Seasonal variations in gonad weights of the boring clams and harvest regulation periods.

概要

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The sea around Okinawa Prefecture is northern border for the distribution of boring clams. Spawning is limited to summer, and the cold seasons are rest periods. In April when the monthly mean seawater temperature reaches above 25°C, the gonads of clams turn active. Some clams mature by the beginning of June. But, there are great individual variations. Matured clams undergo spontaneous or partial spawning. With the arrival of the first typhoon, usually in August, clams discharge all remaining eggs and sperm uselessly. In an exceptional year without typhoons, the complete discharge of eggs and sperm occurs when there is a great decrease in water tempera-tures by November. The actual spawning seasons of the boring clams are from June to August. This is the period when the harvesting of the boring clams is prohibited in Okinawa Prefecture.