Spatial variation in bigeye tuna Thunnus obesus size at sexual maturity in the eastern Pacific Ocean

Understanding the reproductive characteristics of a species is of crucial for accurate stock assessment and management plans to ensure sustainable fisheries. In this study, the size at 50% sexual maturity (L₅₀) parameters in different bio-ecological provinces were estimated for bigeye tuna, Thunnus obesus, sampled from the Eastern Pacific Ocean tuna fisheries-dependent survey from 2013 to 2019. The overall sex ratio of the catch during the sampling differed significantly from 1:1. Bigeye tuna exhibit sexual dimorphism in the growth of males and females, with a clear shift in predominance from female to male with increasing sizes. In the North Pacific Sub-tropical Gyre (east) (NPST-east), North Pacific Tropical Gyre (NPTG), Pacific North Equatorial Countercurrent (PNEC), and Pacific Equatorial Divergence (PEQD), females (meals) reached sexual maturity round 102 cm (106 cm), 106 cm (100 cm), 125 cm (110 cm), and 113 cm (110 cm), respectively, the estimated L₅₀ of bigeye tuna was 124.08 cm, 121.97 cm, 139.92 cm and 132.45 cm, respectively. The degree of populations mixing between equatorial (PNEC and PEQD) and high-latitude regions (NPST-east and NPTG) is extremely small, but it is reasonably high between the NPST-east and NPTG or PNEC and PEQD. These parameters were significantly different, suggesting the occurrence of a spatial difference in the size-at-maturity of bigeye tuna between these bio-ecological provinces. The findings of this study provide the key information for understanding the life history of bigeye tuna in the Eastern Pacific Ocean and will contribute to the conservation and sustainable yield of this species.