Site selection for European native oyster (Ostrea edulis) habitat restoration projects: An expert‐derived consensus

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. 6. This list guides site selection by identifying: a shortlist of measurable factors which should be considered; the relevant data to collect; topics for discussion in participatory mapping processes; information of interest from the existing body of local ecological knowledge; and factors underpinning supportive and facilitating regulatory frameworks.