Manila clam (Tapes philippinarum) culture and sediment interactions were tested by comparing two rearing areas, including an oceanic (‘Le-Ferret’) and a more estuarine (‘Les-Jacquets’) sites in the bay of Arcachon (France). The growth of a calibrated clam population (10-mm spat) was monitored in these two areas with a concomitant sediment-water interface survey over a 1.5-year period. Two sites per area, including control and rearing plots, were sampled on a monthly basis. The potential clam farming impacts by bioturbation and interactions were examined at three sediment depths: 0−1, 1−2 and 2−10 cm. Moreover, the main hydrobiological parameters were measured on a weekly basis to establish a relationship between these parameters and sediment-water interface characteristics. The existence of a gradient between the three depths was revealed for most of the parameters examined, with the exception of silt and organic carbon levels, and this regardless of the area examined. Clam growth showed a rate improvement in the oceanic area, which is characterized by a lower silt content. The clam effect was minimal and the activity identified at the ‘Ferret’ site was in fact due to the presence of a net which acted as a particle trap. No significant relationship was established between water column parameters and those of the sediment-water interface at the two geographical sites examined. These results demonstrate that clam rearing had only a limited effect on the environmental sediment parameters (i.e. water percentage, and phaeopigments and silt levels) from a spatio-temporal point of view. Therefore, a return to environmental conditions existing before the implementation of clam farming is likely to occur upon cessation of this activity.

A study of the growth and mortality of the European oyster Ostrea edulis, including the effects of Marteilia refringens and Bonamia ostreae, was carried out in three oceanic areas in the Bay of Areachon from May 1989 to February 1991. One-year-old spat, free from discase were used for the trials. Six months after beginning the trials, Marteilia refringens had severely infested the cultures. Although Bonamia ostreae was found during the first year, the rate of infestation was low. In the second year Bonamiasis increased and high mortality (>60%) occurred in Ostrea edulis populations. Although the growth of the oysters was good, their condition index was generally poor. Moreover, shell perforations by Polydora sp. were observed. In spring 1990, a low glycogen storage was notedwhich may have been related to the Marteilia refringens infestation although no relationship with the carbohydrate content and mortality was detected. The influence of different rearing procedures on Ostrea edulis survival and development in four commercial batches of oysters, free of discase, was also studied. The growth of Ostrea edulis in single culture was better than in cultures mixed with Crassostrea gigas. However, infestation by Marteilia refringens and Bonamia ostreae in the laid cultures was quite similar to that of the experimental cultures (bags). Due to the persistence of these two parasites, the culture of Ostrea edulis on a commercial scale is not advisable in the Bay of Arcachon.