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 Gran de-Riviere and Gascons in the Baie des Chaleurs. The total biomass of fouling organisms was 2-3 times greater at Gascons than at Grande-Rivi ere and at both sites decreased with increasing depth. Throughout the study, at Grande-Riviere, at the mouth of the bay, the fouling communi ty 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 predo minated in the summer while M. edulis and H. arctica predominated in t he autumn. Factors potentially explaining the success of M. edulis and H. arctica in the fouling community are the loss of hydroids due to s elective 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 muscl e and other soft tissues, but only a slight increase in shell height o f 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% fo r shell height. Because fouling decreases with depth and also varies m arkedly between localities, judicious choosing of culture sites in ter m of fouling development could substantially improve meat production.