1. Faculty/Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hako-date, Hokkaido 041-8611, Japan
2. Biology Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, U.S.A.
3. Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A.
4. Arctic Research Center, Hokkaido University, Kita-21 Nishi-11 Kita-ku, Sapporo, Hokkaido 001-0021, Japan