Effects of the winter Monsoon on the growth, mortality, and metabolism of adult oysters in Kinmen Island, Taiwan

The Pacific oyster, Crassostrea gigas, is cultivated intertidally in Kinmen Island by both a traditional way of growing oysters on the surface of stone blocks, nicknamed “rock oysters”, and a recently introduced and more efficient way of hanging them in clusters on horizontal nylon lines, i.e. “hanging oysters”. We investigated the growth and mortality of both types of oysters from late July to early December 2003, and measured condition index (CI) and rates of oxygen consumption and ammonium excretion for the hanging oyster. Growth of oyster shells stopped in early October for the hanging oysters and early December for the rock oysters. Mortality rates were higher for hanging oysters than rock oysters. Hanging oyster's O/N ratio and CI, determined by a home-made CI meter that measured the volume of oyster's inner shell cavity with high precision, decreased significantly in October. Hanging oysters were apparently in poor physiological condition during the winter Monsoon, which is characterized by cold and persistently strong wind. On Kinmen Island the monsoon season begins in late September and is accompanied by declining seawater temperature and high seston loads in oyster farms. Chlorophyll a concentrations and seston food quality decreased significantly in early October after the winter Monsoon began. The hanging oyster's poor acclimation to the local climate was probably caused by the import of its spat from southwest coast of Taiwan where it is warmer than Kinmen Island in fall and winter. We recommend that oyster growers collect spat locally or from areas of similar climate.