Abstract
The effectiveness of hanging culture of bloody clams, Scapharca kagoshimensis, as a parental stock for natural spawning induction was examined in different lake areas with different temperature. In laboratory experiments the number of eggs released, condition index and sex ratio of clams in three shell-length groups (26, 29, 31 mm) was examined after raising the temperature to more than 25°C. The number of eggs released and percentage of females in the three shell-length groups (26, 29, 31 mm) were 22,000, 61,000 and 158,000 eggs/female, and 13%, 36% and 43%, respectively. Due to the spawning of eggs and sperm, the condition index of clams decreased over 7 days. These results suggested that the clams of at least 31 mm in shell length are most efficient for spawning induction. In field research conducted in Lake Nakaumi, the spawning of mature clams was delayed in areas with water temperature less than 25°C. In contrast, when clams were transferred to areas with water temperature greater than 25°C, spawning could be successfully induced. The present study demonstrated that artificial regulation of spawning of cultured bloody clam can be achieved by placing the clam into natural areas of different temperature.
