Some notes on the swans, and on the main factors that caused their extensive death at Odaito Bay, Nemuro, Hokkaido

Abstract

There are two species of swans in Japan. One of them is the Whooper Swan, Cygnus cygnus and the other is the Eastern Bewick's Swan, Cygnus columbianus jankowskii. Both of them come from the northern and north-eastern parts of Siberia to Japan and spend some six months in this country as winter bird. The earliest arrival in Japan is late in October and the last departure is the 28th of April, according to the past observation records.The main wintering areas of swans are Lake Toro (Kushiro), Lake Furen and Odaito Bay (Nemuro), Lake Tofutsu (Abashiri), Lake Utonai (Tomakomai), etc., in Hokkaido. In Honshu they pass the winter at Ohminato Bay and Kominato Bay (Aomori Prefecture), Lake Hyo (Niigata Prefecture), etc. A small number of swans (1-8 individuals) occasionally visit Arasaki, Kyushu.It seems that Odaito Bay is the most important wintering area in Japan, since about ten thousand Whooper swans stay there for six months (Fig. 1). There is no other comparative wintering area in our country.Odaito Bay is located on the eastern part of Hokkaido, between Shiretoko Peninsula and Hanasaki Peninsula (Fig. 2). The area of Odaito is estimated at about 51, 840, 000 square meters. Many kinds of seaweeds are found in this region, providing suitable food for the swans. In addition, during the winter many speciesof ducks stay in this area.As far as I know, Eastern Bewick's swans are rarely found in Hokkaido, while there are comparatively many of them in Honshu.About 200 Whooper swans died because of the cold wave at Odaito Bay in February, 1967. A period of successive low temperatures continued for two weeks (Fig. 3) The temperature began to go down on the 8th of February. We had never before heard that a great number of swans died from cold and starvation.I visited Odaito Bay two times, just before and immediately after the accident, and found that there had occurred a remarkable reduction in the number of subadult swans.On the 12th of February, 1967, 106 sub-adults (11per cent) and 893 adults (89per cent) out of the total number of 999 swans were observed. On the 26th of February, 1967 only 15 sub-adults (2per cent) and 873 adults (98per cent) out of the total number of 888 swans were found. Incidentally, about 16per cent (329 out of 2, 047) for sub-adults and 84per cent (1, 718 out of 2, 047) for adults were observed at Odaito Bay on the 12th of February, 1966 (from the colour films taken at random).The percentages of the adults and sub-adults were estimated from the colour films which were taken at random at Odaito Bay.In order to determine the cause of death in adult and sub-adult swans autop-sies were taken. There was found no food in any crop of the dead swans. Although no green seaweeds were seen in the gizzards of two sub-adults (Fig. 4), some grits were found. On the other hand, in the gizzard of one adult was found a trace of green seaweeds.We could not find externally any symptoms of disease on the various internal organs.It has been found that when the swans were healthy excreted solid fecal pellets (Fig. 5-A), but when they were weak they excreted diarrheic pellets (Fig. 5-B).It is desirable to stir the surface of the water mechanically or to make ripples by bubbling with an air compressor in order to prevent freezing of the sea water on cold nights so that the swans may have access to the seaweeds.