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

Seasonal variations in condition index (CI), reproductive activity. and bio chemical 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 s ignificant between stations, but chlorophyll a concentrations were signific antly higher at Station Josan in Jaran Bay, an outer open system, than at S tation Osu in Hansan-Koje Bay, a semi-enclosed bay system with restricted f ood availability. CI and dry tissue weight of a standard animal showed a si milar seasonal cycle, with minimum values in late summer and peaks in sprin g at both stations. In the Josan oysters a rapid recovery in these componen ts commenced in November 1996 with the simultaneous accumulation of reserve s (glycogen and protein) after the summer spawning. By contrast the Osu oys ters recovered slowly 3 mo later in February 1997. The values were also con siderably higher in Josan oysters than in Osu oysters. At both stations gam etogencsis was initiated in late autumn and the breeding period was extende d over several months during the summer-early autumn period. Spawning inten sity during summer was, however, stronger in the Josan oysters than in the Osu ones. Food availability seemed to be a major Factor in determining gona d proliferation and thereby CI, when gametogenesis was initiated. Apparentl y. the high accumulation of glycogen and protein was observed in the Josan oysters so that the absolute values for the standard animal were significan tly higher at Station Josan than at Station Osu. These results indicate tha t difference in physiological states of the oysters cultivated in the two b ay 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 readju st the cultivated density of oysters to procure enough wild seeds and condi tion of oysters in Hansan-Koje Bay, taking carrying capacity of the buy int o consideration.