Distribution and geographic differences in stable isotope ratios of diamond squid Thysanoteuthis rhombus (Cephalopoda: Thysanoteuthidae) in Japanese waters

Diamond squid Thysanoteuthis rhombus (Troschel, 1857), which can grow to a dorsal mantle length of ~100 cm in 1 year, is an important fisheries resource in subtropical and temperate waters worldwide. Around Japan, it is an important resource in the East China Sea, Sea of Japan, and western North Pacific Subtropical Gyre, including the Kuroshio area. It is not known if linkages occur between these habitats. To clarify the species distribution, we estimated the habitat of young squid using a generalized additive model based on trawl surveys and analyzed carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) in tissue samples. Based on trawl observations, young squid were distributed around northeastern Taiwan and the Okinawa Islands in June–July and in the Sea of Japan and the Kuroshio extension areas in August–September. We observed ontogenetic changes in the stable isotope ratios. The δ¹⁵N values in large diamond squid (dorsal mantle length > 400 mm) were significantly lower in the Pacific than those in other areas. Considering that the δ¹⁵N of forage fish is low in the Pacific and high in the East China Sea and Sea of Japan, large diamond squid in the northwest Pacific presumably have two distinct habitats.