Gonad maturity stage of female bigeye tuna (Thunnus obesus) in the southern Pacific Ocean

We applied paraffin section method to investigate the reproductive features for female bigeye tuna (Thunnus obesus) in the southern Pacific Ocean (SPO), using ovarian tissues sampled at sea during 2010~2013. Maturity stage and fecundity were estimated under a microscope. The results show that the development of ovary was divided into five stages: 1 (immature), 2 (maturing α), 3 (maturing β), 4 (ovulating) and 5 (ovulated). Female gonad somatic index (GSI) ranged from 0.26 to 8.58 (average 3.56), increasing with the increase of fork length. Length at 50% maturity was 116.32 cm. Average batch fecundity was 1.09×10⁶ pcs, and average relative fecundity was 20.5 pcs · g^-1. Cluster analysis shows that there were three classes in terms of similarity in catch depth, i.e., Stages 2 and 3, Stages 1 and 4, and Stage 5. It is suggested that bigeye tuna reached first sexual maturity at earlier ages in SPO than in other areas, probably due to overfishing.