Bull. Jpn. Soc. Fish. Oceanogr. 82(4), Page 161-166, 2018   Reconsideration of “seasonal cohorts” of skipjack tuna Katsuwonus pelamis around Japanese waters, based on somatic growth estimated with otolith daily increment and tag-recapture data analyses Akihiko Yatsu1† , Daisuke Ochi2, Osamu Honda1, Hidetaka Kiyofuji2 and Sadaaki Kayama3 1 Japan Fisheries Information Service Center, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan 2 National Research Institute of Far Seas Fisheries, Fisheries Research and Education Agency, 5–7–1 Orido, Shimizu-ku, Shizuoka-shi, Shizuoka 424–8633, Japan 3 Nagaisuisan Co. Ltd., 5–24–7 Nagai, Yokosuka-shi, Kanagawa 238–0316, Japan † E-mail: yatsu@jafic.or.jp In order to explore relationships between fishing conditions of immature skipjack tuna around Japanese waters in the fall of a given year and those in the next spring, we reconsidered the previously hypothesized somatic growths of “winter- and summer-hatching groups” (i.e., body–length-groups) during wintering. We used 1) the reported von Bertalanffy growth curve that was estimated with combined analyses of otolith daily increment and the Japanese tag-recapture data, and 2) somatic growth data of wintering immature fish that were estimated from the Japanese tag-recapture surveys during 1978–1993 (adopted from the previous report) and 2000–2001. The results suggest a high possibility of underestimation of the previously reported somatic growths during wintering that were estimated with tag-recapture data, in which fish were released in the fall of 1992 and recaptured in the next year. Also, we proposed 1) plausible connectivity of body–length-groups between the fall of a given year and the next spring, and 2) number of principal hatching periods per year of immature fish migrating to Japanese waters. It is necessary to examine seasonal variability in somatic growths, and re-estimate principal hatching periods of each body–length-group in future studies. Key words: body–length-groups, seasonal cohorts, skipjack tuna, somatic growth