Growth, Reproductive Condition, and Digestive Tubule Atrophy of Pacific Oyster Crassostrea gigas in Gamakman Bay off the Southern Coast of Korea

Spat of Pacific oysters (Crassostrea gigas) were collected from Gamakman Bay, Korea, and raised in a spat hardening facility located in the low intertidal zone of the bay for a “hardening/stunting” period of 10 mo. Seasonal changes in growth, reproductive condition, and digestive tubule atrophy (DTA) of these “hardened/stunted” oysters were monitored for more than a year after transplanting to a suspended longline system in a grow-out area in the bay. After transplantation, the hardened/stunted oysters showed a logarithmic increase in shell size for the first 4 mo, from June to October, and growth remained stable from late fall to early spring. During the 12 mo of the grow-out, the shell size of the hardened/stunted oysters increased from 15.4–74.2 mm, and tissue weight increased from 0.49–12.85 g. Histological analysis revealed that gametogenesis of hardened/stunted oysters commenced as early as February when water temperature remained at 10°C, and spawning occurred from July to September when water temperature reached 25–27°C. DTA assessed from histological analysis was higher from September to February, when the chlorophyll a level in the bay was lower. These data suggest that seasonal fluctuations in water temperature and food availability in the water column are the 2 main environmental parameters governing reproduction and growth of oyster in Gamakman Bay, and DTA could be a useful biomarker for monitoring the nutritional condition of oysters.