Bull. Jpn. Soc. Fish. Oceanogr. 74(sp), Page 46-50, 2010   Changes in the reproductive traits of the Pacific stock of chub mackerel Scomber japonicus and their effects on the population dynamics Chikako Watanabe National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan e-mail: falconer@affrc.go.jp The Pacific stock of chub mackerel Scomber japonicus has declined in abundance since the 1970s and its recruitment has become more variable. A marked increase in length-at-age and a decline in age-at-maturity coincidentally occurred with the population decline. The peak spawning period shifted from April to May/June and became less concentrated to the peak period. In addition to decline in abundance, dominance of young adults of age 2 and 3 in the spawning may have deteriorated the spawning potential of the Pacific stock of chub mackerel. Key words: maturity ogive, Scomber japonicus, growth, spawning