Spawning groups of Japanese jack mackerel Trachurus japonicus in the low stock level period were segregated. Segregation was done by the visual examination of monthly changes of the frequency distribution of fork length and/or partly the monthly changes of the mean gonad index.
In the Kuroshio Current System, two spawning groups were recognized: the spring spawning group spawns in spring mainly from February to April and distributes from southern Kyushu to Kanto region; the summer spawning group spawns in summer mainly from June to August and is present only in Kanto region.
In the East China Sea and Tsushima Current System, two spawning groups are recognized: the winter spawning group spawns in winter mainly December to January and is present in Wakasa Bay and Noto regions; the spring-summer spawning group spawns from spring to summer mainly April to June and is present in northwestern Kyushu region.
Comparison of the population structure of both high stock level (former studies) and low level (present study) shows that basically no differences between the two periods exist.