The European f lat o yster Ostrea edulis is one of the most appreciated mollusks due to its gastronomic, cultural and environmental value, and plays an important role in the marine ecosystem through, e.g., its reef-forming capacity. Despite its historical and ecological importance, flat oyster reefs have completely disappeared from the Belgian part of the North Sea (BPNS). Next to overexploitation and habitat destruction by bottom fisheries, diseases caused by Bonamia and Marteilia parasites contributed to the demise of oyster populations. To evaluate the feasibility of restoring and cultivating flat oysters, within the H2020 UNITED project we started a demonstration project inside a Belgian offshore wind farm . T he goal of this study was two-fold: On the one hand to determine the status of flat oysters introduced in the BPNS and their offspring regarding bonamiosis and marteiliosis, and on the other hand to characterize the fouling biodiversity associated with flat oysters.
Flat oysters implemented were initially certified Bonamia and Marteilia free and originated from Norway (adult oysters) or from England (oyster spat). After six months to two years, these oysters, and their offspring, were sampled (N = 356) during late spring (May – June) or autumn (September – October ) from both nearshore and offshore sites within the BPNS. The associated fouling biodiversity was also sampled and taxonomically identified in situ and in the lab.
Each oyster sample was split in two, one for histology analysis and the other for Real-Time PCR analysis . Only in case oysters tested positive for the detection of Bonamia sp. or Marteilia refringens parasites through q PCR, the histologic slides were further analyzed.
Results & Discussion
In none of the tested flat oyster samples, Bonamia or Marteilia parasites were detected, while the control samples were positive . This is the first time these oyster diseases have been monitored in the BPNS, allowing to demonstrate disease-free status , which is a promising result for oyster restoration and cultivation projects envisaged in the BPNS . Nonetheless, other diseases might be present or emerge in this new oyster population. Thus, it is crucial to continue monitoring by checking health status via histology and qPCR, especially if mortality occurs.
A high species diversity was associated with flat oyster habitat, including other reef-forming species such as the Ross worm Sabellaria spinulosa. However, also high densities of tube-dwelling amphipods such as Jassa spp. and Monocorophium spp. were recorded, posing challenges to offshore aquaculture.
This research is a first, but necessary, step to pave the way for the restoration and sustainable aquaculture of the once abundant flat oyster in Belgium. When situated inside an offshore wind farm, the combination of restoration and aquaculture can be a prime example of sustainable marine multi-use. Furthermore, the restoration of flat oyster reefs brings back a lost habitat with its associated biodiversity and ecosystem services and helps to appreciate again the cultural, ecological and economic value of this once so-important species.