| Summary: | We investigated seasonal variation in the reproductive output and proximate composition of tissues (protein, lipid, and carbohydrate) of the Pacific oyster, Crassostrea gigas Thunberg, at the top (0–2 m) and bottom (3–5 m) of a long-line suspended culture in Gosung Bay, Korea. The water temperature was 2–3 °C higher at the surface than at the bottom from early spring to mid-summer. The chlorophyll a level was also higher at the surface during March and April, when a spring phytoplankton bloom occurred in the bay. The seasonal variation in the proximate composition of oyster tissues differed between the surface and the bottom as well. Carbohydrate levels in oysters at the surface were somewhat higher in fall and winter, when the oysters were actively accumulating carbohydrates in their tissues for future growth and reproduction. Oysters at the surface tended to produce more eggs during the spring to early summer spawning period; the gonadosomatic index (GSI) was significantly higher in surface oysters than in bottom oysters ( p < 0.05). The overall growth and reproduction rates of the surface oysters were higher, even though the bottom oysters were located only 1–3 m below them. Accordingly, our data suggest that culture depth in the traditional long-line suspended culture needs to be re-evaluated to maximize oyster production.
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