Reproductive biology of bigeye tuna (Thunnus obesus) in the tropical western and central Pacific Ocean

Understanding the reproductive biology of bigeye tuna (Thunnus obesus) and quantifying size-specific parameters provides the means for accurately predicting the effects of fishing on the reproductive potential of stock and the basis for improving the assessment and management of bigeye tuna stocks. In this paper, a total number of 886 T. obesus were sampled by pelagic longline fishing in the tropical western and central Pacific Ocean (WCPO, 2°3'-11°17'S, 163°14'-173°35'E) from May 2018 to March 2019 and then their reproductive biology was investigated. The effect of biological factors, such as individual fork length, and temporal and spatial factors, such as fishing depth and month, on the reproductive traits of T. obesus was assessed by a generalized linear model (GLM) and polytomous logistic regression for the ordinal response model, combined with the circular statistics method. The results showed that the mean fork lengths of male and female T. obesus were (132.2±19.1) cm and (125.4±16.8) cm, respectively. The sex ratio (female: male) of sampled T. obesus was (0.54±0.74), and a significant negative correlation between fork length and sex ratio was detected (r=−0.263 P=0.016). Males would dominate the sample group when the fork length is greater than 140cm. The output of the GLM fit showed that there was a significant quadratic relationship between gonadosomatic index (GSI) and fork length (P<0.001), which means GSIs of T. obesus increased with fork length, peaked at a certain fork length (130-140 cm for males and 140-150 cm for females, respectively) and then decreased with fork length. Based on circular statistics, a significant effect of fishing months on GSIs was also detected. It is found that the GSIs of males in the first half of the year are higher than those in the second half (sine significant, P<0.001), while GSIs of females at the end and beginning of the year are higher than those at the middle of the year (cosine significant, P=0.021). The output of the ordered multivariate logistic model fit indicated that gonad maturity stages increased significantly with fork length (P<0.001) and gonad maturity stages of male T. obesus were relatively higher during May and July (cosine significant, P=0.004). In addition, fishing depth has a significant effect on the maturity stage of males (positive correlation, P<0.001). Fork lengths at 50% sexual maturity were (96.13±1.24) cm and (104.93±2.60) cm for males and females, respectively. The findings in this paper reduce uncertainty in current stock assessment models, which will ultimately assist the fishery in becoming sustainable for future generations and will contribute to scientific conservation and sustainable yield of this species.