Annual reproductive cycle and spawning ground formation in Japanese jack mackerel Trachurus japonicus in the Uwa Sea, Japan

To determine the annual reproductive cycle and the formation of spawning grounds in the Japanese jack mackerel Trachurus japonicus in the Uwa Sea, Japan in relation to sea temperature, seasonal changes in the ovary histomorphology and the catch per unit effort (CPUE) of adult fish were analyzed. Japanese jack mackerel in this region initiated vitellogenesis in winter and spawned between late April and early July in 2013–2015, with a quotient analysis suggesting that suitable temperatures for these two processes were 16–20 °C and 16–21 °C, respectively. However, the CPUE of adult fish was consistently high in the central Uwa Sea irrespective of temperature, indicating that the dense distribution of adult fish there is affected by additional parameters, such as phytoplankton productivity and migration. Comparison of the hatching period with the spawning season suggested that individuals recruited to the population in the Uwa Sea are derived not only from the Uwa Sea but also from other sea areas including the East China Sea. Therefore, further research on the hatching sites and recruitment processes of Japanese jack mackerel is required to evaluate the effectiveness of conservation measures for adult fish in the Uwa Sea.