Bull. Jpn. Soc. Fish. Oceanogr. 84(3), Page 178-186, 2020   Age, growth and spawning period of rabbitfish Siganus fuscescens in the eastern Seto Inland Sea, Japan Masayuki Yamamoto1†,Norio Tanada2,Tsuyoshi Mototani3, Yasuhisa Kobayashi4 and Satoshi Katayama5 1 Kagawa Prefectural Fisheries Experimental Station, 75–5 Yashima-higashi-machi, Takamatsu, Kagawa 761–0111, Japan 2 Fisheries Regulation Division, Tokushima Prefectural Agriculture, Forestry and Fisheries Department, 1–1 Bandai, Tokushima, Tokushima 770–8570, Japan 3 Okayama Prefectural Agriculture, Forestry and Fisheries Department Fisheries Division, 2–4–6 Uchisange, Kita, Okayama 701–4303, Japan 4 Department of Fisheries, Faculty of Agriculture, Kindai University, 3327–204 Naka-machi, Nara 631–8505, Japan 5 Faculty of Agriculture, Tohoku University, 468–1 Aramaki-Aza-Aoba, Aoba, Sendai, Miyagi 980–8572 † E-mail: ky0554@pref.kagawa.lg.jp To understand the fisheries biology of rabbitfish Siganus fuscescens in the eastern Seto Inland Sea, Japan, we examined the age, growth, and spawning period of fish collected off Okayama, Kagawa, and Tokushima Prefectures from April to December, 2013. The standard length ranged from 117 to 310 mm. The gonadosomatic index (GSI) which ranged from 0.04 to 40.36 in females and from 0.01 to 30.16 in males, was high from the middle of June to the end of July and low in the other months. Based on histological observation of gonads in June and July, the females with GSI of ≥3.57 and males with GSI of ≥4.79 matured, and females with involution stage were observed in late July. The females with GSI of ≥3.57 and males with GSI of ≥4.79 appeared from June to August and from June to July, respectively. The GSI of both sexes was high from the middle of June to the end of July. These findings suggest that the fish spawned from June to August and the main spawning season was from between the middle of June to the end of July. The check mark on otoliths was formed once a year from June to August. The maximum ages for males and females were 4 and 8 years old, respectively. There was no significant growth difference between males and females. The von Bertalanffy growth equation estimated as follows: SLt=255 (1-e-0.56 (t+0.95)), for where SLt is the standard length (mm) at age t (year). The relationship between standard length (mm) and body weight (g) was expressed as follows: BW=2.87·10-5·SL 3.00. Key words: jgonadosomatic index, otolith crossing section, rabbitfish, Seto Inland Sea