Knowledge of the movements of marine organisms is essential for effective conservation schemes. Here, we investigated the lifetime habitat use of diamond squid, Thysanoteuthis rhombus, collected in the western North Pacific and its marginal seas (the Sea of Japan and the East China sea) during 2021–2022, whose migratory ecology is poorly known, using bulk stable nitrogen and carbon isotope ratios in eye lenses. From the eye lens isotope profiles, the chronology of the isotopic baseline of squid habitat was estimated by removing the effect of size-dependent changes of trophic position. Then, the baseline estimates were compared to the isoscapes of particulate organic matter. The baseline chronologies showed fluctuations during the paralarval and juvenile stages, becoming stable during the adult stage, suggesting that significant movements mainly occur during the early life stages due to current transport, with adults potentially not undertaking long-distance migrations. The squids in the marginal seas mostly originated from outside the subtropical gyre, while the squids in the subtropical gyre had various sources, including outside the gyre and southern and northern parts within the gyre, revealing a complex mixing pattern of the species. These results show that isotope chronology combined with baseline isoscapes are effective tools to understand animal migrations, which can help to manage various cephalopods and fishes.

The eye lenses of diamond squid have a structure consisting of a hemisphere-shaped proximal lens and a more flattened distal lens. The two lenses can be easily divided using forceps when frozen. The distal lens was always used in this study because it was harder and tended to retain its entire shape. Eye lens laminae averaging about 1 mm thick were peeled (delaminated) successively from the outermost edge of the lens using forceps. To deal with the stickiness of the laminae, the eye lenses were delaminated in a semi-frozen state on a dry plastic plate, within 30 minutes from taking the eye lens out from the freezer. Before each peeling, the diameter of the remaining eye lens was measured again, and the forceps were cleaned with tap water and paper towels to avoid cross-contamination. Nine to thirteen segments were removed from one lens in these individuals. To test the difference in isotopic signals between regions, an additional six to ten individuals were selected to analyse only the three segments of the eye lens corresponding to 0–50, 250–300 and 450–500 mm mantle length (ML), calculated based on the relationship between the diameter of the eye lens and mantle length. In addition, the outermost laminae < 1mm thick were occasionally sampled from eye lenses to compare their isotope ratios with those of the mantle from the same individual. 

The delaminated eye lens laminae were freeze-dried and crushed into powder. Generally, 0.4–1.0 mg of the subsample, but only about 0.1 mg for the part closest to the core, was extracted for isotope analysis. The δ¹⁵N and δ¹³C of the samples were determined at the GeoScience Laboratory (Nagoya, Japan), based on a continuous-flow stable isotope ratio mass spectrometer (Delta Plus Advantage, Thermo Fisher Scientific) coupled to an elemental analyser FLASH2000, Thermo Fisher Scientific). The δ¹⁵N and δ¹³C values were reported in δ-notation against the atmospheric N2 standard and Vienna Pee Dee Belemnite reference standard, respectively, and given as a ‰ value. The analytical precisions were ± 0.2‰ for δ¹⁵N and ± 0.1‰ for δ¹³C.