Bull. Jpn. Soc. Fish. Oceanogr. 82(4), Page 167-175, 2018   Age, growth and maturity of jack mackerel Trachurus japonicus off the Pacific coast of southern Joban-Boso area Makoto Taga1,2† and Yoh Yamashita2 1 Ibaraki Prefectural Fisheries Research Institute, 3551–8 Mitsuzuka, Hiraiso, Hitachinaka, Ibaraki 311–1203, Japan. 2 Field Science Education and Research Center Kyoto University, Kita-shirakawa-Oiwakecho, Kyoto 606–8502, Japan. † E-mail: m.taga@pref.ibaraki.lg.jp Spawning season of jack mackerel Trachurus japonicus collected off the Pacific coast of southern Joban–Boso area was examined based on gonadsomatic index (GSI). Period of otolith annual ring mark formation and age were estimated based on transversal sections of sagittal otoliths. The GSI data showed that the matured individuals appeared from June to August, which was considered to be the spawning season. The age at first maturity was estimated as 2 years old in this area. Measurement of otolith marginal increments showed that annual ring marks were formed from May to September, which approximately corresponded to the spawning season. The von Bertalanffy growth curve calculated from fork length (Lt) and age at catch (t) was expressed as follows: Lt=29.4(1-e-0.38(t+1.08)) (0.33 ≤ t ≤ 19.67). According to the growth equation, the present samples from the Pacific coast of Joban–Boso area showed lower growth rate than those from Kumano-nada and its western area, with a high proportion of older fish in the study area. The low growth rate and long life of jack mackerel in the study area is probably attributable to a shorter period of optimal temperature for their growth. Key words: jack mackerel, otolith, growth, maturity, southern Joban, Boso