Bull. Jpn. Soc. Fish. Oceanogr. 84(2), Page 79-88, 2020   Mature characteristic of jack mackerel (Trachurus japonicus) in term low level resource around Sagami Bay and Sagami-nada Shozo Takamura1†,Yuki Suzuki2,Shinga Ogihara1, Sho Furuichi3 and Chikako Watanabe3 1 Kanagawa Prefectural Fisheries Technology Center Sagami Bay Experimental Station, 1–2–1 Hayakawa, Odawara, Kanagawa 250–0021, Japan. 2 Shizuoka Prefectural Reserch Institute of Fishery Izu Branch, 251–1 Shirahama, Shimoda, Shizuoka 415–0012, Japan. 3 Research Center for Fisheries Management, National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, 2–12–4 Fukuura, Kanazawa, Yokohama, Kanagawa 236–8648, Japan. † E-mail: takamura.3w6@pref.kanagawa.jp Sexual maturation and spawning season of jack mackerel Trachurus japonicus caught in the sea areas around Sagami Bay and Sagami-nada were examined based on gonad histological observation and gonad somatic index (GSI) of males and females. The males and females of high value GSI appeared from March to September, and GSI average of males and females was highest in May, and June, respectively. Based on observation of gonad tissue section, the males with matured testes appeared from March to July, and the females with ovaries which contained hydrated oocytes or migratory nucleus appeared from April to July, respectively. The minimum age at first maturity was 0 years old for males, and 1 year old for males in this area. On the other hand, the oldest matured male individual was 11 years old, and the oldest female was 23 years old. Estimated 50% maturity size was 188.2 mm in males and 230.6 mm in females, in fork length. This suggests that males mature at a smaller size and younger age than females. From the results of comparing age and GSI, the tendency that the GSI value decreases was recognized in the old fish, but jack mackerels in this area continue contributing to reproduction for over ten years from the time they mature and reproduce for the first time. Key words: jack mackerel, maturity, gonad histology, age, Sagami Bay, Sagami-nada