Bull. Jpn. Soc. Fish. Oceanogr. 70(3), Page 170-175, 2006   Rapid decrease of egg production of Pacific stock of Japanese sardine, Sardinops melanostictus, and characteristics of persistent spawning ground in Tosa Bay, southwestern Japan Minoru Ishida Kochi Kuroshio Research Laboratory, National Research Institute of Fisheries Science, 6-1-21 Sanbashidori, Kochi-shi, Kochi 780-8010, Japan e-mail: ishidam@affrc.go.jp Biomass of Japanese sardine Sardinops melanostictus decreased drastically in recent years. Annual egg productions of the species in Tosa Bay southwestern Japan were compared to the total egg production of the Pacific stock of Japan during 1978 and 2005, to consider the characteristics of the bay as a spawning ground. Data of total 7,895 plankton net tows by the Kochi Prefectural Fisheries Experimental Station and the National Research Institutes were analyzed. Annual egg production in the bay did not show long term drastic fluctuation which occurred in the total Pacific stock, but displayed two to four years of short interval moderate changes. Average egg production of the maximum five years (1986, 1998, 1982, 2001, 1989) was 13431012 which was only 5.4 times of that of the minimum (2002, 1984, 2003, 1991, 1988) 2531012, contrasting strongly with that of total Pacific stock of 75 times. The proportion of the egg production in the bay in the total to the Pacific stock had been several percent during 1979 and 1995, but increased to several tens percent since 1996, and more than 60% in the recent three years. The reason why the spawning ground in the bay has been sustained is presumed to be preferable temperature and food conditions for early life stages of the species resulted from the oceanographically appropriate location and less fishing pressure conducted by the prohibition of purse seine. Key words: egg production, Japanese sardine, spawning ground, Tosa Bay