Bull. Jpn. Soc. Fish. Oceanogr. 79(1), Page 12-18, 2015   Interannual changes in body size and hatching period of Japanese common squid Todarodes pacificus off the Pacific coast of Hokkaido Toru Sato1,2† and Kenji Sakaguchi2,1 1 Hokkaido Research Organization, Central Fisheries Research Institute, Hamanaka-cho, Yoichi, Hokkaido 046-8555, Japan 2 Hokkaido Research Organization, Kushiro Fisheries Research Institute, Hama, Kushiro, Hokkaido 085-0024, Japan † E-mail: sato-toru@hro.or.jp The body sizes of Japanese common squid Todarodes pacificus, which migrated to waters off the Pacific coast of Hokkaido, exhibited long-term changes. It was hypothesized that the long-term body size changes corresponded to population dynamics and regime shift. In addition, the body-size changes might be attributed to changes in the hatching period. In the present study, we examined a longterm trend in body-size using the specimens that were collected off the Pacific coast of Hokkaido from 1965 to 2008. In addition, the relationship of body sizes to hatching dates was examined for the periods from 1999 to 2008. The annual mean body size tended to be larger during 1974-1984 than during the earlier (1965-1973) and later (1991-2008) periods. Such a decadal trend of body sizes seemed to correspond to the long-term trends in catch per unit effort and the regime shift. The hatching dates during 1999-2008 ranged from November to April, with a main season extending from February to March. Moreover, a negative relationship was found between the hatching dates and body sizes, suggesting the influence of the hatching period on changes in body-size. Key words: Todarodes pacificus, body size, Pacific coast of Hokkaido, hatching period, statolith