Abundance Increases of Large Baleen Whales in the Antarctic based on the Sighting Survey during Japanese Whale Research Program (JARPA)

抄録

　The Japanese Whale Research Program under Special Permit in the Antarctic (JARPA) was designed as a large-scale, long-term monitoring program using line transect surveys. These have been carried out in a consistent way during the austral summer every other year in the IWC (International Whaling Commission) baleen whale management Areas IV (70°E-130°E) and V (130°E-170°W) since the 1987/88 season. The current research area was set in the half of the Antarctic areas located between 35 °E and 145°W in the waters south of 60°S. Major cetacean species sighted in this area were Antarctic minke (Balaenoptera bonaerensis), humpback (Megaptera novaeangliae), fin (Balaenoptera physalus) and blue (Balaenoptera musculus intermedia) whales. A ‘shift in baleen whale dominance’ from Antarctic minke to humpback whales, was observed in Area IV since the 1997/98 season. In the 1989/90 sea son, Antarctic minke biomass (382,000 tons) was higher than that of humpback whales (139,000 tons), but after 15 years, the biomass of humpback (841,000 tons) increased to more than twice that of Antarc tic minke (335,000 tons). Habitat expansion of humpback and fin whales were also observed in Area IV from the first half (1989/90-1996/97) to the latter half of the surveys (1997/98-2003/04). At this moment, the abundance of Antarctic minke whales is stable in Area IV, but increases in abundance and habitat expansion of humpback and fin whales may lead to their competition with Antarctic minke whales. Yearly changes in some biological features also suggest this ‘event’. Further monitoring surveys will be required in order to improve baleen whale management in the Antarctic Ocean.