The diamond squid (Thysanoteuthis rhombus): A review of the fishery and recent research in Japan

Abstract

Abstract We review the fishery for the diamond squid ( Thysanoteuthis rhombus ) in Japan and recent Japanese research conducted on this species. T. rhombus occurs worldwide in tropical and subtropical waters, including near southern and central Japan, where it is targeted by a growing fishery. The annual catch in Japan increased from 339 t in 1989 to almost 6000 t in 2001, and during 1998–2003, averaged close to 4900 t. The main fishing grounds are located in the Sea of Japan, Okinawa Prefecture, and Kagoshima Prefecture, and 90% of the total catch is made in the Sea of Japan and Okinawa. In the Sea of Japan, it is caught with free-floating angling gear called “taru-nagashi” and inshore trap nets; the fishery there usually runs from July to February, with highest catches occurring in September–December. The annual catch in the Sea of Japan during 1998–2003 ranged between 1600 and 3700 t. In Okinawa, it is fished primarily in the daytime with free-floating angling gear called “hata-nagashi”; the fishery there runs mainly from November to April, with highest catches occurring in February–April. The annual catch in Okinawa increased from 15 t in 1989 to 2300 t in 2003. The free-floating egg masses of T. rhombus occur widely in surface waters near southern and central Japan, and paralarval distribution patterns suggest that it spawns over a wide area almost year-round. Part of the population is transported by the Tsushima Current into the Sea of Japan, but little is known about this migration. The vertical distribution of T. rhombus varies regionally; in Okinawa, it occurs mainly at 300–650 m depth during the day and 0–150 m depth at night, while in the Sea of Japan, it occurs mainly at 75–100 m depth during the day and 0–50 m depth at night. Its life span is thought to be about 1 year, and squid larger than 30 cm mantle length grow about 7–10 cm mo −1 .