Analysis of the hatching site and migratory behaviour of the swordtip squid (Uroteuthis edulis) caught in the Japan Sea and Tsushima Strait in autumn estimated by statolith analysis

ABSTRACT

In contrast to the swordtip squid (Uroteuthis edulis) caught in the spring and summer, those caught in the autumn in the Japan Sea and the Tsushima Strait are characterized by thick bodies with large clubs, long tentacles and large suckers. In addition, most females and many males are immature. However, the reasons for these characteristics, as well as the hatching site and migratory behaviour, are not well known. The empirical water temperature of juveniles was estimated at ∼17°C through strontium:calcium (Sr:Ca) ratios in statoliths and information on seawater temperature distribution. The ontogenetic variation in Sr:Ca ratios decreased from hatching to 60 days after hatching and then remained stable. The estimated water temperature was ∼21°C after 60 days. Thus, individuals of the autumn-migrating group most likely hatch in the southern East China Sea in the winter and early spring and then move north-eastwards with currents and pass through the Tsushima Strait into the southern Japan Sea during or after July. However, the squid could be present in these waters without strong currents northeast of Tsushima Island, where thermoclines occur during summer, and become confined in limited layers until vertical mixing occurs in the autumn. The distribution of the squid would thereafter extend to fishing grounds in the southern and south-eastern Japan Sea and then to the Tsushima Strait. This migratory behaviour could be the cause of the observed characteristics because bottom seawater temperatures are much lower in the Japan Sea than in the East China Sea.

RESPONSIBLE EDITOR:

• Francesc Maynou

KEYWORDS:

• Swordtip squid
• statolith
• Sr:Ca ratio
• thermocline

Acknowledgements

We express our sincere thanks to T. Wada and H. Terakado for supplying the Uroteuthis edulis specimens. We are grateful to Industrial Technology Center of Saga for their permission to use the wavelength dispersive spectrometer. Sincere thanks are also expressed to Y. Sakurai and Y. Ikeda, who kindly advised us regarding the results of strontium and calcium measurements of statoliths. T. Watanabe and K. Hirate helped us to gather information about oceanic conditions. We express our gratitude to T. Aketagawa, M. Saito and Y. Ichimaru for careful statolith preparation.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by funding from the Saga prefectural local government and the Fisheries Agency of Japan.