DOI: 10.5061/dryad.m37pvmd9g
title: Data for: Fisheries shocks provide an opportunity to reveal multiple recruitment sources of sardine in the Sea of Japan

description: Understanding recruitment sources is essential for stock assessments of
 marine fish populations. In 2014 and 2019, schools of sardine in the Sea
 of Japan and East China Sea (SJ-ECS), which come to spawn in Japanese
 coastal areas every spring, were shockingly sparse. Abundances of eggs and
 juveniles also declined abruptly, indicating a sharp decline in
 reproduction in the SJ-ECS. However, in spring of 2015 and 2020, age-1
 fish appeared as usual in the coastal areas, challenging the current
 assumption that sardine in the system is a self-recruiting subpopulation.
 To test the self-recruiting hypothesis, we analysed the stable oxygen and
 carbon isotopes (δ18O, δ13C) for otolith areas formed during the first
 spring and summer in otoliths of age-0 and age-1 sardines in 2010 and
 2013–2015 year-classes captured in the SJ-ECS, as indices of temperature
 and metabolic trajectories. Age-0 sardines generally showed a significant
 decrease in otolith δ18O from spring to summer, reflecting seasonal
 warming in the SJ-ECS. However, the majority of age-1 captured in spring
 2011, 2015, and 2016 showed non-decreasing profiles. The δ18O for summer
 thus revealed different migration groups: the “locals" growing up off
 the Japanese coast and the migrating “nonlocals", the former not
 being the main source of recruitment, contrary to previous assumptions.
 The isotope values of the “nonlocals” overlapped with those of age-0
 captured in the subarctic Pacific, suggesting that the “nonlocals” may be
 migrants from the Pacific, or an unobserved northward migration group in
 the SJ-ECS. Only in 2014 did the majority of age-1 consist of the
 “locals”, suggesting that the abrupt decline in catches was caused by the
 absence of the “nonlocals” and accompanying adults. Our results highlight
 the considerable uncertainty in the population structure assumed in
 current stock assessment models for Japanese sardine, thereby requiring
 focused investigations on their migrations for sustainable fisheries.
description: Collected fish were frozen after landing or on board at -20 °C,
 and thawed at a laboratory. After measurements of length and weight, the
 sagittal otoliths were extracted. The otoliths were cleaned using a thin
 brush and rinsed with fresh water. These otolith samples were processed in
 two different protocols, namely low-resolution analysis similar to
 Sakamoto et al. (2020) and high-resolution analysis similar to Aono et al.
 (2023). The majority of the otoliths were analysed using a low-resolution
 protocol. In addition to these, high-resolution data from age-0 fish
 captured in 2015 around Kyushu and the Noto Peninsula (Aono et al., 2023)
 were also included in analyses to allow a more comprehensive
 comparison. For low-resolution analysis, otoliths were
 embedded in Petropoxy 154 (Burnham Petrographics LLC) resin and kept at 80
 °C for 12 h to cure. Otoliths were ground and polished until the core was
 revealed using sandpaper and alumina suspension (BAIKOWSKI International
 Corporation). Using an otolith measurement system (RATOC System
 Engineering Co. Ltd.), daily increments were examined along the axis in
 the post-rostrum from the core as far as possible. Daily increments could
 be identified until the edge for most otoliths of age-0 fish but not for
 those of age-1 fish, likely because the otolith growths became
 significantly slower during winter. The otolith portion formed during 0–60
 dph, representing spring, was identified and milled out using a
 high-precision micro-milling system, Geomill 326 (Izumo-web, Japan). For
 approximately half of the samples, the portion of the otolith formed
 during 106–120 dph was additionally milled out to represent values for
 summer. The milling depth for the spring and summer portions was 50 and
 100 μm, respectively. The <em>δ</em><sup>18</sup>O
 and <em>δ</em><sup>13</sup>C of powdered samples
 were measured using an isotope ratio mass spectrometer (Delta V plus,
 Thermo Fisher Scientific) equipped with an automated carbonate reaction
 device (GasBench II, Thermo Fisher Scientific), and installed at the
 Atmosphere and Ocean Research Institute, the University of Tokyo, Chiba.
 Detailed analytical conditions have been reported elsewhere (e.g., Shirai
 et al. 2018), with minor modifications where 4.5-ml glass vials were used
 (Breitenbach & Bernasconi, 2011). All isotope values are reported
 using delta notation relative to the Pee Dee Belemnite. No correction was
 applied for the acid fractionation factor between calcite and aragonite
 [phosphoric acid–calcium carbonate reaction temperature 72 °C (Kim et al.
 2007)]. Analytical precisions of
 <em>δ</em><sup>18</sup>O and
 <em>δ</em><sup>13</sup>C for international
 standards (NBS-19) were 0.06–0.13 (1σ) and 0.05–0.11 ‰, respectively.
 Because the difference between the acid fractionation factor of calcite
 and aragonite is temperature dependent (Kim et al., 2007), 0.09 ‰ was
 subtracted from the <em>δ</em><sup>18</sup>O value
 to allow comparison with data analysed using another analysing system
 operating at 25 °C. High-resolution analysis was
 performed for 17 otoliths of age-0 and age-1 fish captured in 2015.
 Otoliths were embedded in epoxy resin (p-resin, Nichika Inc.) and kept in
 a dryer at room temperature for more than a day to cure. Otoliths were
 ground and polished until the core was revealed using sandpaper and
 alumina suspension (BAIKOWSKI International Corporation). Unfortunately,
 microstructure analysis was not performed for some of these otoliths. The
 otolith portions that were formed every 5–30 days or 30–160 μm were milled
 sequentially from the edge to the core using GEOMILL326. The
 <em>δ</em><sup>18</sup>O of collected otolith
 powders was determined by a customized continuous-flow isotope ratio mass
 spectrometry system (MICAL3c with IsoPrime100) at the National Institute
 of Technology, Ibaraki College, Hitachinaka, Japan (Ishimura et al., 2004;
 2008; Nishida & Ishimura, 2017). Otolith powders were reacted with
 phosphoric acid at 25 °C, and the evolved CO<sub>2</sub> was
 purified and introduced into the mass spectrometry system.
 <em>δ</em><sup>18</sup>O values of each sample
 were reported in standard δ notation (‰) relative to the Vienna Pee Dee
 Belemnite (VPDB) standard. Analytical precisions were ± 0.1 ‰ for both
 <em>δ</em><sup>18</sup>O and
 <em>δ</em><sup>13</sup>C. For otoliths for which
 microstructure analysis was not performed, the corresponding age range for
 each milling area was later estimated from the distance from the core
 using the mean relationship between otolith radius and age of other fish
 captured in the same year, season, and region. For comparison with the
 low-resolution data, the high-resolution data and data from Aono et al.
 (in revision) needed to be rescaled to spring (0–60 dph) and summer
 (106–120 dph) resolution. Therefore,
 <em>δ</em><sup>18</sup>O and
 <em>δ</em><sup>13</sup>C data for which the median
 of the corresponding age range falls in 0–60 and 106–120 dph were
 averaged, linearly weighted by the width of the milling area.
description: # Data for: Fisheries shocks provide an opportunity to reveal multiple
 recruitment sources of sardine in the Sea of Japan
 [https://doi.org/10.5061/dryad.m37pvmd9g](https://doi.org/10.5061/dryad.m37pvmd9g) The original data of otolith growth and isotope analysis for sardine in the Sea of Japan are presented here. ## Description of the data and file structure This csv file contains metadata of the collection and the results of otolith growth and isotope analyses. The empty cells are the missing values. For each row, the following data is shown: ID: individual identifier Year: Year of collection Month: Month of collection Loc_num: Collection area identifier (0: West Kyushu, 1: Oki Islands, 2: Noto Peninsula, 3: Subarctic western North Pacific). SL (mm): The size of fish in standard length (mm). age: Fish age estimated from SL and collected months. OR60: Otolith radius in micrometers at 60 days post-hatch, estimated from microstructure analysis. OR120: Otolith radius in micrometers at 12 days post-hatch, estimated from microstructure analysis. d18O_60: Oxygen stable isotope value (VPDB) for the area formed from hatch until 60 days post-hatch. d13C_60: Carbon stable isotope value (VPDB) for the area formed from hatch until 60 days post-hatch. d18O_120: Oxygen stable isotope value (VPDB) for the area formed from 106 to 120 days post-hatch. d13C_120: Carbon stable isotope value (VPDB) for the area formed from 106 to 120 days post-hatch. Memo: Additional information.