National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu, Shizuoka 424-8633, Japan
Contributions of different spawning seasons to the fishery stock of age-0 Pacific bluefin tuna Thunnus orientalis were estimated. Spawning season estimated from otolith daily increments of 477 individuals from 17 to 93 cm in fork length ranged from March to October at the earliest which is longer than previously reported. Age-0 fish caught in Japanese fisheries in 1993-1997 were divided into two sub-cohorts by comparing monthly length data with the growth curves derived from otolith age data. Fish of one sub-cohort which were fertilized up to early July, probably in the area around Taiwan to the Nansei Islands, accounted for most (76%) of the age-0 catch. Fish of the second sub-cohort were fertilized after mid July, probably mainly in the Sea of Japan, and accounted for on average 24% of the age-0 catch, and as much as 40% in 1994.