Feeding ecology of the swordfish Xiphias gladius in the subtropical region and transition zone of the western North Pacific

Abstract

This is the first time a quantitative analysis has been carried out on feeding habits of swordfish Xiphias gladius (n = 455), mainly ranging from 1200 to 2100 mm in eye-fork length, in the western North Pacific. Based on these data, we examined the feeding ecology of this species in relation to its seasonal south-north migration between subtropical and transition waters. The main X. gladius prey size was 80 to 500 mm, and the size spectrum of the prey shifted to a smaller range from spring to summer. In winter and spring, X. gladius was distributed in the subtropical region and fed mainly on the winter-spring cohort of neon flying squid Ommastrephes bartramii, followed by Pacific pomfret Brama japonica. In summer and autumn, X. gladius migrated to the transition zone between the Kuroshio front and subarctic boundary and fed mainly on the winter-spring cohort of O. bartramii, which also migrates into the transition zone from the subtropical region in summer. Other common prey species in summer and autumn were the gonatid squid Gonatopsis borealis and the myctophid fish Diaphus gigas, which are endemic to transition and/or subarctic regions. B. japonica almost disappeared from the stomachs of X. gladius during these seasons because this species migrates in summer into the subarctic region, north of the range of X. gladius. Therefore, seasonal changes in stomach contents of X. gladius could be explained by the difference in seasonal south-north migration patterns between predator and prey. The seasonal south-north migration of X. gladius seems to be a feeding migration, following the migration of the winter-spring cohort of O. bartramii. © Inter-Research 2009.