1. Faculty of Fisheries, Hokkaido University, Minato-cho 3-1-1, Hakodate, Hokkaido, 041-8611, Japan, Laboratory of Organic Geochemistry (LOG), Tokyo University of Agriculture and Technology, Saiwai-cho 3-5-8, Fuchu, Tokyo, 183-8509, and Japan, Hokkaido National Fisheries Research Institute, Fisheries Research Agency