1. Salmon Research Department, Fisheries Resources Institute, Japan Fisheries Research and Education Agency, Nakanoshima, Toyohira, Sapporo 062-0922, Japan
2. Department of Zoology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan
3. Nikko Field Station, Fisheries Technology Institute, Japan Fisheries Research and Education Agency, Nikko, Tochigi 321-1661, Japan