Interannual, seasonal, local and body size variations in reproduction of the prawn Penaeus (Marsupenaeus) japonicus (Crustacea : Decapoda : Penaeidae) in the Ariake Sea and Tachibana Bay, Japan

The reproductive biology of female Penaeus japonicus Bate was investigated in the Ariake Sea sand Tachibana Bay (located outside the Ariake Sea), Japa n from 1994 to 1996. Interannual, seasonal, individual female body size and spatial influences on the incidence of spawning were examined. The proport ion of inseminated females, increased with increasing body size up to 170 m m body length (BL) and decreased thereafter. The minimum size at maturity w as similar between years; 130 to 140 mm BL. The minimum size of individuals with developing ovaries or spermatophores differed from that of ripe femal es by 20 to 25 mm. Spawning occurred in the central part of the Ariake Sea and Tachibana Bay but rarely in the inner part. P. japonicus had a clear re productive cycle. Spawning started earlier and ended later, occurring from mid-May to mid-October, in the eastern central part of and outside of the A riake Sea compared with the western central part, where spawning occurred f rom mid-June to mid-September. Spawning dynamics differed across seasons, b ody sizes and areas but not across years. The seasonal peak in the proporti on of ripe females varied with body size. In small individuals (130 to 169 mm BL), no peak was observed, whereas in large individuals (>170 mm BL) the proportion of ripe individuals peaked in June. The proportion of ripe indi viduals increased with increasing body size and was high outside the Ariake Sea. Factors causing the variation in spawning dynamics are discussed.