Stocking density and growth of the Pacific oyster ( Crassostrea gigas) and the Sydney rock oyster ( Saccostrea glomerata) in Port Stephens, Australia

Abstract

Effects of stocking density on changes of total mass and tissue mass of two commercially important species, the Sydney rock oyster ( Saccostrea glomerata) and the Pacific oyster ( Crassostrea gigas) were examined at Port Stephens, an estuary 250 km north of Sydney (eastern Australia). The lowest density used (80 individuals/tray; ∼50% tray-area coverage) represented a lower stocking density than that generally used by local oyster farmers. The medium density represented a stocking density generally used by local oyster farmers (120 individuals/tray; ∼75% tray-area coverage). The high density used contained 160 individuals/tray (∼100% tray-area coverage). The experiment ran for 9 months, from September 1999 to June 2000. Both species showed a continuous increase in total mass (shell plus soft tissues) but density effects were absent. Somatic growth showed a different pattern. For S. glomerata, positive somatic growth was observed during spring and autumn and for C. gigas, only during autumn. Thus, shell growth and somatic growth were uncoupled. Density effects on somatic mass were significant but small. Generally, differences in somatic mass between the animals kept at a low and medium density were insignificant, but somatic mass of individuals kept at a high density were significantly smaller. In terms of production of total mass of S. glomerata, differences between the medium and high stocking density were not significant but production at these densities was significantly larger than at the low density. Production of C. gigas was significantly different among densities, the largest production at the high, and the smallest at the low density. Total production of C. gigas was slightly but not significantly larger than that for S. glomerata at the low and medium density, and significantly larger at the high density. The significance of this study is that it confirms the generally anecdotal information concerning the negative relationship between stocking density and growth of oysters and makes this information available to the scientific community.