Results of the cetacean prey survey using a quantitative echo sounder in JARPA from 1998 / 99 to 2004 / 2005

Krill biomass estimation surveys using a quantitative echo sounder have been conducted in JARPA since 1998/99 season to achieve one of the main objectives of JARPA which is elucidation of the role of whales in the Antarctic marine ecosystem. The survey was conducted concurrently with cetacean survey. Similar biomass estimates were obtained in Area IV in the 1999/2000 (34.2 million t) and 2001/2002 (34.1 million t) seasons. In Area V, biomass in the 2000/2001 (20.7 million t) and in the 2002/2003 (22.6 million t) seasons were similar but biomass in 1998/1999 (29.7 million t) was higher than the other two years. Higher biomass in 1998/99 could be explained by seasonal effect and area coverage differences. Biomass density in each stratum as well as overall biomass (7.0 million t) in Area V were low in 2004/2005 even if no krill biomass estimate in NorthWest stratum because of prevailing poor weather conditions for acoustic survey was taken account. Biomass in Area IV was higher than Area V. There was a unique opportunity to compare the biomass estimate from two independent vessels in the Ross Sea region in 2004/2005. One vessel, Kaiyo Maru, conducted standardized krill biomass estimation survey adopted by CCAMLR using both RMT net sampling and acoustic survey while the other vessel, Kyoshin Maru #2, only conducted acoustic survey as in previous JARPA cruises. The results of biomass estimates from two vessels were comparable. The result suggested that biomass estimations in previous JARPA cruises which thoroughly relied on acoustic were reliable. Because krill biomass surveys in the whole of Area IV and Area V were rarely conducted in the past, krill data collected by JARPA were quite important to understand krill-baleen whale relationships in the Antarctic. INTRODUCTION It is well known that krill is the key species in the Antarctic marine ecosystem because krill is a major link in the transfer of energy from primary producers to larger organisms such as baleen whales (Laws, 1985; Murphy et al., 1988). Krill have been known as the major food source of baleen whales in the Antarctic (Kawamura, 1994). “Krill surplus” caused by intensive commercial harvesting of large whales, blue (B. musculus), fin (B. physalus) and humpback whales (Megaptera novaeangliae), has been central theorem of the Antarctic ecosystem study. “Krill surplus” resulted in increasing in amount of available food to other krill feeders such as Antarctic minke whales (B. bonaerensis) (Laws, 1977). But it appears that the situation is changed in recent years. After the ban of commercial whaling of large whales in 1987, it was reported that abundance of those species increased in recent years. For example, abundance of blue whales increase 8% per year at the circumpolar level (Branch et al., 2004) though the abundance was still low comparing with pre exploitation population size. Some stocks of humpback whales in southern hemisphere have been showed remarkable recovery even to near pre-exploitation level (Johnston and Butterworth, 2004). To test the magnitude of interaction, preliminarily baleen whales-krill interaction model was developed but the interpretation was limited at this moment because of paucity of information including biomass estimation of krill (Mori and Butterworth, 2006). Krill density showed both short and long term changes. In short term, krill density showed large year to year fluctuation at decadal scale in response to environmental variability such as sea ice extent and oceanographic conditions (e.g. Pakhomov, 2000; Hewitt and Demer, 2003). It was reported that krill density have showed statistically significant decreasing trend in the southwest Atlantic since 1976 (Atkinson et al. 2004) though the magnitude of decrease should be studied further to draw the conclusion because wide varieties of net types were used in the analysis. Given the krill density change information, krill biomass survey should be conducted regularly in the same region. To assess the magnitude of interspecific competition among baleen whales for krill and the consequences of the competition