The Sea of Amakusa playing the role of a distributor of swordtip squid (Uroteuthis edulis) migrating from the East China Sea to the east and west sides of Japan

We investigated the monthly numbers of tracers moving from the southern East China Sea to the east side of Japan via the Sea of Amakusa, on the west side of Japan, by tracer experiments. We estimated the migratory route of large Swordtip squid (Uroteuthis edulis) caught in Sagami Bay, on the east side of Japan, by the micro-increment counts and variation of strontium:calcium ratios of the statoliths to test our hypothesis that the Sea of Amakusa plays the role of a distributor of this species to the east and west sides of Japan. Results of the monthly numbers of the tracers were generally consistent with the season of peak catches on the east side. The older tracers reaching the east side of Japan were found to have passed through the Sea of Amakusa at larger rates. Moreover, the estimated empirical water temperature history of the large squid caught in Sagami Bay showed two peaks, in winter and early summer, implying that the squid experienced the Kuroshio current twice and that during spring, they stayed in waters whose temperature was a few degrees lower than that of the Kuroshio; that is, in the Sea of Amakusa. Frequent and extensive marine observations will be needed to fully understand the distribution of this species around Japan.