Bull. Jpn. Soc. Fish. Oceanogr. 83(1), Page 1-9, 2019   Annual reproductive cycle and shell length frequency distribution of the clam Mactra chinensis in the dredge fishing ground in Mikawa Bay Kohei Kawamura1†, Satoshi Yamada2, Yasuo Nakashima3, Hirotake Hattori4 , Masahiro Narita1 and Yasuhiro Hirazawa1 1 Fishery Division, Nishimikawa Regional Office, Aichi Prefectural Government, 1–4 Myodaijihonmachi Okazaki, Aichi 444–0860, Japan 2 Fishery Division, Higashimikawa Regional Office, Aichi Prefectural Government, 5–4 Hacchodori Toyohashi, Aichi 440–0806, Japan 3 Fisheries Administration Division, Department of Agriculture, Forestry, Fisheries, Aichi Prefectual Government, 3–1–2 Sannomaru Naka-ku, Nagoya, Aichi 460–8501, Japan 4 Marine Resources Research Center, Aichi Fisheries Research Institute, 2–1 Toyoura Toyohama, Minamichita, Chita-gun, Aichi 470–3412, Japan † E-mail: kouhei_kawamura@pref.aichi.lg.jp The annual reproductive cycle and shell length (SL) frequency distribution of the clam Mactra chinensis was examined based on specimens collected from the dredge fishing ground in Mikawa Bay from February 2017 to February 2018. Mature specimens were collected mainly from April to July. The gonads began to degenerate from August and most specimens collected from November to February were immature. These results indicate that the major spawning season of M. chinensis in Mikawa Bay is from April to July. Based on cohort analysis using SL data, most specimens collected from February to May were commercial-sized (>40 mm SL), and new cohorts (<30 mm SL) recruited in September 2017. The fraction of the newly recruited cohorts increased from September to November 2017. They showed rapid growth, attaining the commercial size by February 2018. These results indicate that the early juveniles of M. chinensis settle down from spring to early summer, and grow up to the commercial size within one year. Key words: Mactra chinensis, annual reproductive cycle, condition factor, shell length frequency distribution, Mikawa Bay