National Institute of Polar Research

Revealing long-term changes in the marine environment and ecosystems of the Southern Ocean

April 7, 2025
Tokyo University of Marine Science and Technology
Hokkaido University
National Institute of Polar Research
Wageningen Marine Research

Large-scale multidisciplinary ecological survey reveals long-term changes in the eastern Indian sector of the Southern Ocean

Background

Krill is considered to be the largest animal biomass in the wild on Earth and is an important prey species for baleen whales and seabirds, making it a key species in the Antarctic marine ecosystem. Fishing for Antarctic krill began in the Indian Ocean sector in the 1970s and it has become an important target species in the Southern Ocean. Since the 1990s, the main fishery has been in the Atlantic Ocean sector, and research and surveys of Antarctic krill have been concentrated in this sector.

Large-scale surveys for Antarctic krill in the eastern Indian Ocean sector, south of the Australian continent (80–150°E) have not been conducted since the 1996 Australian BROKE survey (Baseline Research on Oceanography, Krill and the Environment). In recent years, the demand for krill oil as a health food ingredient has increased, and the catch limits have regularly been reached in the main krill fishing ground in the Atlantic. This has led to a trend toward expansion of the Antarctic krill fishery, which now includes limited operations in the Indian Ocean sector. The populations of humpback whales that feed on Antarctic krill was reduced to the verge of extinction due to commercial whaling, but since the International Whaling Commission (IWC) banned the whaling in 1966, the population has recovered in 2015, the population in the Indian sector recovered to the level before commercial whaling began.

The BROKE survey also included oceanographic observations using CTD (conductivity (salinity), temperature and bathymetry) sensors. Since then, changes in bottom water characteristics have become apparent, and detailed, wide-area oceanographic observations were necessary to elucidate the causes and understand trends.

Against this background, a large-scale multidisciplinary ecosystem survey was conducted in the eastern Indian sector of the Southern Ocean (80–150°E) by the Japanese research vessel, Kaiyo-maru (Fisheries Agency of Japan, Figure 1), during the 2018-2019 austral summer (KY1804 survey, an acronym for the fourth survey of the vessel in the Japanese fiscal year 2018).

Methods

The KY1804 survey was conducted by researchers from more than 20 research institutes and universities in five countries, including the Tokyo University of Marine Science and Technology (TUMSAT), Hokkaido University, the National Institute of Polar Research (NIPR), the Japan Fisheries Research and Education Agency (FRA), the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO, Australia), Shanghai Ocean University (China), Wageningen Marine Research (WMR, the Netherlands), the Scripps Institution of Oceanography (USA), and the Southwest Fisheries Science Center (SWFSC, USA). The research was conducted with two main objectives: (1) estimation of standing stock (biomass) of Antarctic krill in the survey area and (2) oceanographic observations in the area to detect any long-term changes.

Along with these main objectives, as with BROKE, we attempted to detect changes in the marine ecosystem between 1996 and 2018–2019 by conducting a survey covering from primary producers to top predators.

The research vessel departed Tokyo on 16 November 2018 and returned after 127 days on 22 March 2019. The vessel called at Fremantle, Melbourne, and Sydney (all in Australia) for resupply. The research area is south of 60°S and between 80°E and 150°E. This is one of CCAMLR's management areas, Division 58.4.1.

Eighteen survey tracklines were placed in the research area (Figure 2), and data from the quantitative echosounder, which were necessary for estimating the biomass of Antarctic krill, were recorded along these lines. Several types of nets were also towed to collect data on zooplankton distributed in the area. A CTD equipped with water samplers collected biological and oceanographic data from the sea surface to depths of up to 3,000 m or more.

Results

The biomass estimate of Antarctic krill from this survey was regarded as the best available estimate in this area by CCAMLR in 2021. The biomass estimates of Antarctic krill from this survey and BROKE in 1996 were comparable, although there are some caveats in the interpretation as the timing of the survey and estimation methods differed.

Compared to 1996, the southern boundary of the Antarctic Circumpolar Current (SB) has shifted 50–120 km southward in the eastern part of the study area (112–150°E), and water temperature has increased. The freshening trend of the Antarctic Bottom Water (AABW) in the Australian–Antarctic Basin was reversed during the 2010s. This may be due to the slowdown of the melting of ice shelves around the Amundsen Sea, which is the upper reaches of the Antarctic Coastal Current, and as a result, the supply of freshwater may have been reduced.

In the eastern part of the study area, the dominant macrozooplankton were salps in 1996, but this changed to amphipods in 2018–2019. This change may be related to the southward shift of the southern boundary of the Antarctic Circumpolar Current, although it could also be due to the difference in the timing of the surveys. In addition, we collected sea surface organisms using a sea surface net, which is not commonly done in the surveys in the Southern Ocean. It was found that Antarctic krill were abundant near the sea surface.

Other studies included investigation on vertical and horizontal mixing of sea waters, biogeochemical studies of floating ice and seawater, studies of primary production, and abundance and prey consumption estimates for seabirds and cetaceans.

The results of these studies were reported in 14 scientific papers and published as an online special issue of the peer-reviewed journal "Progress in Oceanography" on 10 March 2025. In addition to this special issue, 12 other scientific papers have been published to date. A list of these papers is provided later in this press release.

URL of the online special issue
https://www.sciencedirect.com/special-issue/10LJ8PLCQ72

Future Perspective

Through this survey and research, we were able to obtain knowledge on the current status of the ecosystem in the eastern Indian sector of the Southern Ocean, where few large-scale ecosystem surveys have been conducted. The findings from this research are expected to be utilized in academic studies such as monitoring of climate and marine ecosystem changes, as well as in various considerations by international organizations such as CCAMLR.

Acknowledgments

The survey and research were supported by the Fisheries Agency, the Japan Fisheries Research and Education Agency, and the Cetacean Research Institute of Japan. As indicated in the respective papers, individual studies were also supported by the JSPS Grants-in-Aid for Scientific Research (KAKENHI) and other grants from the Japan Society for the Promotion of Science (JSPS).

Contact

Contact regarding research

Tokyo University of Marine Science and Technology
Department of Ocean Sciences
Associate Professor Hiroto Murase
Email: hmuras0@kaiyodai.ac.jp

Hokkaido University
Institute of Low Temperature Science
Professor Shigeru Aoki
Email: shigeru@lowtem.hokudai.ac.jp

Field Science Center for Northern Biosphere
Associate Professor Daiki Nomura
Email: daiki.nomura@fish.hokudai.ac.jp

Graduate School of Fisheries Science
Assistant Professor Kohei Matsuno
Email: k.matsuno@fish.hokudai.ac.jp

National Institute of Polar Research
Bioscience Group
Assistant Professor Nobuo Kokubun
Email: kokubun@nipr.ac.jp

Center for Antarctic Programs
Assistant Professor Daisuke Hirano
Email: hirano.daisuke@nipr.ac.jp

Wageningen Marine Research
Dr. Fokje L. Schaafsma
Email: fokje.schaafsma@wur.nl

Contact for interviews

Tokyo University of Marine Science and Technology
Public Relations Office
E-mail: so-koho@o.kaiyodai.ac.jp

Hokkaido University
Public Relations & Communications Division
Office of Public Relations and Social Collaboration
E-mail: en-press@general.hokudai.ac.jp

National Institute of Polar Research
Public Relations Section
E-mail: koho@nipr.ac.jp

List of papers published in Progress in Oceanography

Abe, K., Matsukura, R., Yamamoto, N., Amakasu, K., Nagata, R., Murase, H., 2023. Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E) in the 2018–19 austral summer. Progress in Oceanography 218, 103107. https://doi.org/10.1016/j.pocean.2023.103107

Aoki, S., Yamazaki, K., Hirano, D., Nomura, D., Murase, H., 2023. Distribution of stable oxygen isotope in seawater and implication on freshwater cycle off the coast from Wilkes to George V Land, East Antarctica. Progress in Oceanography 217, 103101. https://doi.org/10.1016/j.pocean.2023.103101

Hamabe, K., Miyashita, T., Nagata, R., Sasaki, H., Murase, H., 2024. Abundance of humpback whales in the eastern Indian sector of the Southern Ocean in 2018/19 using opportunistic sighting survey data with a note on the occurrence of other cetaceans. Progress in Oceanography 226, 103296. https://doi.org/10.1016/j.pocean.2024.103296

Katsumata, K., Yamazaki, K., 2023. Diapycnal and isopycnal mixing along the continental rise in the Australian–Antarctic Basin. Progress in Oceanography 211, 102979. https://doi.org/10.1016/j.pocean.2023.102979

Kokubun, N., Hamabe, K., Yamada, N., Sasaki, H., Nishizawa, B., Watanuki, Y., Murase, H., 2025. Abundance and estimated food consumption of seabirds in the pelagic ecosystem in the eastern Indian sector of the Southern Ocean. Progress in Oceanography 231, 103385. https://doi.org/10.1016/j.pocean.2024.103385

Matsuno, K., Sugioka, R., Maeda, Y., Driscoll, R., Schaafsma, F.L., Driscoll, S., Yamaguchi, A., Matsukura, R., Sasaki, H., Murase, H., 2024. Spatiotemporal changes in the community and demography of mesozooplankton in the eastern Indian sector of the Southern Ocean during austral summer 2018/2019. Progress in Oceanography 229, 103360. https://doi.org/10.1016/j.pocean.2024.103360

Matsuno, K., Sumiya, K., Tozawa, M., Nomura, D., Sasaki, H., Yamaguchi, A., Murase, H., 2023. Responses of diatom assemblages and life cycle to sea ice variation in the eastern Indian sector of the Southern Ocean during austral summer 2018/2019. Progress in Oceanography 218, 103117. https://doi.org/10.1016/j.pocean.2023.103117

Murase, H., Abe, K., Schaafsma, F.L., Katsumata, K., 2025. Overview of the multidisciplinary ecosystem survey in the eastern Indian sector of the Southern Ocean (80–150°E) by the Japanese research vessel Kaiyo-maru in the 2018–19 austral summer (KY1804 survey). Progress in Oceanography 103456. https://doi.org/10.1016/j.pocean.2025.103456

Nomura, D., Sahashi, R., Takahashi, K.D., Makabe, R., Ito, M., Tozawa, M., Wongpan, P., Matsuda, R., Sano, M., Yamamoto-Kawai, M., Nojiro, N., Tachibana, A., Kurosawa, N., Moteki, M., Tamura, T., Aoki, S., Murase, H., 2023. Biogeochemical characteristics of brash sea ice and icebergs during summer and autumn in the Indian sector of the Southern Ocean. Progress in Oceanography 214, 103023.
https://doi.org/10.1016/j.pocean.2023.103023

Schaafsma, F.L., Matsuno, K., Driscoll, R., Sasaki, H., Van Regteren, M., Driscoll, S., Matsukura, R., Sugioka, R., Urabe, I., Murase, H., Van Franeker, J.A., 2024. Zooplankton communities at the sea surface of the eastern Indian sector of the Southern Ocean during the austral summer of 2018/2019. Progress in Oceanography 226, 103303. https://doi.org/10.1016/j.pocean.2024.103303

Shiomoto, A., Sasaki, H., Nomura, D., 2023. Size-fractionated phytoplankton biomass and primary production in the eastern Indian sector of the Southern Ocean in the austral summer 2018/2019. Progress in Oceanography 218, 103119. https://doi.org/10.1016/j.pocean.2023.103119

Tozawa, M., Nomura, D., Yamazaki, K., Kiuchi, M., Hirano, D., Aoki, S., Sasaki, H., Murase, H., 2024. Oceanographic factors determining the distribution of nutrients and primary production in the subpolar Southern Ocean. Progress in Oceanography 225, 103266. https://doi.org/10.1016/j.pocean.2024.103266

Urabe, I., Matsuno, K., Sugioka, R., Driscoll, R., Driscoll, S., Schaafsma, F.L., Yamaguchi, A., Matsukura, R., Sasaki, H., Murase, H., 2025. Spatio-temporal changes in the macrozooplankton community in the eastern Indian sector of the Southern Ocean during austral summers: A comparison between 1996 and 2018–2019. Progress in Oceanography 231, 103414. https://doi.org/10.1016/j.pocean.2025.103414

Yamazaki, K., Katsumata, K., Hirano, D., Nomura, D., Sasaki, H., Murase, H., Aoki, S., 2024. Revisiting circulation and water masses over the East Antarctic margin (80–150°E). Progress in Oceanography 225, 103285. https://doi.org/10.1016/j.pocean.2024.103285

List of papers published in other peer-reviewed journals

Aoki, S., Yamazaki, K., Hirano, D., Katsumata, K., Shimada, K., Kitade, Y., Sasaki, H., Murase, H., 2020. Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s. Scientific Reports 10, 14415.
https://doi.org/10.1038/s41598-020-71290-6

Foppert, A., Rintoul, S.R., Purkey, S.G., Zilberman, N., Kobayashi, T., Sallèe, J.-B., van Wijk, E.M., Wallace, L.O., 2021. Deep Argo reveals bottom water properties and pathways in the Australian-Antarctic Basin. Journal of Geophysical Research: Oceans 126, e2021JC017935. https://doi.org/10.1029/2021JC017935

Hirano, D., Mizobata, K., Sasaki, H., Murase, H., Tamura, T., Aoki, S., 2021. Poleward eddy-induced warm water transport across a shelf break off Totten Ice Shelf, East Antarctica. Communications Earth & Environment 2, 153. https://doi.org/10.1038/s43247-021-00217-4

Maschette, D., Wotherspoon, S., Murase, H., Kelly, N., Ziegler, P., Swadling, K.M., Kawaguchi, S., 2025. Resource potential and maturity estimates of Euphausia superba in East Antarctica. Frontiers in Marine Science Sci. 12. https://doi.org/10.3389/fmars.2025.1448250

Matsukura, R., Doiguchi, H., Yamamoto, N., Abe, K., Amakasu, K., Fukuda, Y., Hasegawa, K., Mukai, T., Murase, H., 2024. Association of density and sound-speed contrast with maturity stage of Antarctic krill in the eastern Indian sector of the Southern Ocean. Fisheries Science 90, 893–905. https://doi.org/10.1007/s12562-024-01819-5

Roemmich, D., Sherman, J.T., Davis, R.E., Grindley, K., McClune, M., Parker, C.J., Black, D.N., Zilberman, N., Purkey, S.G., Sutton, P.J.H., Gilson, J., 2019. Deep SOLO: A full-depth profiling float for the Argo Program. Journal of Atmospheric and Oceanic Technology 36, 1967–1981. https://doi.org/10.1175/JTECH-D-19-0066.1

Schaafsma, F.L., Driscoll, R., Matsuno, K., Sugioka, R., Driscoll, S., van Regteren, M., Sasaki, H., Matsukura, R., van Franeker, J.A., Murase, H., 2024. Demography of Antarctic krill (Euphausia superba) from the KY1804 austral summer survey in the eastern Indian sector of the Southern Ocean (80 to 150˚E), including specific investigations of the upper surface waters. Frontiers in Marine Science 11. https://doi.org/10.3389/fmars.2024.1411130

Tamura, T.P., Nomura, D., Hirano, D., Tamura, T., Kiuchi, M., Hashida, G., Makabe, R., Ono, K., Ushio, S., Yamazaki, K., Nakayama, Y., Takahashi, K.D., Sasaki, H., Murase, H., Aoki, S., 2023. Impacts of basal melting of the Totten Ice Shelf and biological productivity on marine biogeochemical components in Sabrina Coast, East Antarctica. Global Biogeochemical Cycles 37, e2022GB007510. https://doi.org/10.1029/2022GB007510

Thomas, G., Purkey, S.G., Roemmich, D., Foppert, A., Rintoul, S.R., 2020. Spatial Variability of Antarctic Bottom Water in the Australian Antarctic Basin from 2018–2020 captured by Deep Argo. Geophysical Research Letters 47, e2020GL089467. https://doi.org/10.1029/2020GL089467

Tozawa, M., Nomura, D., Nakaoka, S., Kiuchi, M., Yamazaki, K., Hirano, D., Aoki, S., Sasaki, H., Murase, H., 2022. Seasonal variations and drivers of surface ocean pCO₂ in the seasonal ice zone of the Eastern Indian sector, Southern Ocean. Journal of Geophysical Research: Oceans 127, e2021JC017953. https://doi.org/10.1029/2021JC017953

Yamamoto, N., Amakasu, K., Abe, K., Matsukura, R., Imaizumi, T., Matsuura, T., Murase, H., 2023. Volume backscattering spectra measurements of Antarctic krill using a broadband echosounder. Fisheries Science 89, 301–315. https://doi.org/10.1007/s12562-023-01678-6

Yamazaki, K., Aoki, S., Katsumata, K., Hirano, D., Nakayama, Y., 2021. Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica. Science Advances 7, eabf8755. https://doi.org/10.1126/sciadv.abf8755