Main spawning season of yellowfin tuna, Thunnus albacares, in the western tropical Pacific Ocean based on the gonad index

The main spawning season of yellowfin tuna (Thunnus albacares ) in the western Tropical Pacific Ocean was examined using specimens over 100 cm caught by Japanese longline and purse seine vessels. For females, percentage of fish with gonad index (GI) of 2.0 and over was used as an index of spawning season. Seasonal change of log transformed GI was also examined for females and males. The sexual maturity of yellowfin tunas caught by the longliners was substantially lower than that by the purse seiners. No regular seasonal change in sexual maturity was observed for the longline samples, but a regular seasonal pattern occurred from year to year for the samples by the purse seiners. It is assumed that those differences were caused by movement of matured fish from midwater to surface layer for spawning. Based on purse seine samples, the main spawning season was estimated to be from November to April.