Swimming behaviour and migration of a swordfish recorded by an archival tag

Abstract

An archival tag equipped with sensors for temperature, depth and luminous intensity is an excellent method to elucidate behaviour and migration of marine organisms. Herein, what we believe is the first successful archival tag experiment with swordfish in the world is reported. The swordfish, which was harpooned with an archival tag encased in a plastic capsule, was released in July 1999 off the east coast of Japan. The fish was recaptured by a harpoon fishing vessel in June 2000, only 103 km from the tagging location and weighed approximately 120-kg. By comparing the water temperature data of the archival tag with oceanographic data, a cyclic seasonal migration between the food-rich Oyashio cold current area (40–45°N) during summer and the subtropical wintering area (10–20°N) was suggested. During most of the days of observation and for the majority of the time within a day, tagged fish swam in cold water (3–6°C) and deeper than approximately 200 m; at night, fish generally stayed near the warmer surface waters (21–27°C). The greatest swimming depth was approximated to be 900 m, deduced from ambient water temperature data. The swimming depth and behaviour pattern changed in response to the ambient water temperature.