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.
Spawning season of jack mackerel Trachurus japonicus collected off the Pacific coast of southern Joban–Boso area was examined based on gonadsomatic index(GSI). Period of otolith annual ring mark formation and age were estimated based on transversal sections of sagittal otoliths. The GSI data showed that the matured individuals appeared from June to August, which was considered to be the spawning season. The age at first maturity was estimated as 2 years old in this area. Measurement of otolith marginal increments showed that annual ring marks were formed from May to September, which approximately corresponded to the spawning season. The von Bertalanffy growth curve calculated from fork length (Lt) and age at catch (t) was expressed as follows: Lt=29.4(1-e-0.38(t+1.08)) (0.33≤t≤19.67). According to the growth equation, the present samples from the Pacific coast of Joban–Boso area showed lower growth rate than those from Kumano-nada and its western area, with a high proportion of older fish in the study area. The low growth rate and long life of jack mackerel in the study area is probably attributable to a shorter period of optimal temperature for their growth.