Bull. Jpn. Soc. Fish. Oceanogr. 82(1), Page 14-25, 2018   Comparison of stock assessments based on calendar year and fishing year: An example of the Tsushima Warm Current stock of chub mackerel Soyoka T Muko1† and Hiroyuki Kurota1 1 Seikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 1551–8 Taira-machi, Nagasaki 851–2213, Japan † E-mail: symuko@affrc.go.jp We compared stock assessment for the Tsushima Current stock of chub mackerel based on the fishing year with that based on the calendar year to evaluate how the definition of “year” influenced the estimation of resource fluctuation. The total catches of chub mackerel in the East China Sea, the Sea of Japan, and Korean waters did not have a large difference between the two definitions of year. While most catches consisted of Age-0 and Age-1 in the calendar year basis, over 80% of catches was regarded as Age-0 in the fishing year basis. The fishing year basis estimated less stock biomass and less spawning stock biomass by virtual population analysis tuned to resource abundance indices than the calendar year basis, but no marked difference was found in the trends of the abundance estimates. In contrast, the estimated number of recruitments was about the same between the two bases of year. Retrospective patterns demonstrated that stock biomass was continuously overestimated in both types of year. The degree of overestimation for the fishing year basis was less than that of the calendar year basis in most years, however, it happened to be larger in a few years. Allowable biological catch was also overestimated in such years. Lastly, we listed the characteristics of the stock assessment results based on the fishing year and discussed their future challenges. Key words: tunned VPA, ABC, retrospective analysis