Seasonal Growth, Reproduction, and Health of the Pacific Oysters Cultured in an Intertidal Rack System in Garorim Bay, West Coast of Korea

This study investigated seasonal variation in tissue growth, reproductive status, energy reserves, and digestive tubule condition in the Pacific oyster, Magallana gigas, cultured in an off-bottom intertidal rack system in Garorim Bay on the west coast of Korea. Monthly samples of adult oysters were collected from February 2009 to December 2010. Condition index (CI), total carbohydrate content, reproductive stage, female reproductive effort estimated by indirect enzyme-linked immunosorbent assay (ELISA), and digestive tubule atrophy (DTA) were measured concurrently in individual oysters. Gametogenesis was largely synchronized between sexes and was strongly associated with seasonal sea surface temperature (r=0.84, p<0.0001). Carbohydrate content peaked in early spring and declined during the spawning season. In females, carbohydrate content was negatively correlated with gonad somatic index (GSI) (r=−0.30, p<0.0001), suggesting that oysters investing more heavily in egg production tended to have lower remaining carbohydrate reserves. GSI showed interannual variation in spawning timing, peaking at 37.9% in June 2009 and 37.0% in July 2010. DTA showed temporal variation, with relatively elevated values in late summer–early autumn 2009 and mid-winter 2010. The late-summer 2009 increase in DTA coincided with a marked decline in salinity; however, monthly mean analyses did not establish a causal relationship between salinity and DTA. Together, these results indicate that the seasonal physiological condition of intertidal rack-cultured oysters reflects a dynamic balance between reproductive investment, carbohydrate reserves, and digestive gland condition, with this balance potentially modified by seasonal environmental variability. These results provide a cross-sectional, multi-parameter baseline for seasonal monitoring of reproductive status, energy reserves, and digestive tubule condition in intertidal rack-cultured oysters in Garorim Bay.