Author(s): Tatsuya Yurimoto

Abstract: In Japan, the peak spawning period of the Pacific oyster, Crassostrea gigas, is from June to August. The ideal water temperature for spawning is 23–25 °C, and rapid temperature and salinity changes such as those caused by weather changes are known to stimulate spawning events. In this study, at approximately 15:50, on May 9, 2015, we observed the release of a turbid white solution from a wild population of the C. gigas oysters attached approximately at sea level to the harbor quay in Nagasaki Bay. Weather data for the area were obtained from the website of the Japanese Metrological Agency, and analyzed to identify changes in air temperature, rainfall, and tidal rhythm that occurred during or just prior to the observed event. Therefore, it was determined that on the day of the event, there was an increase in air temperature during the afternoon, because rainfall occurred in the morning. By the time of the event, the air temperature had reached 23 °C, which is considered the optimal temperature for C. gigas spawning. Thus, the observed event was identified as a spawning event.

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