Fouling development and its effect on the growth of juvenile giant scallops (Placopecten magellanicus) in suspended culture

We examined the development of the fouling community on pearl nets and its effect on the growth of juvenile Placopecten magellanicus at Grande-Rivière and Gascons in the Baie des Chaleurs. The total biomass of fouling organisms was 2-3 times greater at Gascons than at Grande-Rivière and at both sites decreased with increasing depth. Throughout the study, at Grande-Rivière, at the mouth of the bay, the fouling community was dominated by two bivalves, Mytilus edulis and Hiatella arctica, and the hydroid Obelia gelatinosa was third in abundance. In contrast, at Gascons, 40 km inside the bay, the hydroid Tubularia larynx predominated in the summer while M. edulis and H. arctica predominated in the autumn. Factors potentially explaining the success of M. edulis and H. arctica in the fouling community are the loss of hydroids due to selective grazing by nudibranchs, the rapid growth rate of the bivalves and their ability to inhibit settlement of competitors. Changing the pearl nets regularly results in a sharp increase in the yield of muscle and other soft tissues, but only a slight increase in shell height of the cultured scallops. For example, for a 30-mm scallop suspended at 9 m in depth for 4 months, the increase in muscle mass is 68% greater in cleaned than in fouled nets whereas the difference is only 4.8% for shell height. Because fouling decreases with depth and also varies markedly between localities, judicious choosing of culture sites in term of fouling development could substantially improve meat production.