Bull. Jpn. Soc. Fish. Oceanogr. 70(4), Page 240-248, 2006   Estimation of Cnidaria and Ctenophora biomass and vertical distribution using the Video Plankton Recorder II (VPRII) in the meso- and epipelagic layers of the Oyashio and Transition zone off eastern Japan Tadafumi Ichikawa†1, Kyohei Segawa2 and Makoto Terazaki3 1 National Research Institute of Fisheries Science, Fuku-ura, Kanazawa-ku, Yokohama 236-8648, Japan 2 National Research Institute of Far Seas Fisheries, Fuku-ura, Kanazawa -ku, Yokohama 236-8648, Japan 3 Ocean Research Institute, University of Tokyo, Minamidai, Nakano-ku, Tokyo 164-8639, Japan †e-mail: wamusi@affrc.go.jp To reveal the accurate biomass and detailed vertical distribution of Cnidaria and Ctenophora, new Video Plankton Recorder (VPRII) and plankton net were towed while obtaining environmental (CTD) data from 25 to 500 m depth during July 2003 and June to July 2004 at 5 stations in the Oyashio and Transition zone off the Joban, Sanriku and Eastern Hokkaido coast, Japan. The vertical distributions of Hydromedusa and Ctenophora biomass in every 10 m depth obtained from the VPRII images were intermittent. The biomass peaks of Hydromedusa in the vertical distribution were found in the North Pacific Intermediate Water (NPIW). While the peaks of Ctenophora were both in the surface layer and NPIW, and the surface peak seems to have been associated with their food source because it was located directly below the surface fluorescence peak. To supplement the underestimation of larger size organisms by VPRII measurement, we used the <10 mm sized Hydromedusa and Ctenophora from VPRII, whereas the >10 mm Hydromedusa and Ctenophora and whole size Siphonophora was used from the plankton net data. Cnidaria and Ctenophora biomass estimated from VPRII images and plankton net samples averaged over 25 to 500 m were from 0.021 to 0.063 mgC m-3 in the Oyashio and Transition zone, and these biomasses were 14 times higher (0.3 to 260, SD=44, n=40) than only using the plankton net samples. The ratio of Cnidaria and Ctenophora to Copepoda biomass was only from 0.2 to 0.7%. However, it seems that the biomass of Cnidaria and Ctenophora was higher than in our estimation by either depth or location due to the intermittent distribution and underestimation of larger Ctenophora biomass in our methods. Key words: