Bull. Jpn. Soc. Fish. Oceanogr. 74(1), Page 1-12, 2010   Structure of the marine ecosystem of the Ross Sea, Antarctica —Overview and synthesis of the results of a Japanese multidisciplinary study by Kaiyo-Maru and JARPA— Mikio Naganobu1†, Hiroto Murase2, Shigetoshi Nishiwaki2, Hiroki Yasuma3, Ryuichi Matsukura3,Yoshimi Takao4, Kenji Taki5, Tomonari Hayashi5, Yuko Watanabe6, Takashi Yabuki7, Yasuo Yoda8, Yoshifumi Noiri3, Mariko Kuga3, Kohei Yoshikawa9, Nobuo Kokubun10, Tetsuo Iwami11, Kiyoshi Itoh12, Mutsuo Goto2, Tatsuya Isoda2, Koji Matsuoka2, Tsutomu Tamura2 and Yoshihiro Fujise2 Received August 13, 2009; Accepted September 16, 2009. 1 Fisheries Research Agency, 15F Queen’s Tower B, 2-3-3 Minato Mirai, Nishi-ku, Yokohama, Kanagawa 220-6115, Japan 2 Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan 3 Hokkaido University, 3-1-1 Minatocho, Hakodate, Hokkaido 041-8611, Japan 4 National Research Institute of Fisheries Engineering, 7620-7 Hasaki, Kamisu, Ibaraki 314-0408, Japan 5 National Research Institute of Far Sea Fisheries, 2-12-4 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japan 6 Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan 7 Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan 8 Tokai University, 3-20-1, Orido, Shimizu, Shizuoka, Shizuoka 424-8610, Japan 9 University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan 10 National Institute of Polar Research, 10-3, Midoricho, Tachikawa, Tokyo 190-8518, Japan 11 Tokyo Kasei Gakuin University, 2600, Aihara, Machida, Tokyo 194-0292, Japan 12 Environmental Simulation Laboratory, 2-4-1 Arajyukucho, Kawagoe, Saitama 350-1124, Japan † naganobu@affrc.go.jp A multidisciplinary marine ecosystem study in the Ross Sea region of the Antarctic was conducted in austral summer in 2004/05 as a Japanese national program. A fisheries research vessel, Kaiyo-Maru, and JARPA (the Japanese Whale Research Program under Special Permit in the Antarctic) vessels were engaged in the study. The main focus of the study was to elucidate the ecological interactions among biological organisms, especially between krill and whales. The surveys consisted of (1) oceanographic observations, (2) net sampling of planktons and fishes, (3) hydroacoustic surveys of krill (4) sighting surveys of top predators (birds and cetaceans) and (5) sampling of Antarctic minke whales. To understand the distribution pattern of various biological organisms in relation to oceanographic conditions, a simple oceanographic index, Mean TEMperature from the surface to 200 m (MTEM-200) was introduced. Species compositions changed against the north-south gradient of MTEM-200. For instance, Antarctic krill were mainly distributed in waters between 0 and -1°C while ice krill were distributed in waters colder than -1°C. Humpback whales were mainly distributed in the waters warmer than 0°C. Antarctic minke whales were mainly distributed in waters around -1°C in a continental shelf slope frontal zone. Comparison of stomach contents of minke whales and net samples as well as results of spatial modeling suggested that oceanographic conditions played an important role in defining predator-prey relationships. The data collected through this multidisciplinary study provides vital information to understand the structure of the marine ecosystem in the Ross Sea where data are still sparse. Key words: Balaenoptera bonaerensis, Euphausia superba, Euphausia crystallorophias, food web, environment-prey-predator relationship