Oyster reefs are among the most threatened marine habitats globally. In Europe, oyster reefs have been extirpated from most locations within their historical range. Active restoration of the native oyster (Ostrea edulis) in Europe has grown substantially in recent years. In sharing experiences between oyster restoration projects in Europe at the Native Oyster Restoration Alliance conference, NORA2, in Edinburgh in May 2019, it became apparent that a number of similar barriers are experienced. This study identified the top 40 questions, which, if answered, would have the greatest influence on the policy and practice of oyster restoration in Europe. Initially 71 people were consulted across 28 institutions and 11 European countries to generate 194 questions. An established process of one round of pre‐workshop voting followed by a one‐day online workshop and two post‐workshop rounds of voting resulted in the final 40 questions. Questions were broadly grouped into the following 10 themes: baselines, site selection, restoration methods, quantifying benefits, disease management, biosecurity, genetic diversity and population differentiation, policy and management, novel technologies, and current and future threats. We anticipate that this list will provide a starting point for developing collaborative projects across the NORA network, as well as assisting policy makers and funders with identifying key areas that need to be addressed in order to overcome existing barriers to scaling up oyster restoration in Europe.

There is a growing effort throughout Europe to restore populations of native oysters (Ostrea edulis), with the ecological objective of enhancing ecosystem biodiversity and resilience. The introduced parasite, Bonamia ostreae, caused catastrophic mortalities during the 1980s, furthering the decline of this species, and is now present throughout much of the natural range of O. edulis. It is therefore important that restoration attempts avoid further introduction and spread of this parasite, which can cause lethal infections of O. edulis. This article presents a comprehensive overview of the scale and distribution of current infection, transmission pathways, and preventive measure guidelines, focusing on the seas, inlets, and estuaries of north‐west Europe, where most ecological restoration attempts for the native European oyster have occurred so far. This is critical information for restoration project planning in which the risk of Bonamia infection must be taken into account.

1. The European native oyster (Ostrea edulis) is a threatened keystone species which historically created extensive, physically complex, biogenic habitats throughout European seas.2. Overfishing and direct habitat destruction, subsequently compounded by pollution, invasive species, disease, predation and climate change have resulted in the functional extinction of native oyster habitat across much of its former range.3. Although oyster reef habitat remains imperilled, active restoration efforts are rapidly gaining momentum. Identifying appropriate sites for habitat restoration is an essential first step in long-term project success.4. In this study, a three-round Delphi process was conducted to determine the most important factors to consider in site selection for European native oyster habitat restoration projects.5. Consensus was reached on a total of 65 factors as being important to consider in site selection for European native oyster habitat restoration projects. In addition to the abiotic factors typically included in habitat suitability models, socio-economic and logistical factors were found to be important. Determining the temporal and spatial variability of threats to native oyster habitat restoration and understanding the biotic factors present at a proposed restoration site also influence the potential for project scale-up and longevity.

No abstract