Bull. Jpn. Soc. Fish. Oceanogr. 83(2), Page 75-86, 2019   Inter-annual variability in crude fat content in flesh of large-sized Pacific saury Cololabis saira and estimated biomass of Pacific saury in the waters off northern Japan in the falls of 2012-2017 Akihiko Yatsu1†, Kiyotaka Takahashi1, Kazuyoshi Watanabe1 and Osamu Honda1 1 Japan Fisheries Information Service Center, 4–5, Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan † E-mail: yatsu@jafic.or.jp In order to examine mechanisms of inter-annual variability in age-aggregated biomass of Pacific saury Cololabis saira in the waters off northern Japan, which has been declining in recent years, we measured crude fat content in flesh (Fc), body (knob) length (Bl) and body weight of 796 individuals of large-sized (age-1) Pacific saury captured from the waters off northern Japan from September to December during 2012–2017. Statistically significant negative and positive correlations were detected between capture date (Cd, number of days elapsed from January 1st in each year) and Fc, and between Bl and Fc, respectively. The best multiple regression model in terms of AIC, with Fc as a response variable and Cd, Bl, and year as explanatory variables, indicated that effects of year-specific intercept, Cd, Bl and an interaction between Cd and year were statistically significant. Year-specific intercept, representing Fc level, indicated statistically significant positive correlations with Japanese annual catch and standardized CPUE of Pacific saury. The cause(s) of this phenomena were explored mainly by using survey-derived biomass in the high seas during June–July and age-1 fish ratio in estimated stock abundance, and oceanographic conditions off the southeastern coast of Hokkaido Island. As a result, we proposed a hypothesis that Fc level in August affects timing and location of the start of southward migration and subsequent migration routes of age-1 Pacific saury. We also discussed potential areas where fat is accumulated during the northward migration from May to August. Key words: Pacific saury, crude fat content, multiple regression analysis, biomass, inter-annual variability