Restoration of the European flat oyster Ostrea edulis using elevated broodstock reefs

Abstract

The precarious status of flat oyster Ostrea edulis stocks in Europe is widely acknowledged. To build a scientific basis for oyster restoration, an elevated experimental reef stocked with O. edulis was established within Poole Bay (Dorset, UK). Oysters were planted out on twenty four oyster reef modules (80cm above sea bed) and compared with oysters held on the sea bed close to each reef module to test the hypothesis that a reef habitat enhances physiological performance of O. edulis and improves local biodiversity. The environmental data indicated that there was no significant difference in temperature or salinity between the elevated reefs and sea bed. Whilst total suspended solids were significantly higher at the sea bed than at 80 cm above the sea bed at every sampling interval. The filtration rates of oysters varied with elevation (reef/sea bed) and months. Filtration rates of reef oysters were significantly higher than sea bed oysters. Respiration rates varied among months but were not significantly affected by elevation. Elevation and month also affected the total number of haemocytes and the granulocyte population; reef oysters had significantly higher numbers of haemocytes than sea bed oysters. Althought geographical variation in filtration rate was also observed, this study suggested that the improved physiology of reef oysters in summer was still notable when comparing between different populations. An increase in filtration rate for reef oysters may, however, increase the likelihood of uptake of the protozoan parasite, Bonamia sp.. However, haemocyte and haemolymph protein data suggested that there was no differential impact on physiology between reef and sea bed oysters as a result of Bonamia sp. infection. Fifty-four epifaunal species including oyster spat were found on oyster valves in reef boxes whilst only 23 species with no oyster spat were found on oyster valves in oyster cages laid on sea bed. The epifaunal community on oyster valves on reefs was significantly different from those sea bed valves but also had some species in common. Sixty-five mobile species were observed in reef boxes, whilst 47 species were recorded in oyster cages. These differences in faunal communities observed on the elevated reefs and sea bed implied that the presence of elevated reef habitats created by O. edulis valves can enhance or accelerate local diversity in Poole Bay. As current stocks of European flat oysters (O. edulis) in Europe have declined in both abundance and distribution, the results of this pilot study suggest that the culture of oysters on elevated reef structure represents at least a partial solution to improve O. edulis physiology for restoration in Europe.

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