Ecological restoration of European flat oysters in the German Bight

berenger.colsoul [ at ]


Several marine ecosystems currently face severe degradation, in the form of habitat loss. As a consequence, humans are undertaking initiatives to restore species and habitats to restore and preserve ecosystem services and functions. Although there have been many initiatives to restock commercial marine species for fisheries and aquaculture, the restoration of marine habitats is a relatively new discipline. To recover ecosystem conditions that maintain their structure and function, ecological restoration was conducted and implemented by the Alfred-Wegener-Institut Helmholtz Zentrum für Polar- und Meeresforschung (AWI) and the Bundesamt für Naturschutz (BfN) for re-establishing lost and ecologically relevant biogenic oyster reefs in the frame of marine conservation measures in the German North Sea. From 2016 to 2019, the AWI-led and BfN-funded RESTORE project actively investigated the technical and biological feasibility of restoration, from which this thesis originates. In this context, three key topics (and their associated subtopics), relevant for the development of a successful restoration programme, are addressed in this thesis: I) Oyster supply - How can we provide ecological restoration efforts with substantial amounts of appropriate Ostrea edulis seeds (i.e. gametes, larvae and spat)? Which production techniques and knowledge exist? Which are appropriate for restoration? II) Supply of essential settlement substrate for the oyster life cycle - Which types of substrate to use in accordance with biological traits of O. edulis? Which types of substrate to use in accordance with legislative restrictions? III) Biosecurity aspects of oyster restoration - How to avoid the transfer of pathogens or invasive species during ecological restoration projects (focusing on seed production and substrate transfer)? A review paper on the reproductive biology of O. edulis and on existing seed production techniques relevant for aquaculture and restoration (Chapter I) provides the knowledge basis for successful production, tailored to the demands of ecological restoration. It reviews four majour seed production processes of O. edulis, discusses them in the context of different aquaculture and ecological restoration scenarios (e.g. techniques to minimise disease transmission, or to manage genetic variability), and identifies critical knowledge gaps that need to be closed to facilitate stable and substantial O. edulis seed production. Twenty substrate types are evaluated in the context of seed supply in natural environments (to enhance recruitment in the field) as well as in hatchery seed production (Chapter II). The approach is complementary (in situ and in vitro tests) and shows clear differences in settlement preferences of O. edulis larvae in relation to substrate type and environment. The results indicate that substrate selection (for practicioners) is essential to optimise O. edulis restoration practices. Biosecurity measures are investigated for the substrate supply chain (Chapter III) as well as for hatchery production (Chapter IV). The lack of established and recognised practical measures regarding the potential risks of translocations of non-native species, diseases and/or pests highlights the early stage of ecological restoration in Europe. A new method for sorting and processing shell substrate from France for restoration projects in Germany is presented here. Based on different treatments, first conclusions are drawn and future research directions suggested for the practice of importing shells for re-establishment at sea.

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Colsoul, B. Ecological restoration of European flat oysters in the German Bight PhD thesis,

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