The distribution and early growth of juvenile Pacific bluefin tuna (PBF) Thunnus orientalis were studied based on trawl surveys conducted around Sado Island, eastern Sea of Japan, from late August to early September in 2017 and 2018. Juvenile PBF were mainly collected in an area between the Tsushima Warm Current and land where the surface water (5-m depth) temperature ranged from 22.8 to 27.2 °C. The fork length (FL) of the fish ranged from 12 to 320 mm, and there were modes at 140–170 mm FL in the frequency distribution for both years. In 2018, there was also a mode at around 50 mm FL. Otolith microstructure analysis revealed that hatch date mainly peaked in early and mid July in both years, and in early and mid August in 2018, and corresponded with the modes of FL. In addition, differences in growth were observed between year classes, and between size classes in 2018. Total gill raker number increased with FL and was almost constant at around 100 mm FL. This study provides an update of the distribution of juvenile PBF in the Sea of Japan, which was found to be related to the Tsushima Warm Current, and indicates the importance of the eastern Sea of Japan as a nursery ground for PBF that hatch from July to August.

中文翻译：

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日本东海佐渡岛附近太平洋蓝鳍金枪鱼Thunnus orientalis幼鱼的分布及早期生长

2017 年和 2018 年 8 月下旬至 9 月初在日本东海佐渡岛附近进行拖网调查，研究了太平洋蓝鳍金枪鱼 (PBF) Thunnus orientalis 幼鱼的分布和早期生长。在对马暖流和陆地之间，地表水（5 米深）温度范围为 22.8 至 27.2 °C。鱼的叉长（FL）范围为 12 至 320 mm，并且在这两个年份的频率分布中都存在 140-170 mm FL 的模式。2018 年，还有一种模式在 50 mm FL 左右。耳石微观结构分析表明，孵化日期主要在这两年的7月上旬和中旬以及2018年的8月上旬和中旬达到顶峰，与FL模式相对应。此外，观察到年级之间的增长差异，和 2018 年的大小等级之间。总鳃耙数量随着 FL 增加并且几乎恒定在 100 毫米 FL 左右。本研究提供了日本海中与对马暖流有关的幼鱼 PBF 分布的最新情况，并表明日本东部海作为 7 月至 7 月孵化的 PBF 温床的重要性。八月。