Biology comparison of bigeye tuna (<italic>Thunnus obesus</italic>) between the south and north in the eastern Pacific Ocean

Abstract

Understanding the biological characteristics of bigeye tuna is an important prerequisite for assessing population resources and formulating and implementing conservation management measures to maintain the sustainable utilization of the population. The characteristics of reproduction, growth, and feeding of bigeye tuna (<italic>Thunnus obesus</italic>) are significantly affected by environmental conditions. Different oceanographic characteristics (e.g., current system, ocean stratification) between southern and northern regions from the equator in the eastern Pacific Ocean are likely to lead to the heterogeneity of biological characteristics of bigeye tuna over these areas. To understand the population characteristics of bigeye tuna in the eastern Pacific Ocean over long periods and space, in the present study, we examined a range of biological information (fork length, dressed weight, feeding intensity, gonad maturity, and sex ratio) based on fishery-dependent data collected by the resource survey of a tuna longline fishing operation from 2013 to 2019. The results showed that (1) the fork length of bigeye tuna in the north area ranged from 72 cm to 229 cm, with a dominant fork length of 80–160 cm and a mean fork length of (121.53±29.11) cm. The female–male ratio was 0.91∶1. Stages III and IV were the dominant stages in the gonad maturity of males and females, respectively. (2) The fork length of bigeye tuna in the south area ranged from 63 to 209 cm, with a dominant fork length of 110–150 cm and a mean fork length of (134.87±25.73) cm. The female–male ratio was 0.62∶1. Stages III and V were the dominant stages in the gonad maturity of males and females, respectively. (3) Stages 1 and 2 were dominant in the feeding intensity of the bigeye tuna, and the percentage of empty stomachs for the south and north regions were 46.98% and 3.22%, respectively. The feeding intensity increased with fork length. (4) An ANCOVA showed a significant difference in the relationship of fork length and dressed weight between the south and north areas (<italic>P</italic>&lt;0.05). (5) An ANOVA showed no significant difference in the gonad maturity between males and females (<italic>P</italic>&gt;0.05). (6) The proportion of surviving individuals after haul-back was relatively high, while the dead individuals only accounted for 8.52%.