Description: The European oyster (Ostrea edulis) is under significant threat across its natural distribution range and even functionally extinct in some regions, such as in the German North Sea. Due to its ecological significance in terms of biodiversity and other ecosystem services, the species, and the habitat it provides, are defined as highly endangered by the OSPAR Convention. Restoration measures are gaining momentum in Europe and conclusive recommendations for large‐scale biogenic reef restoration are relevant for example within the Marine Strategy Framework Directive and the Habitats Directive. This study examined whether present‐day environmental conditions of sublittoral offshore waters are ecologically suitable for the return of European oysters. Seed oysters (shell length ~2 mm) were deployed in cages in offshore field experiments in 10–26 m water depth. Survival, growth, and condition were investigated over the course of 2 years. Survival was high, even over winter. Growth was excellent, with oysters reaching a mean length of 55.0 ± 7.2 mm shell length and 19.2 ± 6.1 g wet weight after 2 years. The formation of firmly aggregated oysters was observed and confirms O. edulis as a reef‐building species. The overall condition of oysters in the field was excellent, identified by high condition indices and early reproductive activity. These findings are highly relevant for future restoration measures in the North Sea as they confirm that present‐day environmental conditions and small, hatchery‐produced seed oysters are suitable of supporting sustainable and successful restoration efforts even in sublittoral offshore waters.

Description: 1. 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. 2. 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. 3. 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. 4. This is critical information for restoration project planning in which the risk of Bonamia infection must be taken into account.

Description: The European flat oyster (Ostrea edulis) is an ecosystem engineer that provides important biogenic reef habitat with associated ecosystem functions and services. Most stocks have been commercially exploited and degraded; some are functionally extinct. Ecological restoration now aims to recover these degraded, damaged or destroyed ecosystems. Availability of seed oysters and substrate for successful larval recruitment has been identified as a major limiting factor for restoration projects in Europe. In substrate‐limited areas, restoration approaches have to involve the restoration of suitable substrates. The present study provides an evaluation of such potential substrate types. Various categories were investigated through hatchery and/or field experiments: (1) marine bivalve shells; (2) inorganic materials; (3) sandy sediment; (4) 3D sandstone reefs; (5) wood materials; and (6) limed materials. The respective settlement rates (settled larvae per cm2) indicate settlement preferences. Hatchery experiments showed significant preferences for bivalve shells and inorganic materials. Best settlement rates were observed on Mytilus edulis shells, followed by O. edulis shells as well as on slaked lime and on baked clay. Settlement was significantly higher on bottom‐oriented areas of bivalve shells and 3D reefs in laboratory experiments; however, this was not substantiated in the field experiments. Field experiments showed significant settlement preferences between substrate categories (bivalve shells, inorganic materials and wood materials). Best settlement rates were observed on baked clay, followed by slaked lime and bivalve shells. Wooden materials did not perform. Settlement rates and substrate preferences of larvae in controlled environments (laboratory, hatchery) differed from rates in the natural environment (field). This study provides a list of substrate types considering these specific environments. The relevance of these results for ecological restoration in the field and potential applications in seed oyster production are discussed.

Description: The conservation and active restoration of European flat oyster (Ostrea edulis) populations are a major focus of ecological restoration efforts to take advantage of the wide‐ranging ecosystem functions and services this species provides. Accordingly, additional and new demands for seed oysters have arisen. In commercial aquaculture (mariculture), the production of O. edulis is still largely based on natural seed collection. Considering the specific requirements, related to ecological restoration, such as the absence of pathogens and the preservation of high genetic diversity, the current supply is insufficient. Despite the development of breeding and controlled reproduction techniques for this species since the late 1930s, seed production today is mainly based on empirical concepts. Several of the issues that producers still face are already subjects of research; many others are still unanswered or even unaddressed. This review provides a summary of all available knowledge and technologies of O. edulis seed production. Furthermore, it provides a detailed reflection on implications for restoration, future challenges, open questions and it identifies relevant research topics for sustainable seed supply. The study covers the following aspects on (i) biology of the species, (ii) stressors – including pathogens and pollutants, (iii) genetics, (iv) history of production technologies, (v) seed production in polls, (vi) seed production in ponds and (vii) seed production in hatcheries. Future research needs on sex determinism, gametogenesis, cryopreservation, nutrition, selective breeding, pathogens and disease, and the development of reliable protocols for production are highlighted.

Description: 1. According to the EU Marine Strategy Framework Directive (2008/56/EC), marine protected areas (MPA) should contribute to a good environmental status of the Europeans seas. Measures maintaining or restoring a favourable conservation status of protected species and habitats are mandatory according to the EU Habitats Directive (92/43/EEC). 2. Identification of suitable sites for ecological restoration measures within MPAs is a crucial step towards successful conservation and sustainable MPA management. In terms of species restoration, it is important to restore the respective species with the best possible environment for growth, survival, fitness, and successful recruitment. 3. This study provides a comprehensive list of site-selection criteria for ecological species restoration. Three general categories were chosen: (1) ecological history: evidence for the historical distribution; (2) feasibility of restoration: regulating framework and logistics; and (3) environmental conditions: quality of abiotic and biotic factors. A total of 16 site-selection criteria were identified and applied to biogenic reef restoration, namely for reefs of the native European oyster Ostrea edulis, in the German Bight. 4. The Natura 2000 area Borkum Reef Ground was identified as a suitable site for oyster restoration. It is one of three MPAs in the German Exclusive Economic Zone of the North Sea, which have been declared as Nature Conservation Areas according to national legislation. The conservation objectives include maintenance or, if necessary, restoration of the habitat type ‘reefs’. As a reef-building species, the European oyster O. edulis is of particular importance for this habitat type in terms of nature conservation.