Bottom culture is a promising strategy for final grow-out of the Portuguese oyster Crassostrea angulata

Almost all the Portuguese oyster Crassostrea angulata in China are grown in nearshore farms using suspended culture. Problems such as overstocking and environmental degradation have occurred in many suspended oyster farms. In this study, two batches of C. angulata, which were pre-grown in suspended culture in Dapeng Cove, South China Sea, were transplanted onto the seabed at the mouth of the bay. The first batch of small-sized C. angulata was deployed on March 16, 2019 (bottom culture A), while the second batch of large-sized oysters was deployed on July 6, 2019 (bottom culture B). Temporal variations in the survival and growth of these oysters were tested against their counterparts that continued growing in suspended culture. Results indicated mortality of oysters in bottom culture A was mainly a result of crab predation, while summer mass mortalities in the suspended culture and bottom culture B were a result of stress from reproduction and high temperatures. Oysters in bottom culture A exhibited smaller but cleaner shells than those in suspended culture. The soft tissue of C. angulata in all treatments continued to increase until August 22. Accelerated increases in tissue weight and condition index (CI) from early July to late August were mainly attributed to gamete formation, while subsequent decreases in tissue weight and CI likely resulted from the spawning activity. Oysters in bottom culture A consistently performed better in terms of tissue growth and CI than those in suspended culture. Individuals in both the suspended culture and bottom culture B exhibited very similar growth patterns after July 6, when overcrowding at the oyster farm was partially resolved. The results of this study indicate that bottom culture is a promising strategy for producing meaty and clean C. angulata within a short duration. This method also provides a feasible solution to the overcrowding problem in suspended oyster farms. Nevertheless, a simple, efficient, and economical method to reduce crab predation in bottom culture is needed.