1. Department of Environmental Veterinary Sciences, Graduate School of Veterinary Medicine, Hokkaido University, N18 W9 North Ward, Sapporo 060, Japan, Department of Environment Conservation, Ehime University, Tarumi 3-5-7, Matsuyama 790, Japan, National Research Institute of Far Seas Fisheries, Orido 5-7-1, Shimizu 424, Japan, The Sere Group Ltd., 138 Road 2 West, Kingsville, Ontario N9Y 2E5, Canada, and Kochi University, Akebono-cho 2-5-1, Kochi 780, Japan