Influence of migratory route on early maturation of swordtip squid, Uroteuthis edulis, caught off western Kyushu Island, Japan

Abstract

Mature female Swordtip squid (Uroteuthis edulis) of ca. 100 mm mantle length are found in Tachibana Bay (western Kyushu, Japan) from June to August, although many mature females caught in the eastern Tsushima Straits (the entrance to the Japan Sea) are over ca. 150 mm. To reveal the cause for the difference, statolith analyses and Lagrangian tracer experiments were conducted. The results show that the small mature female squid would have hatched in the southern East China Sea in winter and migrated to the Sea of Amakusa via the Kuroshio Current, as also would many squid caught in the eastern Tsushima Straits. The two major differences between them were found to be the history of their empirical water temperature (estimated from the ratio of Strontium to Calcium in the statoliths) and the depth of the waters where they were caught. The small mature females would have experienced water temperatures rising from 18° to 20°C in the coastal waters off Kyushu, probably over 16 °C even at the bottom layer, for one month or more before being caught. Considering also the results from previous research, a hypothesis is proposed that, after reaching the minimum age of maturity, the squid could begin (and continue) to mature when the water temperature in the bottom layer (or in the deepest zone to which this species can dive) exceeds ca. 16 °C, and take about one month to reach full maturity. The method used here to explain the size differences detected among siblings born in the same locality but following migratory paths with very different temperature conditions is a useful and the results also suggest another small mature squid that appear in the southern East China Sea in spring may migrate from the warmer waters, which can be contributed to the stock management of this species.