Bull. Jpn. Soc. Fish. Oceanogr. 69(1), Page 37-44, 2005   Growth, Maturation and Mass Mortality of Konosirus punctatus in Lake Shinji, a Coastal Lagoon, Japan Yu ISHITOBI†1, Jun-ichi HIRATSUKA2, Hiromichi KUWABARA2, Masumi YAMAMURO3, Yoshiyuki NAKAMURA4 and Shimpei MORIWAKI5 1 Shimane Prefectural Institute of Public Health & Environmental Science, 582-1 Nishihamasada, Matsue 690-0122, Japan 2 Shimane Research Group of Wildlife, c/o Dr. Iwao Sakamoto, Department of Medicine, Shimane University, 89-1 Enyamachi, Izumo 693-0021, Japan 3 Marine Resources and Environment Department, AIST, 1-1-3 Higashi, Ibaragi 305-0046, Japan 4 Marine Environment and Engineering Department, Port and Air port Research Institute, 3-1-1 Nagase, Yokosuka 239-0826, Japan 5 Shimane Prefectural Fisheries Experimental Station, 25-1 Setogashima, Hamada 697-0051, Japan†e-mail: ishitobi-yu@pref.shimane.jp In order to know the growth and maturation of Konosirus punctatus in Lake Shinji, monthly examination was carried out. Fishes were collected using a set net at the western end of the lake from March 2002 to March 2003. Large, medium and small size groups of K. punctatus occurred in March, April and July, respectively. The large group collected from March to June was divided into two groups. We found 4 age groups of K. punctatus from the frequency distribution in standard length. Spawning season was estimated to be from May to June from the seasonal change in GSI. Condition factor (CF) of the large and medium groups decreased after the spawning season to September. The large group exhibited greater decrease in CF than the medium group, but no decrease in the small group. Mass mortality of K. punctatus occurred in June 2002 when GSI rapidly decreased. It was confirmed that K. punctatus died when the bottom layer was not in anaerobic conditions. The large size fish occupied a larger part of dead fish than the medium fish. The mortality was considered to be closely related to spawning and decrease in CF. Key words: Konosirus punctatus, growth curve, GSI, condition factor, Lake Shinji, mass mortality