Bull. Jpn. Soc. Fish. Oceanogr. 74(sp), Page 27-34, 2010   Temporal variability in reproductive traits and fluctuation in Pacific cod Gadus macrocephalus population Yoji Narimatsu Tohoku National Fisheries Research Institute, Fisheries Research Agency, Hachinohe Station, 25-259 Shimomekurakubo, Same, Hachinohe, Aomori 031-0841, Japan e-mail: nary@affrc.go.jp Annual changes in life history traits and their effect on reproductive success were reviewed in exploited demersal fishes, especially in Pacific cod off the Pacific coast of Northern Honshu, Japan. In Pacific cod, age at maturity plastically changed with recruitment size. Early maturing females showed no different characteristics in relative fecundity and onset of spawning compared to older females. On the other hand, early maturing females showed disadvantages in future growth and egg size compared to older ones. In comparisons with other demersal species, the recent fluctuating pattern of the Pacific cod population was discussed in relation to the variability of life history traits. Key words: population level, age at maturity, reproductive success, reproductive characteristics, Pacific cod