The acidity of seawater well off the Kii Peninsula near 137 degrees east longitude has been increasing since 1984, according to the Meteorological Agency.
The rise is believed due to an increase in carbon dioxide from industrial emissions, the agency said.
If acidity continues to rise, calcium carbonate levels in plankton, shellfish, shells and the skeletons of crustaceans will fall. As a result, the number of fish that rely on these small creatures for food will decrease, eventually hurting the fishery industry.
There are also concerns that acidification could accelerate global warming because the ocean’s capacity to absorb carbon dioxide will fall.
Two observation ships from the agency conduct surveys on acidity every winter at 137 degrees east longitude and between 3 and 34 degrees north latitude.
The surveys look at the hydrogen ion concentration index of water taken at a depth of 4 meters.
The results of the survey are posted on the agency’s website, which will be updated each May starting next year.
In its original state, seawater is slightly alkaline, with the index standing around pH 8. But the index has shifted by 0.02 every decade.
The pace of acidification is faster in the sea around Japan. The figure fell to 8.118 this year from 8.175 in 1984 at 30 degrees north latitude.
At 20 degrees north latitude, near Okinotorishima, the country’s southernmost island, the index dropped to 8.097 from 8.141, while it fell to 8.068 from 8.096 at 10 degrees north latitude, near Yap Island, part of the Federal States of Micronesia.
Seawater at 137 degrees east longitude was surveyed because of the Kuroshio Current and the subtropical gyre.