DOI: 10.5061/dryad.msbcc2g4c
title: Data for: Stable isotopes in eye lenses reveal migration and mixing patterns of diamond squid in the western North Pacific and its marginal seas

description: 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.
description: 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 &lt; 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 δ<sup>15</sup>N and
 δ<sup>13</sup>C 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 δ<sup>15</sup>N
 and δ<sup>13</sup>C 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 δ<sup>15</sup>N and ± 0.1‰ for
 δ<sup>13</sup>C.