Understanding reproductive potential of any species is of great importance to their resource assessment and management. In this study, the reproductive biology of bigeye tuna, Thunnus obesus, was studied based on 1,283 samples taken from the Chinese longline vessels in the eastern and central Tropical Pacific Ocean during February through November 2006. The female-male ratio was 1.0 : 1.5 and males were predominant in all length classes except for the length class of 166 - 170 cm (fork length). Males dominated in sizes larger than 171 cm, all specimens of 192 cm or larger were males. The main spawning period of bigeye tuna occurred between March and November. Gonadosomatic rate of males were larger than that of females to specific length classes. Statistically, female and male bigeye tuna had no significant reproductive seasonality. The observed minimum length at sexual maturity for female bigeye tuna was 94 cm. Length at 50% sexual maturity of female bigeye tuna was estimated at 107.8 cm, and maturation rate was 0.106 cm-1. The results derived in this study provide the information critical to our understanding of key life history parameters of bigeye tuna in tropic Pacific Ocean.
