Effects of culture media and stocking density on biofouling, shell shape, growth, and survival of the Pacific oyster (Crassostrea gigas) and the Manila clam (Venerupis philippinarum) in suspended culture

Abstract

Suspended culture of bivalves offers a number of advantages over beach (intertidal) culture, but is often hindered by two issues: biofouling and suboptimal shell shape. This study assessed the efficacy of two novel culture media in Pacific oyster (Crassostrea gigas) grow-out and Manila clam (Venerupis philippinarum) nursery tray culture. We examined the combined effects of culture media type (expanded clay aggregate and lava rock), media volume (0, 15, 30, and 45Lm−2), and bivalve stocking density (226, 453, and 679indm−2 for oysters; 3019, 9057, and 15,094indm−2 for clams) on biofouling levels, shell characteristics, growth, condition, and survival of bivalves. Adding either media type to culture trays significantly reduced mussel fouling on oysters and effectively eliminated clam entanglement in mussel byssus. Clay aggregate was more effective at controlling tube worms and combined fouling on oysters than lava rock. Oyster shell shape index was not affected by either media type and decreased with increased stocking density. Oyster cup depth index decreased at higher media volumes and stocking densities, likely as a result of space restrictions. Reduced growth and condition were observed in both clams and oysters as media volume and stocking density increased. The presence of culture media slightly decreased oyster survival and improved clam survival. Oysters in all treatments exceeded minimum industry marketability standards. The benefits of culture media for biofouling control were realized at the lowest media volume (15Lm−2) suggesting that the efficacy of lower volumes of media should be assessed in the future.