Antarctic seals make good use of warm autumn seawater to secure food. Polar Research Institute and Hokkaido University investigate

Antarctic seals make good use of the warm seawater (warm water) that flows from the open ocean to the coast in the fall to secure food. The results of these observations were clarified by the National Institute of Polar Research and the research group of Hokkaido University. As part of the observations by the 58th Japanese Antarctic Research Expedition (2016-2018), it is said that it was the result of investigation by attaching a low-load recorder to the Weddell seals that live around Syowa Station.

Weddell seal with a recorder on its head. Photographed by Mr. Akihiko Kokubun of the National Institute of Polar Research (provided by the Institute)

According to the Ministry of the Environment, five species of seals such as leopard seals and Weddell seals and fur seals of the Antarctic fur seal inhabit the South Pole. These six species are protected by the Seal Conservation Treaty. About 60% of the world's seals live in the Antarctic Ocean. Of these, the Weddell seals mainly inhabit the vicinity of Syowa Station. Males are about 280 cm long, females are about 330 cm long, and weigh 400-450 kg. The body color changes depending on the season, and it becomes brown as a whole in spring, and in other seasons, blackish brown and pale yellowish white spots are seen all over the body. It appears on ice during the breeding season from spring to summer, but mainly lives under ice in winter. The research group consisting of young researchers such as Assistant Professor Akihiko Kokubun of the Biosphere Research Group of the Polar Research Institute and Associate Professor Shigeru Aoki of the Institute of Low Temperature Science, Hokkaido University will start from the fall (March to April) of 2017. Until (September), eight Weddel lizards were fitted with the latest water temperature salt recorder (CDT tag), and a previously unknown coastal marine environment survey was conducted after autumn. In addition to location information, the CDT tag measures salt content and water temperature, and at the same time records the diving depth and transmits the data by satellite communication. The weight of the device is 580 grams, which is lighter than the weight of the seals surveyed (average 326 kg), and the load on the seals is small because they fall off together with the seals after a certain period of time when the seals lose their hair. Analysis of the data obtained by the research group revealed that hot, low-salt water was found on many shallow coastal seabeds (100-150 meters deep) in the fall, and deeper (maximum) as the seasons progressed toward winter. The actual condition of the dynamic marine environment along the Antarctic coast was revealed, such as the fact that it had subducted to a depth of 400 meters). In addition, we calculated the index showing how efficiently the seals secured food from the record of diving depth, and analyzed how it affected the feeding behavior. As a result, it became clear that seals were able to efficiently obtain food for krill and sardines in low-salt, high-salt water at a higher temperature than in low-temperature, low-salt water.

A schematic diagram of the marine environment on the continental shelf near Syowa Station from autumn to winter. Colored arrows indicate the flow of seawater, and white arrows indicate the expected flow of nutrition (illustrated by Shinichiro Kinoshita, provided by the National Institute of Polar Research and a research group at Hokkaido University).

Many large animals such as seals and penguins inhabit the coast of Antarctica even in the sea area covered with sea ice. It has been pointed out that one of the factors that makes this possible is the existence of abundant organisms associated with the influx of warm water rich in nutrients from the deep layers of the open ocean. However, it was difficult to conduct oceanographic surveys by ship due to the thick ice, and it was not well understood under what circumstances large animals in extremely cold waters could live.