Size-specific spatiotemporal dynamics of bigeye tuna (Thunnus obesus) caught by the longline fishery in the eastern Pacific Ocean

Abstract

This study aimed to understand the size-specific spatiotemporal dynamics of bigeye tuna (Thunnus obesus) in the eastern Pacific Ocean from 1991 to 2017. A spatiotemporal delta-generalized linear mixed model was applied to catch rate data by size class collected from a Japanese longline fishery. The results showed that the density of bigeye tuna varied spatially across the eastern Pacific Ocean, with spatial segregation by size. The mean predicted density by size in the study area (100–150 °W, 10 °S–10 °N) indicated that adult fish (fork length (FL) > 152 cm) were mainly distributed in the middle of this study area, 110–140 °W in both hemispheres. For the intermediate size stage (FL > 115 to < = 152 cm), a high-density area was found in both the western and eastern parts, but mainly in the northern hemisphere and east of 120 °W. A high-density area of the juvenile stage (FL < = 115 cm) was found in the southwestern part of the study area. Spatial restructuring by size in response to El Niño and La Niña was also observed. El Niño and La Niña caused adult fish to distribute in the eastern and western EPO, respectively, and the smaller fish were distributed in the opposite area. Oceanographic conditions affect fish distribution and movement differently according to size, and the trends in relative abundance by size were different. We discuss the reasons for the segregation by size and the different responses of each fish size category to oceanographic conditions.