Photo/Illutration A rise in water temperatures is blamed for bleaching of table coral off the Izu Peninsula of Shizuoka Prefecture. (Provided by Kirin Sekito)

The surface ocean temperatures in waters near the Japanese archipelago reached a record high from June to February for the third consecutive year, negatively impacting the fisheries and aquaculture industries.

According to the Japan Meteorological Agency, record high temperatures for three years running was the first time that has occurred since the agency began tracking surface sea temperatures in 1982. 

In 2023, the average ocean surface temperature in waters near Japan was 1 degree higher between June and August than normal, which is the 30-year average of the same period from 1991 to 2020, the agency reported. 

As for the September-November period and the December-February period, the water temperature was 1.9 degrees higher and 1.1 degrees higher, respectively.

The largest difference of 0.8 degree in the surface ocean temperature in the June-August period, compared with the normal, was recorded in 2022.

The largest disparity for the September-November period came in 1998 and 1999, when it was 0.7 degree.

For the December-February period, the biggest discrepancy of 0.7 degree was logged in 1999.

The record was set as Japan and elsewhere recorded unusually high temperatures last year.

According to the agency, the average temperatures for 2023 in the northern and northeastern regions were the highest since the agency began collecting data in 1946.

As for western Japan, the mercury reading was on par with the highest recorded.

The sea surface temperature is expected to remain high this year, given the agency’s forecast for this year’s air temperature being as high or higher than in 2023.

The agency said higher temperatures were particularly noticeable on the eastern coastline of the main island of Honshu, a result of global warming and a shift in the direction of the Black Current.

The Black Current, a warm current flowing from south to north, changes its direction to eastward and flows away from Japan after approaching the Boso Peninsula, which separates Tokyo Bay from the Pacific Ocean.

But since spring 2023, the Black Current has continued to flow along the coast of the northeastern Tohoku region, rather than traveling eastward.

“It is highly unusual that the Black Current goes that far to the north and it remains a continuing trend,” a JMA official said.

The shifting tide of the current is bringing a variety of fish to the Tohoku region that have not been previously caught.

Miyagi Prefecture’s fisheries technology center reported that fishermen at the Ishinomaki fish market earlier this year caught species of fish that usually inhabit the waters off southern Japan, including giant trevally and spangled emperor, the first such catches there.

These species of the fish were believed to never migrate beyond the seas of the Kanto region.

A monitoring spot on Tashirojima island in Sendai Bay logged 13.5 degrees of the average surface sea temperature in the middle of March, 4 degrees higher than in the same period in 2023 and 6.3 degrees higher than normal.

“The water temperature level last month is unprecedented,” an official at the center said. “It is difficult to predict how its trajectory will move.”

High sea temperatures in recent years are taking a toll on fisheries and aquaculture industries, according to the Fisheries Agency.

Large hauls of yellowtail and blowfish have been reported in Hokkaido, while catches of cold water fish, such as salmon, have plunged in the sea off the Tohoku region.

The higher water temperature is also affecting the growth of seaweed, which normally fares better in colder water.

(This article was written by Takuya Miyano and Eiji Zakoda.)

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