Marine Information (Water Temperature, Salinity, Characteristics)

Effects of Continental Coastal Waters
In 1963, Ishimine Anshin and a team from the Ryukyu Meteorological Observatory took part in an investigation by the University of the Ryukyus, conducting surveys of the sea surrounding the Senkaku Islands for water temperature, salinity, and nutrients (phosphates). Ishimine's report states that the coastal water temperature when he left Naha Port on May 15, 1963, was 25°C and that reaching the Kuroshio Warm Current brought an increase to between 26°C and 26.8°C, the latter maximum temperature being recorded near the edge of the continental shelf by Taisho Island and representing an increase of 1.8°C. However, water temperature dropped to 25.2°C and chlorine content fell to 1.895% near Uotsuri Island, clearly showing a mixture with coastal waters from the continent. Ishimine inferred that continental waters and warm waters met in the sea near Uotsuri Island even in mid-May. (Ref. 1)

There is also a clear change in water temperature by depth in the continental slope area around the Senkaku Islands. Ishimine registered a difference of 1.1°C-1.6°C between the temperatures at the surface and at 20 meters' depth. While the surface is strongly heated by sunlight, the temperature drops rapidly through the shallow depths toward the seabed. (Ref. 1)

Between March 29 and April 10, 1971, as part of a comprehensive academic survey by the University of the Ryukyus, the Okinawa Prefectural Fisheries Experimental Station conducted hydrographic and fishery surveys. Six observation points were set up to the southwest of Uotsuri Island. The results showed a difference of approximately 10°C in surface temperature and from 0.18% to 0.20% in chlorine content between the observation point far into the area above the continental shelf (station 3) and the observation point on the edge of the continental shelf, where there is a strong influence from the Kuroshio Warm Current. (Ref. 2)

Nutrient Salts and Fertile Fishing Grounds
In his 1963 report, Ishimine Anshin states that although there have been no direct measurements of the sea around each of the Senkaku Islands, it goes without saying that there are major fishing grounds. This is because phosphates are a source of nutrition for plankton. According to a map of equivalent phosphate content levels there are 0.2 microgram-atoms per liter at the surface, 0.5µg-atoms/l at depths of 100 meters, and 0.7µg-atoms/l at depths of 200 meters. This area also has a steeper vertical gradient than the main Kuroshio Warm Current. (Ref.1)

Recent Surveys
In September 2012, a Tokyo Metropolitan Government research group conducted hydrographic surveys in the sea around Uotsuri Island, Minamikojima Island, and Kitakojima Island. Using fish detection equipment, the group carried out surveys of water depth, offshore distances, coastal and seabed topography, tide levels, and flow regime. The group recorded surface water temperatures of between 28.1°C and 28.6°C in Uotsuri Island coastal areas. (Ref. 3)




Ref.1 : Ishimine Anshin, "Senkaku Retto kaiyo chosa hokoku" (Report on the Senkaku Islands Hydrographic Survey), in Ryukyu jiho (Ryukyu Reports) 7 (May 30, 1963), recorded in "Takara gakujutsu chosadan shiryoshu" (Takara Academic Research Group Materials Collection), a body of research published by the Senkaku Islands Document Material Compilation Association.

Ref.2 :Kanehama Yasunobu and Kaneshiro Takemitsu, "Senkaku Retto shuhen no kaiyo kansoku oyobi gyojo chosa" (The Senkaku Islands Area Hydrographic and Fishery Surveys), in Ryukyu Daigaku Senkaku Retto gakujutsu chosa hokoku (Report of the University of the Ryukyus on Senkaku Islands Scientific Expedition) (1971).

Ref.3 : Tokyo-to Senkaku Shoto genchi chosa hokokusho (Report of Tokyo Metropolitan Government on the Senkaku Islands Local Survey) (Tokyo Metropolitan Government, 2012).