources, including from outside the UK, with France and the Netherlands treating M. gigas as a legally naturalised species. The naturalised status is justified, potentially simplifies regulation and enables aquaculture production to provide nutritious and sustainable food while supporting employment. The presence of M. gigas can have positive environmental impacts by improving water quality, diversifying the seascape and providing living breakwaters for contemporary coastal defence. Positive effects of non-native species are notably missing from habitat-regulation assessments. While acknowledging the important role of non-native species in biodiversity loss, the potential negative effects of M. gigas have not universally materialised and efforts to reduce its wider spread in England will fundamentally fail due to natural spread across Europe and the UK from substantive larval connectivity. UK policy on M. gigas should be revised to reflect the socioeconomic benefits of Pacific oysters to shellfish production and the evaluation of the legally prescribed ecological status of protected sites requires updating. Location-specific management interventions should consider a dynamic ecological status that focuses on ecological function, the provision of services and the realised impacts of non-native species instead of a rigid focus on the identity of a species.