Improvements in the physiological performance of European flat oysters Ostrea edulis (Linnaeus, 1758) cultured on elevated reef structures: Implications for oyster restoration

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 out planted on twenty four oyster reef modules (80 cm 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 enhanced physiological performance. Filtration and respiration rates, condition index, haemolymph protein concentration, haemocyte counts and gonad maturation were measured as indicators of physiological performance. During the first 15 months of oyster reef deployment, water samples were collected at regular intervals at the sea bed and at a height of 80 cm from the sea bed to determine chlorophyll a concentration, total suspended solids and bacterial abundance. Total suspended solids were significantly higher at the sea bed than at 80 cm above the sea bed at every sampling interval, while bacterial abundance adjacent to the sea bed was significantly higher than 80 cm above the sea bed in August and November 2013 when temperature was 18 °C and 15 °C, respectively. The filtration rates of oysters varied with elevation (reef/sea bed) and months. Filtration rates of 'reef oysters' (oysters on elevated reefs) were significantly higher than 'sea bed oysters' (oysters held on the sea bed). Respiration rates varied among months but were not significantly affected by elevation (reef/sea bed). 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. 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.