Bull. Jpn. Soc. Fish. Oceanogr. 74(sp), Page 35-45, 2010   Temporal and spatial changes in the reproductive characteristics of female Japanese sardine Sardinops melanostictus and their effects on the population dynamics Haruyuki Morimoto Japan Sea National Fisheries Research Institute, Fisheries Research Agency, 1-5939-22 Suido, Chuo, Niigata, Niigata 951-8121, Japan e-mail: woodbook@affrc.go.jp Changes in the reproductive characteristics of the female Japanese sardine Sardinops melanostictus sampled mainly in February and March from 1990 to 1996 in Tosa Bay and off southwestern Japan, one of the main spawning grounds on the Pacific coast of western Japan during a period of declining stock size were examined. The females distributed in the offshore area of Tosa Bay and the frontal region of the Kuroshio Current spawned more and larger eggs than those distributed in the coastal area of Tosa Bay in 1990, 1991 and 1992 although these offshore females became almost extinct in 1993. Both batch fecundity and yolk volume of the translucent hydrated oocytes in the ovaries of female sardines in Tosa Bay increased from 1990 to 1996, during which the improvement of fatness in the body mass was found. Similarly, those of females in offshore areas increased from 1990 to 1992 with the advance of their fatness. These results suggest that the body length at any stage of yolk-sac larva, which depends on the egg size, around 1990 was smaller than that in later years, and that the reproductive abilities of females around 1990 were inferior to those in later years. Key words: Japanese sardine, batch fecundity, egg size, maternal effect