Tagging studies on the diamond squid (Thysanoteuthis rhombus) in the western Sea of Japan

This paper describes the tag-recapture experiments conducted on the diamond squid (Thysanoteuthis rhombus) in the western Sea of Japan in October and November during 2001-2006 to elucidate its horizontal migratory ecology. A total of 2,751 squid were tagged and released using disc-type tags off Tottori and Hyogo prefectures by researchers (Group A, 791 squid in 9 experiments) and fishers (Group B, 1,960 squid in 5 experiments), and 163 were recaptured (121 in Group A and 42 in Group B). Recapture rates were 0-22.7% (overall rate= 15.3%, experiment mean= 11.7%) in Group A and 0-6.4% (overall rate=2.1%, experiment mean=2.5%) in Group B, which are high compared to the results of previous tagging studies on oceanic and neritic squids. The high recapture rates were presumably due to regional oceanographic-fisheries peculiarities in the western Sea of Japan where catches of T. rhombus tend to be high in warm nearshore areas. Recaptures occurred both west and east of the release sites each year, but more squid were caught to the east. Distances between release and recapture sites ranged from 0.5 to 513 km, and mean movement speeds were 0-78 km/d, which suggest that T. rhombus is a slow inactive swimmer.