1Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Hokkaido 041-8611, 2Marine Fisheries Research and Development Center, Fisheries Research Agency, Yokohama, Kanagawa 220-6115, 3National Research Institute of Fisheries Science, Kanazawa-ku, Yokohama, Kanagawa 236-8608, 4Graduate school of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
California headlightfish Diaphus theta is the most abundant mesopelagic fish in the subarctic Pacific. Day- and nighttime field acoustic data and biological samples were obtained in January 2000, in the Pacific area off eastern Hokkaido to estimate fish density and total biomass of D. theta. Sound scattering layers composed mainly of immature D. theta (63 mm length on average) were widely distributed around the shelf edge (>300 m water depth). Relatively dense schools were observed below the depth of 250 m during the daytime, although the schools were scattered widely above 50 m depth during the nighttime. Fish density was estimated to be 17.3 g/m2 on average, which was more than several times higher than that estimated by conventional methods such as net sampling gears. Total biomass in the survey area (2,083 km2) was estimated to be about 35,900 t.