Bull. Jpn. Soc. Fish. Oceanogr.  68(1), Page 20-26, 2004   Estimation of the Spawning Ground of Jack Mackerel in the East China Sea Based on the Catch Statistics and Biometric Data Mari YODA† , Seiji OHSHIMO and Yoshiaki HIYAMA Seikai National Fisheries Research Institute, Fisheries Research Agency, Nagasaki 851-2213, Japan †e-mail: yoda@fra.affrc.go.jp The spawning ground of jack mackerel (Trachurus japonicus) in the East China Sea was estimated by using the catch statistics of the Japanese large- and medium-type purse seine fishery from 1996 to 2000. Biometric data were obtained from speciments caught by purse seine and angling in the East China Sea and the area west of Kyushu from 1998 to 2001. From the results of histological examination, we found that females larger than 3 in the gonadosomatic index (GSI) were in maturing stage. Using this criterion, GSI data, and catch statistics, the catch amount and CPUE (kg/set) of spawners were calculated by month and by size category of catch. The spawning period was different among size category, the large and medium fish (fork length: larger than 26 cm) spawn during November to June and the small fish (fork length: between 20 and 26 cm) spawn during January to May. Spawners appeared in the area west of Kyushu from November to June. In the middle and southern area of the East China Sea, spawners appeared mainly from January to June. Sea surface temperature of the spawning ground areas ranged from 15-25°C. Key words: Trachurus japonicus, spawning ground, East China Sea, GSI, catch statistics