Population dynamics of portunid crab Charybdis bimaculata in Ise Bay, central Japan Teruyoshi NARITA, Monthon GANMANEE AND Hideo SEKIGUCHI* Faculty of Bioresources, Mie University, Tsu, Mie 514-8507, Japan ABSTRACT: The present study dealt with seasonal and interannual variations in the abundance and biomass, and spatio-temporal distributions of the portunid crab Charybdis bimaculata dominant in Ise Bay, central Japan. The abundance and biomass of the crab decreased in summer when the oxygen-poor water developed in central or inner parts of the bay, and then increased through new recruits from autumn (October–November) to the following spring (March–May) when the oxygen- poor water disappeared. Berried females were collected mainly from spring to autumn. Recruits were collected in any season. Particularly in winter, most recruits were located in the innermost part of the bay. According to the cohort separation based on size frequency distribution in carapace width of the crab specimens, the cohorts that were derived from spawning in spring to summer largely contributed to establishing and maintaining the benthic populations for the following year in the bay, whereas those from other seasons failed to recruit because of serious damage caused by the oxygen-poor water. Most crab individuals one year post hatch contributed to spawning and then died by the winter of the same year. KEY WORDS: Charybdis bimaculata, Ise Bay, megabenthos, population dynamics. INTRODUCTION The portunid crab Charybdis bimaculata is com- monly found and dominant among benthic crus- taceans in Japanese coastal waters such as Tokyo Bay, 1,2 Ise Bay, 3–6 Osaka Bay, 7,8 Yuya Bay, 9 Beppu Bay 10 and the Seto Inland Sea. 11 Although many individuals of the crab are collected by small trawl nets in these coastal waters, little ecologic informa- tion is known to date because the commercial value of the crab is very low for trawl fisheries. The crab has an important role for the production of marine fishery resources because it is an important food organism for the frog flounder Pleuronichthys cornutus 12 and the conger eel Conger myriaster 13 in the above coastal waters. It is important to know the population dynamics of this crab for successful use of fishery resources in order to clarify the megabenthos community structure in Ise Bay, central Japan. Based on short-term observations, seasonal variations in the abundance and biomass, spawning season, sex ratio 2,11 and food habits of the crab 14 have been reported, whereas those based on long-term observations have been reported only by Hossain 5 and Narita et al. 6 According to these studies, 5,6 the abundance and biomass of the crab decreased in summer when oxygen-poor water developed in the central or inner parts of the bay, and then increased from autumn to the follow- ing spring when the oxygen-poor water disap- peared. To clarify the population dynamics of the crab in the bay, it is necessary to examine its migra- tion or larval recruitment events in relation to the development/disappearance of the oxygen-poor water in the bay, based on detailed observations through the life cycle of the crab. The aims of the present study were to clarify the population dynamics of the portunid crab Charyb- dis bimaculata in Ise Bay, based on long-term observation during the 10 years from 1993 to 2002. Specific aims were: (i) to examine seasonal and interannual variations in the abundance and biomass, the occurrence of berried females, and spatio-temporal distributions of the crab; (ii) to separate different cohorts for examining the fates of these cohorts in relation to the development/ disappearance of the oxygen-poor water in the bay; and (iii) to make clear when and where new recruits are located within the bay. *Corresponding author: Tel: 81-59-231-9555. Fax: 81-59-231-9540. Email: sekiguch@bio.mie-u.ac.jp Received 6 February 2007. Accepted 12 September 2007. FISHERIES SCIENCE 2008; 74: 28–40 © 2008 Japanese Society of Fisheries Science doi:10.1111/j.1444-2906.2007.01494.x