Late winter vertical distribution of mesopelagic fish larvae in the Kuroshio Current region of the western North Pacific

Abstract

Vertical distribution patterns of five abundant mesopelagic fish larvae (Diaphus slender type, Notoscopelus japonicus, Myctophum asperum, Lipolagus ochotensis and Sigmops gracilis) were firstly described in the western North Pacific Kuroshio Current region; samples were taken at discrete depths (0–150 m) during both day and night on 28 and 29 February 1996. Habitat depths of all larvae collected did not differ between day and night. Diaphus slender type, N. japonicus and M. asperum occurred mainly in the 25–80-m, 30–75-m and 35–80-m depth layers, respectively, all of which were within the mixed layer above the thermocline at a depth of 80 m. The distributions of L. ochotensis and S. gracilis were centered in the 30–100-m and 55–100-m layers, respectively, depths that were between the mixed layer and the thermocline. Habitat depths of all species or types overlapped primarily in the 55–75-m layer; overlapping distributions were likely related to the vertical mixing of the Kuroshio waters in winter because previous report indicated that larvae of mesopelagic fish were generally habitat-segregated by depth in the stable water column that develops during the summer thermal stratification. The fish species had ontogenetic vertical migrations, with smaller individuals distributed primarily in the mixed layer and larger larvae in the thermocline.