Seasonal variations in condition, reproductive activity, and biochemical composition of the pacific oyster, Crassostrea gigas (thunberg), in suspended culture in two coastal bays of Korea

Abstract

Seasonal variations in condition index (CI), reproductive activity. and biochemical composition of the oysters, Crasostrea gigas, in suspended culture in different nutritional conditions were compared between two bay systems (Jaran Bay and Hansan-Koje Bay) of the southern coast of Korea from January 1996 to September 1997. Differences in temperature and salinity were not significant between stations, but chlorophyll a concentrations were significantly higher at Station Josan in Jaran Bay, an outer open system, than at Station Osu in Hansan-Koje Bay, a semi-enclosed bay system with restricted food availability. CI and dry tissue weight of a standard animal showed a similar seasonal cycle, with minimum values in late summer and peaks in spring at both stations. In the Josan oysters a rapid recovery in these components commenced in November 1996 with the simultaneous accumulation of reserves (glycogen and protein) after the summer spawning. By contrast the Osu oysters recovered slowly 3 mo later in February 1997. The values were also considerably higher in Josan oysters than in Osu oysters. At both stations gametogencsis was initiated in late autumn and the breeding period was extended over several months during the summer-early autumn period. Spawning intensity during summer was, however, stronger in the Josan oysters than in the Osu ones. Food availability seemed to be a major Factor in determining gonad proliferation and thereby CI, when gametogenesis was initiated. Apparently. the high accumulation of glycogen and protein was observed in the Josan oysters so that the absolute values for the standard animal were significantly higher at Station Josan than at Station Osu. These results indicate that difference in physiological states of the oysters cultivated in the two bay systems are strongly related to site-dependent variation in the storage-utilization cycle of energy reserves (particularly glycogen), depending on food availability. Our findings also suggest that it is necessary to readjust the cultivated density of oysters to procure enough wild seeds and condition of oysters in Hansan-Koje Bay, taking carrying capacity of the buy into consideration.