Migratory routes of the swordtip squid Uroteuthis edulis inferred from statolith analysis

Abstract

Despite the commercial importance of swordtip squid Uroteuthis edulis in Tsushima Strait, which lies between southwestern Japan and South Korea, data on its hatching grounds and migratory routes are lacking. To infer this information, we first revealed a significant negative correlation between ambient water temperatures and strontium:calcium (Sr:Ca) ratios in squid statoliths through tank experiments. Next, we counted statolith microincrements in spring and summer migratory groups of the species to elucidate their hatching dates, after measuring Sr:Ca ratios from the nucleus to the edge of each statolith. Based on these methods, spring and summer groups appear to inhabit the southern East China Sea as juveniles, and move northeastward with currents along shelf edges in the Kuroshio region. However, individuals in the spring group are apparently affected by cold waters in the northern East China Sea before reaching the Tsushima Strait, likely resulting in the unique characteristics of this seasonal group. In contrast, those in the summer group are likely unaffected because of higher water temperatures in summer than in spring. This study is the first to infer the migratory routes of the spring and summer groups of the swordtip squid.