Horizontal distribution and habitat conditions of surface migratory myctophid juveniles in the Kuroshio region off southern and central Japan

Abstract Surface migratory myctophids migrate nocturnally from the mesopelagic to the near-surface layers and are the dominant components of oceanic neuston fish assemblages. We examined the horizontal distribution and habitat conditions of juvenile surface migratory myctophids in the Kuroshio region in late winter from 2003 to 2016. In total, 9098 myctophid juveniles, including 17 species, were collected in the surface layer at night. Of these, the eight most abundant species accounted for 99.0%. They showed five distinct distribution patterns closely related to the position of the Kuroshio axis. Dasyscopelus orientalis was restricted to the inshore side of the Kuroshio axis (INK), where sea surface temperature (SST) was lowest but zooplankton biomass (ZPB), a proxy for food availability, was highest. The centre of distribution of D. asper and D. obtusirostris was in the area of the axis (KUA), where the highest SST and intermediate ZPB were observed, although they were also abundant in INK. Dasyscopelus spinosus was collected mainly in the area south of Kyushu Island adjacent to upstream of the Kuroshio. The distributions of Symbolophorus evermanni, Centrobranchus nigroocellatus , and Myctophum nitidulum were restricted to the offshore side of the axis (OFK), where intermediate SST and lowest ZPB were observed. Hygophum reinhardtii was collected abundantly in both OFK and KUA. Spatial habitat segregation of juveniles would result from differences in spawning grounds and larval and juvenile transport routes. The Kuroshio contributes to the biodiversity of myctophids along the Pacific coast of southern and central Japan by creating adjacent but distinct habitat conditions.