Relationship between empirical water temperature and spring characteristics of swordtip squid (Uroteuthis edulis) caught in the eastern Tsushima Strait

We analysed the data of swordtip squid Uroteuthis edulis caught in the eastern Tsushima Strait, from April to September between 2012 and 2018, to consider the relationships among mantle length and body and gonad weights, associated with the estimated seasonal migratory routes and itineraries improved by the quantitative results of the tracer experiments. Our analyses have enabled us to reveal the characteristics of kensaki-type squid and the reasons for their appearance. We identified the kensaki-type U. edulis as male individuals, caught mainly in April–June, with long slender bodies, probably adapted to empirical duration of the cold sea water temperature in the Sea of Amakusa. Researchers and fishermen have referred to the seasonal migrating group including such males as a spring-migrating group. However, the females belonging to the spring-migrating group had no kensaki-type characteristics. Moreover, contrary to that achieved by the males, the female squid had acquired greater maturation in the spring than in any other seasons. These data demonstrated that the females continued to grow to maturation even in the cold sea waters, implying a different female strategy for reproduction from that of male squid. We are concerned that the commercially valuable kensaki-type squid may decrease in the future because the water temperature in the northern East China Sea is gradually rising, probably due to global climate change.